Neil Sheeley (BS '60, PhD '65), Astrophysicist and Heliophysicist

Considering the roads not taken in one's educational and career path can be a means of sharpening our appreciation of the choices made. In the discussion below, Neil Sheeley muses about not going to Princeton, despite his mother's urgings, where she hoped he would become a "normal person" - perhaps a lawyer. Instead, Sheeley embraced his self-identity as an "egghead" and went off to Caltech to pursue research in number theory and eventually heliophysics. He never looked back.

At Caltech, Sheeley learned from the giants; Feynman, Leighton, Apostol, and his research focus on solar wind built on the foundational work conducted by Robert Millikan and George Ellery Hale even before the inception of the Institute. A "complete Techer," Sheeley finished both his undergraduate and graduate degrees at Caltech, before embarking to the newly-created Kitt Peak National Observatory and its solar telescope, which promised a new generation of heliophysics research focusing on the source of coronal mass ejections and the distribution of solar wind. From there, Sheeley joined the Naval Research Laboratory (NRL). His descriptions of the freedom to pursue fundamental physics offers a good lesson on the willingness of the U.S. military to support purely basic science - whether or not it has some application to the national defense.

During his long career at NRL, Sheeley conducted research on coronagraph observations, surface fields and their heliograph extensions, and the associations between coronal mass ejections and interplanetary events. He also made important contributions in the area of solar flares and the dangers they pose to the electric grid. In 2009 Sheeley was awarded the George Ellery Hale Prize from the American Astronomical Society, and the event compelled him to wax poetic on Hale's far-reaching research, the giant advances since achieved in heliophysics, and the remaining questions left to future researchers.

DAVID ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It is Friday, January 13th, 2023. I am delighted to be here with Dr. Neil R. Sheeley Jr. Neil, it's great to be with you. Thanks so much for joining me today.

SHEELEY: It's nice to be with you, David. Thank you.

ZIERLER: Neil, to start, would you please tell me your current or most recent title and institutional affiliation?

SHEELEY: When I retired from the Naval Research Laboratory here in Washington, D.C., I was an astrophysicist. That was my official job title. I was in the Space Science Division of the Naval Research Laboratory. I retired on November 30, 2016. I was 78 years old at the time, so I stayed on a long time. [laugh]

ZIERLER: Neil, by virtue of your science and your affiliation at NRL, is astrophysicist really the best way to describe your career?

SHEELEY: I'm not sure, because I've done a number of things over the years. When I came out of graduate school at Caltech, I was doing solar physics. My first job was in Tucson at the Kitt Peak National Observatory, where I continued to do ground-based solar physics. When I started working there, I had the title "Junior Astronomer". [laugh] I guess they changed that title after a while because they thought it was too pejorative [laugh]. I remember thinking that they should call the younger members of the Administrative Division, "Junior Businessmen". After a year or so, they asked me what I wanted to be called, and, thinking of my Caltech diploma, I said, "I'm a physicist." So I became an Assistant Physicist, and then an Associate Physicist. When I joined NRL, I think they called me a Research Physicist for a while. Then some people came to interview the staff members to see if their jobs matched their titles. When their project was over, [laugh] I became an Astrophysicist. I thought that Astrophysicist was more accurate at that time because that's what I was doing. I was not doing nighttime astrophysics, but I was doing astrophysics of the sun.

ZIERLER: Neil, over the course of your career at NRL, was it mostly fundamental research that you were involved in, or were there more applied aspects of your position?

SHEELEY: It was pretty much fundamental or curiosity-driven scientific research, but within a larger area where the discoveries could be applied. So it was similar to the approach that had been used at Bell Labs, where the transistor work was fundamental, but had applications in communications. My NRL colleague, Yi-Ming Wang, and I have just finished writing an essay for the 100th anniversary of the Naval Research Laboratory. They asked us to write an essay on the Wang-Sheeley-Arge Solar Wind model. We concluded by saying that when we made our solar wind speed model, which is now part of the National Weather Service Forecasting Office, there was no internet, and there were no websites to post our forecasting capability on. The only reason we made this discovery was because highly motivated people were put together with suitable funding, and I was allowed to work on curiosity-driven research. So the work at NRL was analogous to what Tom Rosenbaum described in his end-of-year 2022 letter at Caltech — the Idea Factories of Bell Labs and Caltech. NRL is also an Idea Factory, and we wanted to say something upbeat to help keep it going for another 50 years. [laugh] Our discovery had an application, and NRL liked that, but we didn't get there by saying, "Let's invent a forecasting model that we could put on a National Weather Service website." There were no websites.

Astrophysics and the Naval Research Laboratory

ZIERLER: Neil, let me ask an almost too-obvious question. Why would the Naval Research Laboratory support astrophysics?

SHEELEY: I think it's historical. I was hired by Richard Tousey, who was in the Optics Division of the Naval Research Laboratory at one time. I imagine that's because there was a need to see well from ships both during the day and at night. However, at the end of World War II, the US military recovered a number of V-2 rockets from Germany, and NRL was given the opportunity to fly payloads on them. At that time, the Director of the Naval Research Laboratory was E.O. Hulburt, who had already made a name for himself studying the upper atmosphere and its effect on radio communication (another solar-related subject). As I understand it, Hulburt challenged Tousey to provide a V-2 payload. Tousey designed a photographic spectrograph to observe the extreme ultraviolet spectrum above the Earth's atmosphere, and on October 10, 1946, he obtained a spectrum that extended to 2100 Angstroms, well below the atmospheric cutoff around 3000 A. Later, in 1959, he obtained a spectrum of the Lyman alpha line at 1216 A as well as a Lyman alpha image of the Sun. He went on to obtain images of the Sun in a variety of ultraviolet emission lines using a slitless spectrograph, which is how I first came to know of him.

I think it was around this time that NRL's Herbert Friedman flew X-ray payloads in which the observations were radioed back to Earth. He was the first to observe solar X-rays and study their effects on Earth's atmosphere, and he was one of the first to observe non-solar X-rays. (A group at the American Science and Engineering Corp. in the Boston area made the first X-ray observation of a non-solar source, and one of its members, Herb Gursky, eventually replaced Herb Friedman when he retired as Superintendent of the Space Science Division at NRL.) Not only did Friedman's solar X-ray observations help to distinguish the relative effects of X-rays and extreme ultraviolet radiation on the Earth's atmosphere, but also his patrol X-ray observations with the SOLRAD satellites provided a way to monitor radiation from nuclear bomb tests. So that's another example of why the Navy supported astrophysics in general and solar physics in particular.

The area of geomagnetic disturbances provides another example. Things that happened on the Sun affected the Earth's magnetic field, and these geomagnetic disturbances were of interest to the Navy. When I returned from the Skylab mission in Houston, I was directed to a paper by NRL Ocean Scientist Perry Alers who had been measuring the magnetic field in the vicinity of seamounts in the Atlantic Ocean. I guess he did it by dragging magnetometers across the ocean floor. He had found magnetic variations and wondered if they were caused by a solar source, and if so, if the source were a flare or a coronal hole. As I recall, those geomagnetic disturbances were caused by high-speed streams from coronal holes that we had observed during the Skylab mission. There are many examples of this kind of relevance.

In the 1970s, relevancy seemed to dominate our work. I think it was around the time that Jimmy Carter was President, which was 1977 to 1981. Anyway, around that time everything had to be relevant. Luckily, during Skylab, we had discovered ways of predicting geomagnetic activity by looking at coronal holes. These are regions where the magnetic field extends outward from the Sun and doesn't return. It doesn't bend over and come back to the surface but extends outward indefinitely. Of course, to balance the flux, you'd have two holes: one with positive flux, and another with negative flux somewhere else. In any case, if these regions were at the equator, then the Sun's rotation would periodically direct their high-speed streams toward the Earth to produce recurrent geomagnetic disturbances. So we were learning how to predict geomagnetic disturbances. Historically, newspapers would have headlines that said, "Solar flare to light sky," or something like that, very dramatic. What the news writers didn't know was that coronal holes can also light the sky because their geomagnetic disturbances are also accompanied by auroras.

Here is one of my favorite relevancy stories: It began when Peter Gentieu, a scientist at the Goddard Space Flight Center, returned from a long observing run (about a month) at Fort Churchill, Canada, where his objective was to launch a rocket into an aurora and obtain ultraviolet spectra. During that observing run, there was no aurora visible from Ft. Churchill. But soon after he returned to Goddard, there was an aurora, and, of course, he missed it. He had spent a month at Ft. Churchill, occasionally encountering (or watching out for) polar bears that wandered into town, and had nothing to show for it, except the travel expenses for him and his scientific team.

Somehow Peter heard that auroras can be caused by coronal holes as well as solar flares, and he asked me what caused the aurora that he missed. I looked at the Kitt Peak infrared helium images and photospheric magnetograms, and I found a coronal hole that might have been responsible for a geomagnetic disturbance and aurora. The timing was correct, and the magnetic polarity was negative, which favored geomagnetic activity in the spring season of the year. So we decided to watch for the return of that coronal hole during the next rotation of the Sun. If a negative-polarity hole of sufficient size came around the limb, then we would calculate when it would reach disk center and add three days of Sun-Earth transit time to estimate when its high-speed stream would reach Earth. The coronal hole returned the next rotation and all signs looked favorable for another geomagnetic disturbance. Peter Gentieu contacted someone at Wallops Island, where the rocket was kept, and they decided to fly the rocket to Fort Churchill on the day before the aurora was predicted to occur. On the next day, the aurora occurred, they launched the rocket, and obtained the auroral spectra. The experiment was a success and the observing run was much shorter and less expensive than the previous one. In addition, I was able to use that story as an example of relevancy when I wrote proposals to obtain NASA funding for my own solar research. Perhaps I have given too many examples.

ZIERLER: No, no. Neil, actually, to push further on the distinction between fundamental and applied research at NRL, just as a thought experiment, had you worked at a university or a different government agency, do you think that you would've pursued more or less the similar kinds of research?

SHEELEY: I worked at the Kitt Peak National Observatory for several years, and I was doing what might appear to be less-relevant research. I was studying sunspots, watching little magnetic flux elements move out from sunspots, taking spectra, and doing lots of things like that. If I had stayed there, I probably would have continued to do curiosity-driven research. But how do we know? As NSF funding became more difficult to obtain, even staff members at KPNO began to look for funding. (This is something we never were allowed to do in the early years. We were supposed to help visitors and do our scientific research, and I let the KPNO Director worry about obtaining funds from the NSF. I recall a meeting of our little Solar Division in which Keith told us that we had $200K that we would lose unless it were spent by Friday. Then he asked if anyone had anything they would like to buy.) If I had been there during the later years, I probably would have looked for funding too, especially to support the solar-based geomagnetic forecasting that was relevant to NSF and NASA missions.

Recall that the golden age of curiosity-driven research ended at Bell Labs when funding became scarce after the breakup of AT&T in 1984. I worked at Bell Labs four summers during the years 1956-1960, and I think that most of that research that I encountered was at least partly related to the goals of the telephone company. I worked in a transistor group for two summers, solving transistor circuit equations. Another summer, I worked with millimeter waveguides, using a klystron oscillator to feed a millimeter waveguide system containing a ferrite crystal. Faraday rotation of the polarized radiation caused a measurable change in the output power of that waveguide system. I think that Bell was thinking about using millimeter waves for communication at one time. So that activity probably was relevant to AT&T communication goals too. In the summer of 1960, I worked with Sol Buchsbaum studying cyclotron resonance in a magnetized plasma. I don't know how that experiment was related to communications. However, Sol Buchsbaum was a very talented scientist, and at Bell Labs, very talented people, like Sol, Claude Shannon, and John Pierce, seemed able to do curiosity-driven research. On the other hand, the experience Sol acquired doing first-class scientific research at Bell Labs made him well prepared to provide scientific advice to presidents and government committees during his later years. He received the National Medal of Science from President Reagan and served as Chair of the White House Science Council under presidents Reagan and George W. Bush, among other things.

ZIERLER: Neil, to turn that question around, at NRL, do you feel, either directly or indirectly, that you contributed at all to the national defense?

SHEELEY: Yes, I think that I contributed at least indirectly to the national defense. For example, if Navy systems are affected by geomagnetic storms, then being able to predict when those storms occur would be a useful contribution. I did not work on offensive activities, like designing drones to go off on bombing missions. But I did things that helped the Navy. On one occasion I went to a Naval base in Norfolk, and explained some things that I had learned about how the Sun affects the Earth, to Naval personnel there. I was able to help because I was doing solar-terrestrial research (now called space-weather research), which concerns the way that the Sun affects the Earth and its surroundings. Also, part of my research was done under contract with NASA, not only for my own projects, but also as a Co-Investigator for NRL experiments on NASA missions like Skylab, SOHO, and others. So, in addition to helping the Navy, I also helped NASA, and in some cases my discoveries and accomplishments may have helped both organizations at the same time.

Princeton and Not Becoming a Lawyer

ZIERLER: Neil, let's go back now, and establish some historical context. In high school, how did you get the idea to apply to Caltech, or how did you even learn about Caltech?

SHEELEY: I was wondering about that myself recently. I don't remember how I first learned about Caltech. Of course, the 1955 Time magazine article had an effect. I wouldn't say it was the whole thing. The Time magazine article was a cover article called The Purists. It was about scientists that had done applied research at Los Alamos and similar places during the war, and were returning to curiosity-driven research at universities, and especially Caltech. Ira Bowen was the director of the Mount Wilson Observatory at the time, and he said, "For the dickens of it, that's what we're doing here. We're doing astronomy for the dickens of it".

But I should step back and talk about math because, as for many scientists that you've probably interviewed, math was central to my life in those early years. That's how I got started. I would say that in sixth or seventh grade, math was my least-favorite subject. But by eighth grade, I was getting math books out of the library, and going to my friends to show them the "neat things" that I had found, often about the puzzles we were working on.

Two ideas come to mind: Mrs. Wilkens math class and my father's "Calculus Made Easy" book by Thompson. The George Washington Junior High School in Ridgewood, New Jersey, went up to ninth grade. Even though ninth grade is technically the first year of high school, that was the last year of GW. So at the start of ninth grade, Mrs. Wilkens gave us an Algebra 1 book. At the end of the first week, she asked for feedback. She said something like, "Class, we've been doing this now for one week. What do you think? Do you have any questions?" I said, "This was a really good book that you gave us, Mrs. Wilkens, but I finished it. Do you have any more books?" She said, "Yes, I do. I have a more advanced algebra book that a publisher sent me as a trial. You can read that book and work the problems." So I went through that book reading everything and working all the problems myself without any help that I can recall. At the end of the year, she sent me over to Ridgewood High School to take the final exam in second year algebra. I didn't have any trouble with that exam, and had the impression that I did very well. Later, I think that someone, probably Mrs. Wilkens, confirmed that I had done well, especially compared to some of the other students. But she did not tell me the grade, so I didn't ask her what it was.

The second point had to do with my father and his Calculus Made Easy book. He had grown up with radio and was a radio amateur. He did his service during World War II, not in the Navy, but working for a contractor, the Submarine Signal Company, installing, maintaining, and operating radar on Navy ships in the Pacific. At that time, there weren't very many people who knew about radar, but the contractor had the clever idea that he could find them through amateur radio. He found my father and several others in the directory of radio amateurs. Later, when Dad was on a ship in the South Pacific, he received a request from the draft board to report for a physical in Boston. According to that story, he and some of his shipmates enjoyed composing a reply, saying that he would be glad to report to Boston for a physical if the draft board would arrange for his transportation out of the war zone. But eventually he returned, found a new job, and renewed his amateur radio activities, which leads to my second point about math.

My father was not particularly skilled at math, and he had purchased a few calculus books. I don't know what he intended to learn from them. Maybe they helped him do the circuit calculations or the antenna theory that he encountered in his amateur radio activities. Or maybe they helped him interpret Maxwell's Equations that I saw in one of the books he had received while working for Submarine Signal Company at the end of the war. In the post-war years, I found several of those math books in a bookcase in our house. One of them was Calculus Made Easy by S. P. Thompson. Inside the cover, there is a small tag saying, "Old Corner Book Store, Inc., Boston, Mass." So he must have bought that book in Boston while we still lived in New Hampshire.

Thompson was a very learned person, a fellow of the Royal Society of London, and practical. He said, if this book "falls into the hands of professional mathematicians, they will (if not too lazy) rise up as one man, and damn it as being a thoroughly bad book". (The preface of that second edition was written in October 1914. In a modern world with more women mathematicians, he would have said "rise up as one person" instead of "rise up as one man".) His point is that his book omits all the hard stuff in calculus - the things that I would later learn from Tom Apostol's book at Caltech (real and complex number systems, set theory, limits, continuity and topological mappings together with lots of epsilons and deltas). Thompson just explained derivatives and integrals in a simple way, including how to evaluate them. It was like learning to speak or read English before learning about verbs, adjectives, predicates, or diagraming sentences. So here I was, in ninth grade, doing calculus very easily without knowing that it was supposed to be difficult.

Around this time, I went into New York City with a ninth-grade friend and his father who worked there. I think that my friend had an eye appointment or something, but while we were there, we went to look around in the Barnes & Noble Bookstore. I found three books that I liked - one on vector analysis, one on Riemannian geometry, and one on calculus. The calculus book was Advanced Calculus by Kaplan, which turned out to be the text that Caltech math students used their junior year, and therefore turned out to be very helpful. I thought that Riemannian Geometry by Eisenhart would be helpful for interpreting the equations that I had seen in The New York Times in a front-page article on March 29, 1953 about Einstein and his latest field equations, but I never really learned Riemannian geometry, even at Caltech when I took a course in general relativity from H. P. Robertson.

Our family had moved quite a bit since the war, and my father thought that I should go to one school for the rest of my pre-college education. So when I finished ninth grade at GW in Ridgewood, I began my sophomore year at Mount Hermon School on the Connecticut River in northwest Massachusetts (now called Northfield Mount Hermon School). Right away, I was able to take the advanced math course for seniors, which really wasn't very advanced - at least not after reading Kaplan's book on advanced calculus. In those years, I learned calculus and math pretty much on my own because I liked it, not because it was offered at any particular school. In fact, by my senior year, I had to learn math on my own because I had completed all of the required math courses and I didn't know anyone who could help me with more advanced math.

In the spring of my senior year, I met with Paul Eaton, the Caltech Dean of Students, for an interview. I think that most prospective freshmen were interviewed by members of the Caltech community. Dean Eaton was originally from Maine, and probably did the interviews for students that were applying to Caltech from high schools in New England. During the interview, I asked him whether I would be able to skip some courses at Caltech. He said something like, "Do you mean in physics?" and I said, "No, I don't think I'm that advanced in physics. I mean in math." (I got As in physics at Mt. Hermon, which was sufficient to get into college, but I had progressed much farther in my own self-study of math.) I don't remember what he said then, but the idea was to come see him when I arrived at Caltech.

So on my first day at Caltech, I went to see Dean Eaton in his office at Throop Hall. After a few minutes, he called H. Frederick Bohnenblust, Chair of the Caltech Math Department (and soon to be Dean of Faculty later that year), and Bohnenblust asked Eaton to send me over to his office. When I arrived, Bohnenblust asked me what I had been doing in math, and I told him that I had bought Kaplan's book at Barnes & Noble, and I was working my way through it. He wanted to talk about vector integral calculus and said something like, "Do you know that if you cut an area in a certain way that you can do a line integral around that area by such and such?" (He was explaining that one could do a line integral on the inner and outer edges of a two-dimensional doughnut by cutting the doughnut to form a single boundary, and then recognizing that the contributions on the facing edges of the cut will cancel, leaving oppositely directed contributions from the inner and outer edges of the doughnut.) I didn't know that technique, but thought it was "pretty neat". Then he asked Professor Tom Apostol to come into his office. Apostol talked to me for a while and then said something like, "These are pages of an old sophomore final exam. Could you take them to the library across the hall, and see if you can work the problems? Normally, we give students three hours to complete the exam." I worked them in about an hour and he said that I could skip the first two years of math. (But he did make a comment that I think was very significant. He asked me if I knew the theorem for solving a particular kind of linear differential equation (perhaps an inhomogeneous equation). I told him that I remembered it vaguely, but I knew that I could solve that equation by transforming it to a simpler form that was much easier to solve (probably a homogeneous equation). So I did it the simpler way that I knew.) He suggested that I take the sophomore math elective (given by Basil "Sandy" Gordon). It was a three-term course, consisting of modern algebra and matrix theory, the theory of equations, and mathematical logic.) At the same time, he suggested that I read his typewritten chapters of "Mathematical Analysis," which he was writing to replace Kaplan's book, and meet with him once a week to discuss my reading. In retrospect, I think that Apostol's book was too rigorous - it had all the epsilons and deltas. That's ok. I was able manage my way through epsilons and deltas, but I think that I might have benefitted from less rigor and more applications or additional topics along the lines of S.P. Thompson's philosophy. So, I stopped the independent reading after my first term, and manipulated those epsilons and deltas in my sophomore year when I took the second and third terms of Apostol's Ma108 course, using his new book. I got an A in that course and still refer to Apostol's book from time to time, so I probably benefitted from that course more than I realized.

And now, David, I've taken all this time on math topics and not spent enough time on your question of how I became interested in Caltech[laugh]. I applied to five colleges. (It cost about $5 per application and I was accepted at all five.) I applied to Dartmouth because I was originally from New Hampshire, and I liked skiing and the outdoors. I applied to Stanford because I understood that if I got a certain grade on their math test, I would receive four years of full tuition. I applied to Princeton probably because I'd been reading about Einstein's unified field theory in the New York Times. I don't know why I applied to Caltech; maybe I was influenced by the Time Magazine article. Somewhere (maybe in that Time Magazine article, which was called the Purists), I heard that Caltech was the best school for pure science. I applied to MIT, but thought it was focused more on engineering than on science. Besides it didn't have a football team, whereas the Caltech football team played in the Southern California Intercollegiate Athletic Conference (SCIAC). I didn't play football, but I thought that going to football games was a normal college activity. Stanford said that I was not eligible to take the math test because it was only open to students living in four neighboring western states. I narrowed it down to Princeton versus Caltech. Now, this is an old family story, but it's a funny story. My mother said, "You should go to Princeton. If you go to Princeton, you'll become a normal person." You won't become what we now call a nerd.

ZIERLER: [laugh]

SHEELEY: [laugh] "You'll probably become a lawyer. You'll live around here somewhere, maybe in Philadelphia, and I'll get to see you occasionally. But you'll be a normal person, playing tennis, watching baseball, and doing other normal things." [laugh] My father said, "Oh, no." (My father was from Oregon, and my mother was from New Hampshire, so they had different experiences and opinions.) My father said, "You should go to Caltech. First of all, you'll see another part of the country, which will broaden your experience. Second, if you go to Princeton, you will have to compete socially with the children of very rich people, whereas if you go to Caltech [laugh], you will compete on your own merit, just as you will do in your studies." Then he said, "Besides, if you go to Caltech, you'll get away from your mother" [laugh]—

ZIERLER: [laugh]

SHEELEY: —which was just a joke. But that's a funny story that we often told in our family. [laugh] I accepted Caltech's offer. In those days, you sent a letter across the country, and it arrived in a few days, three days or five days, depending on whether you sent it air mail or by train. But, in the meantime, I got another letter from Caltech saying, "We're pleased to offer you the Caltech Regional Prize for New England. It's $1,000, and is based on your college board scores, and your promise as a future scientist." At that time, the total cost of a Caltech education was $1,800 per year, including room and board, tuition, and miscellaneous expenses like books, according to the catalogue. Then I was concerned that I had already accepted Caltech's first offer without the prize. So [laugh] I better send them a telegram accepting the prize before they get my first letter, accepting without the prize [laugh]. That was really stupid, but it's what I did.

Another interesting thing about going to Caltech was meeting the other students. Each year at Mt. Hermon, I would go to Amherst for a math contest, sponsored by the math club of the University of Massachusetts. My sophomore year, I came in first in the Amherst District in western Massachusetts. My junior year, I was first in the Amherst District again. But my senior year, I came in second in the state, which seemed pretty good. But when I got to Caltech, I found that my roommate had been first in the state. [laugh] He was the valedictorian at Andover Academy, which was a really big accomplishment. But I was thinking, "I'm not chopped liver. Caltech offered me its Regional Prize for New England, so [laugh] I don't have to feel that bad." [laugh] We were both learning humility — that there are a lot of smart people in the world, and many of them are right here at Caltech.

ZIERLER: Neil, what were your initial impressions when you arrived on campus? What do you remember?

SHEELEY: I remember going to breakfast. I thought it was the next day, but maybe it was later on the morning that I arrived. But let's take a step backwards. I had taken the train to Caltech. My father drove me to the Grand Central Station in New York late one afternoon. From there, I took the New York Central's overnight train to Chicago. In Chicago, I transferred to the El Capitan, and spent two nights in a seat that reclined when you wanted to sleep and the intervening day in the observation car playing solitaire, drinking coke, and looking at the view. When I arrived in Pasadena, a group of Caltech students met the train, and took my footlocker - the same footlocker that my father had used when he was on shipboard during the war - and my suitcase, and drove me to Blacker house, which was where I was staying. Those older students were really nice to help out like that. So the first thing I remember was that the older students met the train and took me (and perhaps others) to Blacker. (In retrospect, the older students were probably looking for freshmen to join Blacker House. So this kindness may have been a subtle start to the process called "rotation", in which the new students spent two days eating in each student house - Blacker, Ricketts, Fleming, and Dabney - before deciding which house they would like to join.)

The second thing I remember was when I went to breakfast with a group of students from Blacker House later that morning or the next morning. (I can't remember which day it was.) I do remember that Blacker House President, Mike Bleicher, was leading the group, so maybe "going to breakfast with the frosh" was another step in the subtle approach to "rotation". Of course, I was unaware of this possibility at the time.

We walked through Ricketts Courtyard and along the Olive Walk to an old wooden building that we called "the Greasy". A year or two later, it was replaced by a new dining hall, but we still called it the Greasy. Now, its official name is the Browne Dining Hall, I think.

ZIERLER: Yes.

SHEELEY: That wasn't there. What was there was an old wooden building. There was a hallway in the back of the building, which passed some rooms where graduate students lived. The front of the building had a kitchen and an adjacent room with tables where we ate breakfast. That was my recollection. The breakfast was ok, but the wooden building was probably a dangerous fire trap. If they had a fire in the kitchen, that old wooden building probably would have burned down. Of course, another recollection was meeting with Paul Eaton, Frederick Bohnenblust, and Tom Apostol and getting started with math, probably later that day.

Another recollection was going to student camp. We went on busses to the mountains somewhere - I can't remember exactly where - maybe it was north of San Bernardino or possibly somewhere in the San Gabriel mountains. On my bus, there were other freshmen, real nerds, big-time nerds, trying to impress each other with their deep knowledge of general relativity and other subjects. They had not yet caught on that humility was going to be important at Caltech. I was discouraged, thinking, "Maybe I came to the wrong college. It would be nice to have some people here that are more normal." [laugh] That was one of the things that I thought. Eventually, I did find and make friends with students that seemed more normal. [laugh]

By the way, we didn't use the term, "nerd", in those days. Perhaps the word was "egghead" as opposed to "cool head" to which many of us aspired. But there was a creative word that applies only to Caltech and is part of the Caltech folklore and heritage that I would like to document. It is the word, "troll", taken from the Norwegian fairy tale, "The Three Billy Goats Gruff". In that story, a "troll" lived under the bridge. At Caltech, that bridge is the Norman W. Bridge Laboratory of Physics. Thus, taken literally, "trolls" referred to people who spent most of their lives working in the basement of Bridge, and therefore applied to many Caltech faculty and graduate students. But, taken more generally, "trolls" referred to students who spent all of their time studying, and did not socialize with the rest of us.

Number Theory and the Switch to Physics

ZIERLER: Neil, was it physics from the beginning? Is that what you wanted to pursue, or was it math initially?

SHEELEY: I wasn't sure. I thought that by going to Caltech, I had more options than if I went to Princeton, because Caltech had an electron synchrotron, and it had Mount Wilson and Mount Palomar Observatories. There is a statement in the 1960 Caltech yearbook (The Big T) under my graduation picture, saying in part, "Neil Sheeley came to Caltech to prove that, given enough hard work and optimism, any brilliant mathematician can become a pretty good physicist." I never learned who wrote that comment.

ZIERLER: [laugh]

SHEELEY: [laugh] In that regard, I remember that Tom Apostol once told me that he was sorry that I had decided to major in physics because he thought I had talent in math. That sounds kind of funny, doesn't it? Maybe he really said that he was sorry that I had not decided to major in math because I had talent in math.

ZIERLER: [laugh]

SHEELEY: When I got to number theory, even though I liked doing number theory, I felt that it was like working crossword puzzles. You couldn't earn a living doing it, somehow. Also, number theory wasn't easy. I knew that I would have to devote a lot of effort to it in order to succeed. And that would be at the expense of my other courses, especially physics. I thought I had a better chance by being in physics. So I dropped the course, and except for Tom Apostol's course in advanced calculus (Math 108), that was the end of my math courses.

Then something really interesting happened. I was walking to class one day with Al Hales. Al was a friend from Blacker House, and a very good student. I think he ranked at the top of our class. In those days, sophomore physics (elementary electricity and magnetism) was taught in sections. The sections were obtained by dividing the class of roughly 180 students into about 15 parts, to obtain approximately 12 students in each section, and then assigning different teachers to each section. I was in Dr. Robert King's section (Robert B. King, Caltech Professor of Physics). I liked him. He had a daughter at Occidental College and when Oxy played Caltech in the Rose Bowl, he spent half of his time on the Oxy side and the other half on the Caltech side. Dr. King studied F-values. His father had been a famous F-value astronomer that worked at Mount Wilson. F-values are transition probabilities in atomic species.

Anyway, Al Hales said, "You really should switch into our section. Richard Feynman is teaching it, and we're learning all these really neat things. For example, to get the dipole electric field, you just take the derivative of the monopole field." So, I went to Feynman, and said, "I'm getting an A in Dr. King's section of electricity and magnetism, but I'd like to transfer to your section." He said something like, "Ok. It's alright with me." Then I went to Dr. King, and I told him that Feynman said I could transfer to his section. Dr. King said something like, "I'm sorry to have you leave my section, but I can see why you might want to join Feynman's section. So it's ok." The approval of those two teachers must have been sufficient for the registrar because I became a member of Feynman's section. In retrospect, I am somewhat amazed that I was allowed to do that, but maybe the small size and informality of Caltech at that time made it possible.

So, I had the good fortune of having two terms in Feynman's class. Caltech had three terms: the fall term; the winter term; and the spring term. The fall term was probably when I got the A with Dr. King; we would have to check the records. But during the winter term and the spring term, I was with about 12 other students, listening to Richard Feynman ad lib, because he didn't prepare anything as far as I could tell. This was 1958, well before the famous Feynman Lectures on Physics, which started in the fall of 1960, I think, and for which Feynman was very well prepared (judging from the high quality of those red books). In fact, it's entered my mind more than once that Feynman's section in 1957-1958 might have been the spark that led to his giving the Feynman Lectures to the entire class. By comparison, freshman chemistry had been taught this way for several years with all 180 freshmen attending Linus Pauling's lectures three times a week, before splitting into sections and working problems with a graduate assistant once a week, as I recall from my own freshman year. (I enjoyed Pauling's lectures, which often tended to be physical with atoms, the periodic table, and electron configurations like 1s² 2s² 2p⁶, and so forth. On the other hand, I did not learn much from the recitation sections, and still don't know why adding salt to a very weak vinegar solution makes it more acidic - which I found many years later using a garden pH meter.) So, physics was behind chemistry in that respect, and needed to catch up.

In the April 2006 issue of Physics Today, Matt Sands (then emeritus professor of physics at the University of California at Santa Cruz, but formerly professor of physics at Caltech) presented his version of how the Feynman Lectures originated. In that article, Sands said that when he proposed that Feynman give the lectures, Leighton replied, "That's not a good idea. Feynman has never taught an undergraduate course. He wouldn't know how to speak to freshmen, or what they could learn." I think that either Leighton had forgotten that Feynman had taught our section of sophomore physics, or that Sands misquoted Leighton in that article. But those of us who were in Feynman's section in 1957-1958 know that Feynman did teach an undergraduate physics course prior to giving the Feynman Lectures in 1960.

At the end of each term, we received our final exams on sheets of paper and we worked the problems in so-called blue books - very small notebooks of maybe 10 pages or so. At the end of the winter term, the final exam consisted of 9 problems and we were asked to work any 6 of them. A few days later, when I received my blue book, an A+ was written on the first page in red pencil, but a line had been drawn through it and an A was written alongside it, indicating that the grade had been lowered from an A+ to an A. When I went to Feynman to find out why, he said that he had read my blue book first, but when he read more of them, he had to go back and lower my grade. Originally, I supposed that this was because some very smart students, like Al Hales or John Munson, had obtained solutions to the problems that were somehow better than mine. However, in retrospect, I think that the original A+ indicates that I provided the correct solution to all six problems, and that the downgrading to A indicates that some students worked all 9 of the problems correctly, instead of the mandatory 6.

I remember another incident that occurred toward the end of the spring term. Feynman had spent some time telling us "classic" Feynman stories, like when, as a graduate student, he gave a talk at Princeton. At the end of the talk, someone important, perhaps Dirac, strongly criticized him, and then turned to Einstein and said something like, "Don't you agree Dr. Einstein?" Feynman told us that he would never forget Einstein's reply, which was something like, "No, I agree with Feynman." Anyway, because we spent quite a bit of time with these stories and other interesting scientific diversions, Paul Skov, a little red-haired kid in our class, was worried. We had to take the same final exam as the other sections, and most of the questions were going to be selected by the other teachers. Paul said, "Dr. Feynman, the other classes are reading the text, and working the problems. [laugh] I'm worried that we might not do well on the final." [laugh] Feynman said, "Ok, we'll work some problems."

We did a little bit of cramming toward the end of the year. As I recall, everyone in Feynman's section did very well. I think the lowest grade was a B+, and that was obtained by one of my friends who was a chemistry major. But I think the reason for this success was obvious: Feynman's section was the honor section, so those kids were the best students. Although I had switched from one of the other sections, I was good at electricity and magnetism, so I didn't bring down the average in Feynman's section.

ZIERLER: Neil, did Feynman seem like a celebrity already when you were an undergraduate?

SHEELEY: Oh, yes. In addition to that section of electromagnetism, I'd heard him give several lectures when I was an undergraduate, and I knew that he was a great speaker. But I have two other Feynman stories that I would like to mention.

One day when I was in graduate school, Bill Harrison (another physics grad student) and I were in the physics library where tea and cookies were provided before people went upstairs to the lecture hall. I said to Bill, "We better go get a seat because Feynman's talking today, and you know what's going to happen when he talks." We went up, and we got our seats, and then watched people pour in. By the time that Professor Matt Sands came in, there were no seats left. So he turned over a wastebasket, and used it for a chair. Then people started to cluster in the doorway. From my seat several rows back and up from the front row, I could see people outside the door trying to hear words coming up over the crowd. [laugh] The talk was about gravity, which he described in tensor equations that filled the screen. When he noticed that the audience was a diverse group of people from many walks of life, and not just physics students and faculty, he said, "These are the equations of gravity in secret form."

Another Feynman experience occurred after I left Caltech and went to Kitt Peak for my first job. I came back to Caltech for a brief visit in the fall of 1965, and had stopped by to see Feynman, who had recently won the Nobel Prize. He was opening a package, and said, "You know I just got the Nobel Prize." I said something like, "Yes, Dr. Feynman, you got the Nobel Prize. [laugh] But here at Caltech, we already knew you were great. We felt that the Nobel Committee was lucky to have you [laugh] to give the prize to." Then he started laughing because the package contained a rearview mirror with a note, saying something like, "So you can see behind you.". I didn't catch on to the joke at that time. I thought maybe the rearview mirror was so that he would still be able to see the people he's leaving behind. But that wasn't it. I didn't learn what the joke was until much later when I read Gleick's book about Feynman. As Gleick explained it, the reason for the mirror was so that Feynman could walk backwards when he was receiving the Nobel Prize from the King or whoever it was. He thought that you're not supposed to turn your back on royalty, so he'd have to walk backwards up the steps. The person who sent Feynman the rearview mirror was joking that this rearview mirror was so that you could walk backwards up the steps. [laugh]

ZIERLER: Neil, what were your thoughts on experimental versus theoretical physics when you were thinking about graduate school?

SHEELEY: I liked theory and math-related things way, way better than experimental things. I was not particularly good at designing instruments or building hardware. I could probably do it if I had to. What I liked best, and what I was probably best at, was theory or at least math-related research. But I also had this notion—and I should have asked people for guidance—but I had this notion that it's really important to understand the observations before you start inventing theories about them and doing mathematical calculations; otherwise, you're just playing games. It's much better to know what the facts are. Also, I thought it was important to have some kind of a background or skill that I could sell in the community so that I could get a job. You know what I mean? I got that advice from Chuck Elmendorf at Bell Labs. I don't know if you know about Chuck Elmendorf.

ZIERLER: Sure.

SHEELEY: He guided me through all my summers at Bell Labs, and he give me "fatherly advice", so to speak, although he wasn't my father or any relation or anything like that. But he would say things like, "At Bell Labs, there are lots of places for people who can do experimental work. But if you're going to do theory, you've got to be another Feynman, because the people that we hire for theory are really, really good." I took that kind of seriously because I wanted to have a job when I graduated. [laugh]

ZIERLER: Neil, what advice did you get or considerations did you have in staying at Caltech for graduate school?

SHEELEY: I liked Caltech, and I thought it was the best place. I considered possibly going to Berkeley or Stanford, but I didn't get into either one of them. I think it's because when I went across town to UCLA and took the graduate record exam, I did a bad job. When I mentioned that to my undergraduate advisor, Bud Cowan (Eugene W. Cowan, professor of physics), said, "You should have come to me. I could have gotten you into Berkeley." I received an A in his advanced course in electricity and magnetism, and he probably took that into consideration. But, somehow, I didn't feel like doing that. I thought, I'm right here. I've been accepted to graduate school at Caltech. Why not just stay here? Besides it is sunny and warm here, but cold and rainy at Berkeley, so why leave?

ZIERLER: Was it solar physics and Bob Leighton specifically that you wanted to work with?

SHEELEY: No. [laugh] I thought I'd go into nuclear physics at Caltech.

ZIERLER: Who was doing nuclear physics then? Who would you have worked with?

SHEELEY: Someone, perhaps Willie Fowler or Tommy Lauretsin, steered me to Charlie Barnes. Also, I took two courses in nuclear physics. One course was in theoretical nuclear physics taught by Hans Weidenmüeller - a visiting theorist from Germany- and the other was Willie Fowler's course in nuclear physics. Fowler's course notes were in a huge binder (maybe 2" thick) that reminded me of a big cookbook. It was a large volume of quasi-random nuclear physics facts to learn. Although Weidenmüeller's course was called theoretical nuclear physics, he began by telling us that "theoretical nuclear physics doesn't exist" because we don't have a good theory of the nucleus. By comparison we do have a good theory of the atom. I had enjoyed thinking about electromagnetic fields and wondered if that field approach could be extended to the nucleus. At least, I was leaning in that direction. Originally, I had been considering relativity, especially general relatively and gravitation, thinking of Einstein and the field equations that I had seen in the New York Times. Then when I got to Caltech, I turned to nuclear physics, which seemed more modern and glamorous than gravitation by that time.

So, I started grad school in Kellogg doing nuclear physics, but I didn't last very long. That's why I'm saying that my career is a [laugh] random walk with a math/physics bias. Charlie Barnes said, "Why don't you work with my graduate student, Frank Dietrich." He came to Caltech from Haverford, and then went to Stanford after Caltech. He was doing a neutron time-of-flight experiment, and needed a big collimator to shield the neutron beam from something—I don't know—maybe protons. He wanted me to fill a big can - a cylindrical shell - with paraffin. I had to take these blocks of paraffin, and put them in this little pot on the stove, on the burner, and melt it all down, and pour it into the can. I thought, "Ok, I'll do it," and I spent an hour or so each day melting paraffin. I was trying to be helpful, but melting paraffin just wasn't like doing math. It wasn't what I had come to Caltech to do.

For the next project I was asked to build the chassis and do the wiring for an electrical device that would detect neutron pulses. I did the wiring, but I didn't put the same diligence into that work that I saw an electrician apply when he wired my porch a few years ago. [laugh] The guy who wired my porch did a beautiful job - it was perfect. Every wire was either perpendicular or parallel to the floor. But when you turned my electronic pulse counter upside down, you would see wiring that looked like "spaghetti". So it became obvious that I wasn't going to be an experimental nuclear physicist (or an electrician).

In looking for another area, I remembered Robert King and was aware that one of my physics friends had done a thesis on F-values with him in only two years. So I thought, "I could do F-values. I'll go see Dr. King." But, wait, I had recently attended a talk by Bob Leighton, who had found some interesting things about the Sun that appealed to me more than F-values. Besides, I had liked Bob Leighton when I took his required course in modern physics during my senior year. So I thought, "I'll go see Leighton. He's a good guy." I asked Leighton if he had any theoretical work that I could do for a thesis. He said he didn't, but his graduate student, Bob Noyes, had just passed his final oral exam that morning, and Leighton needed somebody to help him take spectro-heliograms at the 60-foot tower on Mount Wilson. He asked me how I liked getting up early in the morning because that was the best time for observing. I said I didn't like getting up early in the morning, but I'd do it if that would be a path to a thesis. So, I did that, and things just went well. I made interesting discoveries and got my PhD in 3 years.

There was a difference between working with Leighton and working with the group in Kellogg Lab. Leighton was much more formal. Graduate students called him "Dr. Leighton", until they received their PhDs, whereas in Kellogg Lab everyone was on a first-name basis, including Charlie Barnes, Willie Fowler, and Tommy Lauritsen. When lunch time arrived, Leighton put on his suit coat and went down the Olive Walk to the Athenaeum to eat with other faculty members. In the Kellogg Lab, it was less segregated, with faculty members often joining their students at the "Greasy". (Incidentally, Feynman could also be found at the Greasy during those years, often discussing a point from one of his lectures. I remember one occasion in which he illustrated the difference between static friction and sliding friction by sliding a dish across a salt-covered lunch tray while Matt Sands was moving the tray back and forth.) Of course, Leighton did not have the Friday evening lectures and post-lecture parties that occurred in Kellogg. But on special occasions, he invited his students to dinner at his home and showed us spectacular images (like a globular cluster of accumulating stars) through the telescope that he had built and placed on the flat roof of his house in Altadena. (After he moved to a house with a slanted roof in Sierra Madre, he designed and built a special observing platform that was elevated to avoid poor atmospheric seeing near the ground.) Leighton's seriousness, his scientific intuition, and his respect within the faculty gave us confidence that if we followed his suggestions, we would be on successful paths to a thesis and graduation. So, that worked out very well.

ZIERLER: Neil, did you cross paths with Ed Stone? Would the timing have lined up?

SHEELEY: Yes, I did cross paths with Ed Stone. When I was working for Bob Leighton, I had an office across from Leighton's office in the basement of Bridge where several up-and-coming young researchers were always coming by to see Leighton. Ed Stone was one of them. Gerry Neugebauer was another one. Robbie Vogt was another one. Bruce Murray often came over from geology. Later, he became director of JPL, and, of course, Ed Stone did too. Robbie Vogt (Rochus E. Vogt) became Chair of the Division of Math, Physics, and Astronomy at Caltech and was instrumental in LIGO, the Keck Telescopes, and the Owens Valley millimeter wave interferometer. He also was Caltech Provost for three years. I recently learned that Robbie Vogt and Ed Stone had been graduate students together at the University of Chicago, and that Robbie was responsible for bringing Ed to Caltech in 1962. Gerry Neugebauer had lots of interesting jobs too—like Vogt, he was the Chair of the Physics, Math, and Astronomy Division. Also, Neugebauer was Director of the Mount Palomar Observatory, and I think he was a grad-school research advisor of Andrea Ghez, who recently won the Nobel Prize for verifying that a massive black hole lies at the center of our galaxy. Just by coincidence, Gerry's wife, Marcia, is an associate in the Lunar Lab at the University of Arizona, where I now have a visiting scientist appointment. I see her every two or three weeks in our heliophysics zoom sessions.

Magnetic Flux and Heliophysics

ZIERLER: Neil, what would you say the primary contributions or conclusions of your thesis was?

SHEELEY: I believe there were four. [laugh] The general conclusion was that the observations that I made were consistent with Leighton's theory of magnetic flux transport on the Sun. That's the general conclusion. What happened was he had an idea about how magnetic flux would spread from its origin in sunspot groups to form the polar fields and large-scale regions of flux on the Sun. He said, "I know you've been working on other things, but I think that if you were to measure the fluxes in sunspot groups, we could use that to see whether it's consistent with this theory. If you know how much flux is being provided by the sunspot groups, then by subjecting it to this transport, we can see how strong the polar fields get, for example." So, I measured the fluxes in sunspot groups.

In the process of doing that, I found out that flux wasn't spread out uniformly, but it was fragmented in little bits and pieces. Each fragment was small but had a rather substantial amount flux in it. Therefore, the flux per unit area was quite large in these little fragments. In other words, the average field strength was very high, and I got numbers that were in the range 300 to 700 Gauss. At that time, the current thinking was that those fields were 10 Gauss, so they were off by two orders of magnitude. That was one of my thesis results.

Another subject concerned the polar fields themselves. That was around 1963. The Babcocks —Horace and Harold Babcock — had invented the magnetograph, and they observed the reversal of the polar magnetic fields around sunspot maximum during 1957-1959. The question was, would the polar fields reverse every 11 years around sunspot maximum, as it should, according to Leighton's theory? I thought maybe I can go up to the Mount Wilson plate vault on Santa Barbara Street, and look at some of the old data, and see if there's any evidence of that. I looked at three different kinds of images: direct white light images of the Sun as you would see it on a piece of cardboard placed in the telescope's focal plane; and some calcium K-line and H-alpha spectroheliograms—that is, images at spectral wavelengths in the blue and the red.

It turned out that the white-light images were the best because they showed little bright features called faculae without the overhanging chromospheric structures that are visible toward the limb in the K-line and H-alpha images. In those continuum images, the faculae weren't visible at the center of the disc where you're looking deep in the atmosphere, but they were visible toward the limb at all position angles where magnetic fields are present. Not only could you see them on the east and west limbs as solar rotation carried them into view or out of view, but also you could see them at the Sun's poles. But you didn't always see them at the poles either. At sunspot minimum in 1954 and 1964, polar faculae were visible at the poles, but at sunspot maximum in 1958-1959, they were not visible at the poles. Thus, for these and other selected times, it appeared that the numbers of polar faculae varied during the sunspot cycle. They were relatively large at sunspot minimum and fell to zero around sunspot maximum.

I wanted to prove that objectively. So I examined photographic plates that were taken during the spring and fall when the Sun's south and north poles, respectively, were most visible from Earth, and picked the 5 best plates during each season. For the interval 1934-1964, this gave 150 plates for the spring deck and another 150 plates for the fall deck. They were 8"x10" glass plates, stored in paper or cardboard envelopes. For each deck, I placed the envelopes face down so that the dates would not be visible, and then I shuffled the decks. For each plate, I quickly estimated the number of faculae at each pole, and recorded those numbers together with the dates that I obtained from the plate envelopes after counting the numbers of faculae. This process created a randomized list of the numbers of faculae at each pole and their dates. Then, I reorganized the list chronologically, averaged the five measurements for each year, and computed the root-mean-square deviation of these five points from their average to use as an error estimate. These numbers of polar faculae varied with a period of about 11 years, with peaks at sunspot minimum and valleys at sunspot maximum in each cycle. Then I reshuffled the deck of 150 plates and repeated the entire process. The new numbers agreed with the original ones within the measured error bars, indicating the repeatability of the measurements, at least when they were obtained by the same person.

But these periodic curves had a puzzling shape with cusps at sunspot maximum when the numbers of faculae were small. My office mate, Alan Title, said something like, "That's trash, Neil. [laugh] All your measurements are rubbish." [laugh] But Leighton disagreed, saying, "No, I think you should assign a polarity to the numbers of faculae, corresponding to the presumed polarity of the polar field, and that will give a very nice continuous curve." That's what I did, and it worked out really well. Then I extended the measurements back to 1905, when the observations began. So, those polar faculae measurements were a second thesis result. Years later, I extended it forward to 2005 and correlated the numbers of faculae with the strengths of the polar fields since their measurements began at the Wilcox Solar Observatory in 1975. The result was 100 years of polar faculae measurements, calibrated in Gauss. Part of this calibration procedure was done in collaboration with Andres Munoz-Jaramillo, a solar physicist at the Southwest Research Institute in Boulder, Colorado.

ZIERLER: Neil, I'll test your memory. Besides Leighton, who else was on your thesis committee?

SHEELEY: Bob Howard was there from Mount Wilson. Harold Zirin was there. He was a new professor at Caltech at the time. I hadn't worked with him, but I knew him very well. Leverett Davis was one of the members. I believe there was a visitor from Russia. Stashenko was his name - Nikolai Stashenko. I remember him being present one time when I was trying to take spectro-heliograms at the 60-foot tower on Mount Wilson. The image was shaking. When we went to the top of the tower to find the source of the image motion, we learned that it was caused by a drive motor whose vibration was shaking a mirror. We decided that the motor was not supported properly and needed something that would absorb the vibration. I remember thinking that a Russian and an American were working together to solve this problem during the Cold War. We agreed that the support needed to be [laugh] fixed. I remember that.

At the risk a small diversion, I would like to mention another Cold War story. It occurred when I was an undergraduate and had a girlfriend who went to Occidental College. She and some members of her history class participated in an exchange visit with the Soviet Union in the summer of 1960. I met her and one of her Oxy friends on their way through New York on their outbound trip. On her return, she gave me two posters, which I've never seen, before or after. One was in color, and said "на горячому" (na goryachomu) [speaking in Russian] (which is an idiom meaning "gotcha" or caught you "red handed", literally "on the hot") at the top of the poster. I have it here. I can get it out and show it to you.

ZIERLER: Oh, please.

SHEELEY: The other poster was black and white, and said "очистим мир" (ochistim mir) [speaking in Russian], which means "Let's clean the world." It showed a wheelbarrow with nuclear weapons being dumped. (Today, that expression would also apply to pollutants and sources of global warming.) The "на горячому" (na goryachomu) [speaking in Russian] poster showed a red rocket hitting a black U-2 plane. That incident broke up plans for the peace conference between Eisenhower and Khrushchev, that was scheduled to occur in Paris, France. The pilot, Francis Gary Powers, was captured alive. This was an embarrassment for President Eisenhower, who had denied sending a spy plane over the Soviet Union, and it was a "gotcha" for Secretary Khrushchev, who replied that he had both the pilot and the plane in hand. I have that poster here if you could just bear with me a moment. Can you see this all right?

ZIERLER: Yes.

SHEELEY: Actually the "на горячому" (na goryachomu) [speaking in Russian] is toward the bottom, not the top, as I thought it was. You can see the U-2 being hit by the Russian rocket, and you can see the strong Russian hand with the nicely trimmed fingernails, grabbing the evil white hand containing the roll of film. On the film, it says "спегунство СРСР" (spegunstvo SRSR) [speaking in Russian], which probably means spying against the Soviet Union or one of its republics. I took Russian my senior year as an undergraduate, and my first year as a graduate student. In the years after Sputnik, it was popular to study Russian. That was a big diversion from your question about the members of my thesis committee, but it illustrates US-USSR relations around the time that Nicolai Stashenko attended my PhD oral exam. So maybe it wasn't too bad a diversion.

Magnetographs at Kitt Peak

ZIERLER: No, that's fine. That was great. I'm so glad you showed it to me. Neil, after you graduated, were you looking at academic positions and institutional positions also?

SHEELEY: I had been so busy writing my thesis that did not have time to think about that very much. Bob Noyes had gone back to Harvard, and I sent him a letter, asking about the possibility of a position there. But I didn't really want to go back into the snow, and I never did hear back from him. Boulder was a place where people did solar physics in those days. But so many people were doing solar physics there that I thought I would just be one small member of a big community. So I didn't really want to go to Boulder either. Then one day, Leighton said, "Would you like to work at Kitt Peak? They have a large new solar telescope." I'd read about it in Fortune magazine on a flight between New York and Los Angeles. The article was called "The men with the long eyes". That telescope was the world's biggest solar telescope at the time, and they were just getting it online. I said something like, "Oh, yes, that would be interesting. I'd like to do that." Then, while I was standing in his office, Leighton picked up his phone, and called Keith Pierce, the Director of the Solar Division at Kitt Peak. Leighton said, "Would you be interested in having Neil Sheeley come over for an interview?" I had met Keith Pierce during his visits to Pasadena to monitor the progress on the spectroheliograph that was being built by the Boller and Chivens Company in South Pasadena and to consult with Leighton who had vast experience with the Mt. Wilson spectroheliograph. So I went off to Tucson for my interview, and at that time, Keith was less concerned about my thesis than about showing me the telescope and getting me to come there and help them use it, which I did. The Solar Division was a small group. There were only four solar astronomers: Keith, Bill Livingston, Jim Brault, and me. (There actually were five. John Waddell arrived before I did, but he was on sabbatical when I began in the fall of 1965, and John never returned. He died in an automobile accident in Los Angeles on November 14, 1966.)

ZIERLER: Neil, given that you were there really from the beginning, what were the major science objectives of Kitt Peak?

SHEELEY: I don't think there were any. The objective of the Kitt Peak National Observatory was not to do research per se, but to provide and maintain world class observing facilities for the astronomical community — especially for astronomers at universities that did not have large telescopes of their own. In this sense, KPNO competed with Caltech and other places that had their own forefront facilities. Consequently, most of the observing time at Kitt Peak was allotted to visitors, who had to submit proposals and indicate their own science objectives. The rest of the observing time was allotted to the KPNO scientific staff. In principle, I had about 3 days per month, but in practice it was more than 3 days because I (and the other staff members) were usually available to fill in gaps that occurred when visitors had to cancel. On Sunday morning, Keith Pierce could often be found at the main spectrograph, taking spectra of the chromosphere, and always wearing a tie.

As a result, the emphasis was placed on having forefront instruments. So Bill Livingston worked on the magnetograph, and I worked on the spectroheliograph. Later, Jim Brault developed a Fourier transform spectrometer. Keith Pierce had a program to obtain chromospheric spectra at the solar limb using the main spectrograph, and that turned into a definitive atlas of chromospheric spectra and was published in Astrophysical Journal Supplements. As far as the solar program is concerned, I think that most of us were being led by our observations. We were trying to obtain observations with the highest possible spatial, temporal, and spectral resolution, and then getting ideas from those observations. I found that a little frustrating at times because I would come down from the mountain with the world's best image of such-and-such, and wonder what to do [laugh] with it.

There was one time when I set an objective and made a discovery as a direct consequence of that objective. I remembered that Bob Leighton likened a sunspot to a super-large, super-powerful supergranule. Following that idea, I decided to obtain a time sequence of images of a sunspot in the 3883 Angstrom band head of cyanogen to see if I could detect a hypothetical super-powerful outflow. I had explored the ultraviolet with the spectroheliograph, and had become familiar with the band head of cyanogen, CN, at 3883 A. At that wavelength, there were a lot of spectrum lines crowding closer and closer together to form a wide blend of low intensity. I could put a fairly wide slit there, and very rapidly obtain an image that showed brightness variations free of contaminations induced by Doppler shifts of spectral lines. It showed, with about one second of arc resolution, these little bright features, identical to the faculae at the limb that we talked about earlier.

But in this particular case, I took 22 consecutive images over a period of about 2.5 hours on 8"x10" glass plates. I made film transparencies of those plates, registered those film sheets on a light box, and copied the registered sequence onto 35-mm film. Then I sent the 35-mm film to a company in Hollywood that made 16-mm copies. [laugh] I spliced those 16-mm strips into closed loops, so you could watch continuously in what would now be called a loop mode. I laugh to think that we actually used 16-mm films to watch movies back in the 1960s. Of course, we don't do that anymore. [laugh] We do everything digitally. But in those days, we used film.

After preparing this movie, I invited Keith to watch it with me. We went into the conference room, turned off the lights, and I started the projector. I was concentrating on making sure that the film was feeding properly so that it didn't break. Going around and around in this projector, it could easily tear. So, I was aligning it very carefully, and didn't look at the screen quite as soon as Keith did. Right away, Keith said, "Look at the Evershed effect!" Now, the Evershed effect is a well known effect, discovered by British astronomer John Evershed in 1909 at the Kodaikanal Observatory in India. The Evershed motions are visible as horizontal flows on the solar surface that move outward through the penumbras of sunspots. So, I said to Keith, "No, it's not the Evershed effect because this flow is outside the sunspot and the Evershed flow is confined to the penumbra." You would see these little magnetic features moving away from the sunspot. They would go out through what I would later call a "moat" around the sunspot. When they reached the outer boundary of the moat, they would move to the side, as if they were being deflected by a wall.

Later, Karen and Jack Harvey were making similar observations with the Kitt Peak magnetograph and wanted to know what to call these features. I remember that when we were all at a party at Keith's house, Karen said, "You have the right to name those features because you discovered them." I told her that, "I can't think of a good name. I just call them moving magnetic features." So Karen and Jack called them "moving magnetic features" with the abbreviation MMF. Today, if you go to any of the websites that have movies of solar magnetic fields, you can see moving magnetic features moving out through the moats around sunspots.

In the last project before I retired, I used observations from the Solar Dynamics Observatory to study moving magnetic features with higher resolution in both space and time than I had used before. I thought, "I'll just give it one last try before I retire because I still don't really understand this motion since I discovered it 48 years ago." I submitted my results for publication, and the referee didn't like them at all. The paper was eventually published, but the referee said, "Besides, you're using lower case MMF, and MMF is supposed to be capital M, capital M, and capital F." [laugh] It seemed ironic that the person who discovered the effect would be criticized for not abbreviating its name correctly. [laugh] That was funny, but it wasn't so funny that the referee didn't like the paper, and my NRL co-author, Harry Warren, didn't think it was funny either. Eventually the editor selected another referee and the paper was published.

ZIERLER: Neil, what do you think your most significant work was during your time at Kitt Peak?

SHEELEY: During my time at Kitt Peak? That's probably it — the MMFs. What else did I do there? I worked with Gary Chapman to understand why magnetic regions appear bright in some spectral lines. Because the telescope used reflecting optics rather than glass lenses (which absorb ultraviolet light), we were able to explore the ultraviolet down to the atmospheric cutoff at 3000 Angstroms. That's when we realized the importance of the CN band head at 3883A.

But the most significant work might have been related to the Ca II K-line emission. I did a study of the calcium variation over the sunspot cycle. An observer at Mount Wilson, Olin Wilson, started a program to monitor, in effect, sunspot cycles in other stars. He did it using the so-called calcium emission (the emission in the Ca II K-line). When sunspots erupt and then "dissolve", the field doesn't vanish right away. It heats the atmosphere and becomes visible via the resulting brightenings. If you can track these brightenings over time, then you can see the sunspot cycle. So Olin Wilson was looking at the calcium emission in stars, and he had set up a program to do that over many years.

One day, I was taking a sequence of K-line spectra with high spatial, spectral, and temporal resolution. An electric power failure occurred while I was obtaining this relatively short, one-hour sequence of spectra. The telescope drive stopped, and the solar image drifted off the slit during one of my exposures. It drifted so that I accidentally obtained a spectrum of the northern hemisphere averaged over time. This was in 1966 when the northern hemisphere was active, and the southern hemisphere was relatively quiet. Then I realized that I had obtained a spectrum of the active Sun. If I were to do a similar averaging over the southern hemisphere, where there's hardly any activity, I'd get a spectrum of the quiet Sun, like the Sun at sunspot minimum. In one or two days, because my observing run lasted two consecutive days, I could answer the question of whether you can see the sunspot cycle by looking at unresolved calcium emission, as if the Sun were viewed as a distant star. I found that you could, and so I wrote up the results. It was refereed by Leo Goldberg, Director of the Harvard College Observatory, who said, "I have just had the pleasure of refereeing your paper on the sunspot cycle for the Astrophysical Journal and needless to say, I have recommended that it be published as is. It is an extremely important and fundamental contribution. If you happen to have an extra copy of your paper available, I would very much appreciate having it as a reprint." Steve Maran, one of my Kitt Peak colleagues, told me that he thought this accidental discovery was the most interesting result that I had obtained at Kitt Peak since I arrived there, much more interesting than any of my planned experiments. (On the other hand, I did that study soon after arriving at KPNO, and as mentioned above, I found some other interesting things after that - probing the ultraviolet with Gary Chapman, making movies of magnetic features moving outward from sunspots, and so forth.)

That leads nicely to this story that I often tell about Ernest Lawrence Thayer and the poem Casey at the Bat. Thayer was a professional journalist, and his whimsical poem, Casey at the Bat, became a big success. Everyone remembered Thayer, not because of his professional journalism, which was probably very good, but because he wrote this poem, this fun baseball poem, Casey at the Bat. So in my mind, "The Ernest Lawrence Thayer effect" refers to something that turns out to be popular or successful, but may not be what you originally sought. But [laugh] that's all right. It's like the Wang-Sheeley-Arge solar wind model. We didn't set out to invent or discover a solar wind model. It just happened, and then it caught on, and it turned out to be one of our most well known, I think, results. So, like the WSA solar wind model, my paper about the sunspot-cycle variation of calcium emission was another Ernest Lawrence Thayer effect.

Skylab and the Move to NRL

ZIERLER: Neil, tell me about your decision to move over to NRL in 1973.

SHEELEY: It had to do with Skylab. I had already become aware of the Skylab mission, and had met not only the Skylab astronauts but the entire group of astronauts that were being considered for those flights. I think that about 20 of them came to Kitt Peak on two consecutive days. Keith Pierce asked Jack Harvey and me to show them the observatory and the solar image, and to tell them about the Sun. This is a little bit of an aside, but one of them was Walt Cunningham, who recently died on January 5, 2023. There was an article in The Washington Post a couple of days ago by Buzz Aldrin about Cunningham's accomplishments.

By the way, when I'm showing you things, I might as well show you the poster that the Skylab astronauts signed during that visit. When we took them up to the mountain, I had a cardboard poster with some Ca II K-line spectroheliograms mounted on it, showing a large sunspot and the the faculae around that sunspot. (Incidentally, it was a naked-eye sunspot that Marybeth and I saw through the car windshield as we drove west while the Sun was setting into the Los Angeles smog.) Each of those astronauts signed the poster around the edges of the photographs. You can see, for example, Walt Cunningham here, and Story Musgrave, Ed Gibson, Alan Bean, Owen Garriott, Jack Lousma, Charles Conrad, Richard Truly, Joe Kerwin, Paul Weitz—this is fading—Don Lind, Jerry Carr, Bill Lenoir, Rusty Schweickart, and then the people that brought them there, Alan Holt, and Frank Orrall. That experience contributed to my decision to join NRL and participate in the Skylab mission.

So when Richard Tousey asked if I would be willing to join NRL and work on the Skylab project, I accepted and we moved to Houston. At the time, Marybeth, and I were on vacation, visiting relatives on the east coast. We had gone on a path that took us to Mississippi to see my brother-in-law who was temporarily assigned there by the Department of Health and Human Services (HHS), then on to Florida to visit my father who had retired there, then up the coast to New Hampshire to visit my grandmother and other relatives. At my grandmother's house, I received the call from Richard Tousey. (This was before cell phones, and I don't know how he tracked me down. It might have been through Elske Smith, an astronomy professor at the University of Maryland where I was headed next.) Also, it seemed strange, perhaps anachronistic, to be receiving a call about NRL and the Skylab mission when I was in the "telephone room" of the old house where I had spent my childhood during WWII. Tousey said that he heard that I was going to be at the University of Maryland in a few days and wondered if I could make a side trip to NRL. I was on the PhD orals committee for Sou-Yang Liu, a graduate student of Elske Smith. Sou-Yang had spent several months with me, using the Kitt Peak spectroheliograph and spectrograph to study the properties of calcium K-line emission in the Sun.

So on my way to the University of Maryland, I stopped off at NRL. That's when Tousey showed me around the lab and asked if I'd be willing to join their Skylab team. They needed "Czars" for the Skylab Apollo Telescope Mount (ATM) experiment. In principle, the ATM Czar would be a neutral person who would represent and make decisions for the five ATM teams. One team was NRL; one was the Center for Astrophysics at Harvard; one was Marshall/Aerospace (a collaboration between the Marshall Space Flight Center and the Aerospace Corporation); one was the High Altitude Observatory; and another was American Science & Engineering, Inc. (AS&E). These teams were competing with each other for observing time. But when NASA asked them what to do, they needed to "close ranks", and provide an answer. An ATM Czar had to say, "This is what we would like to do." Somebody had to decide.

David Bohlin and I were the two NRL Czars. We actually moved to Houston and lived there for about a year and took part in the mission activities. In those days, everything was "No!" at first. Then pretty soon, it wasn't "No!" anymore. It was probably "Maybe", and eventually it was probably a begrudging "Ok, yes". For example, NASA didn't want outside contractors to be on base at what was then called the Manned Spaceflight Center (and became the Johnson Space Center on February 19, 1973.) Then, they let us have our own control room next to NASA's Mission Operations Control Room (the MOCR). Toward the end of the last mission, we performed exercises in which we actually talked with the crew during flight. The ATM Czar was allowed to have a seat in the MOCR alongside the NASA people who had responsibilities for different aspects of the mission. When the spacecraft came up over a ground station, we could talk with Ed Gibson, for example, and say something like, "There's a helmet streamer over on the east limb. See if you can take a spectrum of it." These real-time interactive experiments occurred toward the end of the final mission, in preparation for air-to-ground communication that would occur in the future Space Shuttle missions. The Skylab mission lasted for about nine months from the initial launch of the space station in May of 1973 to the splashdown of the third manned crew in the Pacific Ocean on February 8, 1974. Participating in this activity was the first thing that I did for NRL.

Then, after we had obtained the Skylab observations, I compared them with the magnetograms and infrared helium images that were obtained at Kitt Peak in support of the Skylab mission. That was interesting, and I learned quite a bit. But I think I learned even more in the next two years because we found that the Kitt Peak infrared helium images showed coronal holes in the same way that the Skylab ultraviolet helium images did. This meant that we could keep tracking coronal holes beyond the nine-month Skylab mission, all the way to sunspot minimum in 1976. By comparing this three-year set of coronal-hole observations with corresponding in situ observations of solar wind speed near Earth, we obtained fingerprint-like evidence that coronal holes were the source of the fast wind which, in turn, was responsible for geomagnetic activity. That was an important result because it provided relevancy for our solar research at a time when research funds were becoming scarce and "relevancy" was becoming the key word. And it helped me to obtain a career appointment at NRL when the hiring freeze was lifted in 1977. That's when we moved to Washington.

ZIERLER: Did bringing you to Washington get you more involved in science policy at all?

SHEELEY: Not at a high level, but I did become involved in deciding what groups would receive the limited funding that was available for solar physics at ONR. I tell the story of how Herbert Gursky, who was the Superintendent of the Space Science Division, came into my office one day, as he often did. He was a real scientist (he had been one of the original AS&E experimenters that detected the first non-solar X-rays), and he liked to see what people were doing around the lab. He asked me to give a talk at the Office of Naval Research across town in Arlington, proposing that our new solar-terrestrial forecasting techniques be combined with their program for funding universities (and a few research institutes). The ONR program was called the Contract Research Program in Solar Physics. I gave the talk, which seemed to go ok, but the proposal was rejected. The reason it was rejected, I think, is because none of the program managers at ONR wanted to have their funding [laugh] mixed in with our NRL solar program, which is understandable.

Then, Gursky asked me to make essentially the same proposal (without the ONR connection) to the RAC, the Research Advisory Committee at NRL, which I did. The RAC accepted the proposal, and gave me five years of funding in the form of a so-called Accelerated Research Initiative. Right after that, I got a call from Ken Davis, who was the head of the Electronics Division at the Office of Naval Research. The Electronics Division was where the solar physics program was located, for some reason. I'm not quite sure why. But he was in charge of it, and he asked me if I would manage that program. [laugh] I started off by saying, "What if I don't? I've just received funding from the RAC." Two additional thoughts crossed my mind, but I did not mention them to Ken: First, I hadn't spent years learning science from Feynman and Leighton and others in order to do management activities. Second, I didn't want to spend my time driving through the Washington traffic, and then looking for a parking space over in Arlington. [laugh] Ken Davis replied, "If you don't, I'll probably phase the program out, and solar researchers will lose ONR funding." I said, "All right [laugh], I'll do it." But I only had to devote about 3% of my time to it. The budget was relatively small - only $360,000. I decided to divide it among the three or four observatories that make magnetic field measurements of the Sun and provide them to the solar community. In addition, that allowed us to use the magnetograms in our flux-transport program at NRL. In effect, we were doing what Gursky originally hoped we'd do, but without the ONR funding. The programs were hooked together.

An important milestone occurred when I had to defend the program to an external review committee at ONR. Among the committee members was Joe Reagan, a Lockheed executive and former space scientist. He said it was great to leverage the ONR program in terms of the NRL forecasting capability. That's fine. But what I want you to do, and what you should do, is make this forecasting technique available to the community through the NOAA and USAF forecasters. That was the first step toward bringing our future forecasting capability to the user community.

The next milestone occurred less than two months later, when Yi-Ming Wang, who I was able to hire with the Accelerated Research Initiative funds from the RAC, made the discovery that changed everything. He confirmed an inverse relation between the speed of the wind far from the sun, and the path that the wind took on its way through the corona.

Breakthrough in Solar Wind Research

ZIERLER: Neil, why does this change everything?

SHEELEY: [laugh] It changed everything because we suddenly had a way of deriving the solar wind everywhere in the heliosphere from magnetograms of the Sun's surface. I guess that "changed everything" is an exaggeration, but that discovery changed our lives. What we would do is use these magnetograms of the Sun's surface as the inner boundary condition on the field that extends outward through the corona and eventually points radially outward into the heliosphere. For a potential field (that is, a current-free magnetic field), this led to a unique coronal magnetic field whose field lines we could calculate. If those field lines bent out, if they flared out like a bugle, then the distant wind would be slow. But if they stayed more or less radial in the corona, then the wind would be fast.

In particular, if there were a big coronal hole at the north (or south) pole of the Sun where the field lines are open, then the field lines at the very center of that hole would be radial, and the speed would be fast. At the edge of the hole where the field lines spread out quite a bit, the wind would be very slow. In fact, the last field line at the edge of the hole near 60 degrees latitude on the surface bends down to the equator at 2.5 solar radii, causing the equatorial wind speed to be slow. Thus, if you were to plot wind speed versus latitude, it would be fast over the pole and slow at the equator. Moreover, the emergence of a low-latitude active region would distort this magnetic geometry, warping the streamer belt and causing fast wind to reach the near-equatorial latitude of Earth. This meant that we could use the magnetograms to calculate the expansion factors in the corona and, therefore, the wind speed in all directions from the Sun as a function of time. Because the fast wind often induces geomagnetic activity when it reaches Earth, we could predict the geomagnetic activity (provided that we knew some other things like the southward component of the interplanetary magnetic field).

Also, we could predict geomagnetic activity several months in advance. Because we had a program that allows us to determine how the flux migrates from its sources in sunspot groups, we could say, "All right, we've got a number of sunspot groups. Three months from now, the large-scale field should look like this, and the solar wind should look like that. Therefore, some days of this interval are going to be geomagnetically active, and some aren't." It gave us a physics-based mechanism for forecasting geomagnetic activity. Since then, our model has been merged with another model that extrapolates the wind further out into the heliosphere and makes it possible to track the effects of coronal mass ejections as well as high speed streams from coronal holes.

The combined model can also be used for predicting when a big geomagnetic storm will occur due to the arrival of a major coronal mass ejection, as well as when recurrent geomagnetic activity will occur from a coronal hole. Among other things, this changed our lives [laugh] because we were recently asked to write it up for the 100th anniversary book as one of NRL's special accomplishments. It may not have been as fascinating as moving magnetic features from sunspots, or the rigid rotation of coronal holes, or some other things that we discovered. But this WSA-ENLIL model is something that is now being used routinely to forecast geomagnetic storms and their harmful effects on communications and the power grid, for example, so it probably has a greater impact on people's lives.

There was a loss of electric power in Texas last year, but not due to a geomagnetic storm. That failure was caused by a major cold wave and accompanying snow storm. But the same effect could have been caused by a solar-induced geomagnetic storm. There was a solar-induced geomagnetic storm in 1989 that caused the blackout of the Hydro-Quebéc power station in Canada and probably caused power to be lost from New England for a while. I'm drifting away from your question. But the discovery changed our lives because we were responsible for at least part of the forecasting model. A year or so before I retired, I attended a seminar given by a young postdoc who came by afterward and asked, "Are you the same Sheeley as the Sheeley of the Wang-Sheeley-Arge model?" [laugh] That's what I mean by how it changed our lives. [laugh]

ZIERLER: Neil, either scientifically or administratively, what interface did you have with NASA?

SHEELEY: I had many scientific colleagues at NASA. At Goddard Space Flight Center, there are several solar researchers with whom I have worked. There are other places: JPL, Marshall Space Flight Center, Ames Research Center. One of my Kitt Peak summer students, Pete Worden, became the Director of Ames Research Center. Long before that, I attended his PhD oral exam at the University of Arizona and his wedding reception in Tucson.

But then another aspect is that we needed NASA funding in order to do our work. NRL did not provide enough funding for us to do all the things that we wanted to do. Most of our space missions were NASA-funded missions. Also, some of my colleagues stopped doing science, and joined NASA as administrators. One of them was Dave Bohlin, the other NRL ATM Czar during Skylab. After Skylab, he moved to NASA Headquarters to become the head of solar physics at NASA. There were others. Dick Fisher, a colleague from my ground-base days, had worked at the Sacramento Peak Observatory, and then, I think, at the Institute for Astronomy in Hawaii. He became director of the Sun-Earth Connections Division, which is now called the Heliospheric Physics Science Division - a name that he suggested. George Withbroe, a Skylab colleague from the Center for Astrophysics at Harvard College Observatory, is another person who left solar research to become Director of NASA's Heliospheric Physics Division. Also, John Klineberg, an aeronautical engineering friend from graduate school at Caltech, was the Director of the Goddard Space Flight Center for several years. And of course, Bruce Murray and Ed Stone were Directors of JPL. I met Thomas Zurbuchen when he was a graduate student at the University of Bern. Later, he became a professor of space science at the University of Michigan. In October 2016, he became NASA's Associate Administrator for the Science Mission Directorate and appeared in the press releases of many successful NASA missions until he left NASA at the end of 2022.

Kent Frewing, an undergraduate friend from Blacker House, spent most of his career as the administrative assistant to the Director of JPL. He recently mentioned that Carol Godfrey, a member of our undergraduate singing group, once accompanied her husband, John Marburger (President George W. Bush's Science Advisor and head of the White House Office of Science and Technology Policy; and later the Director of Brookhaven), on a visit to JPL. At that time, she remembered singing with that group. There were about eight of us. We would go off to high schools and some Caltech undergraduate dances and provide entertainment during the intermission. We would sing popular songs like, "The Day Isn't Long Enough" by the Four Freshmen. There were two girls in our group, Carol Godfrey and Arleen Brandy, who lived close to Caltech, in addition to five undergraduates and one graduate student.

Anyway, there were lots of links to famous people at Caltech. Chuck Elmendorf told me that "when you go to Caltech, you're going to meet lots of people who are going to become famous. You won't necessarily become famous, but you're going [laugh] to meet lots of people who are." He was right. There were lots of people. [laugh] The thing is, you don't appreciate how important your friends and student colleagues may become when all of you are young, and just hanging out together.

There was John Gurdon, a post doc in biology. He had a party in his apartment one evening. I think it was on Wilson Avenue very close to the campus. I don't know why I was there; maybe it was an open house party that one of my graduate student friends told me about. I recall a punch called gluhwein (whose pronunciation has been permanently etched in my memory, but whose spelling I just learned when I looked it up online). I have a vague recollection that gluhwein was an after-ski drink, so the party may have been an after-ski party. Anyway, after I graduated and went to Tucson, I was reading Time magazine and found an article about John Gurdon having cloned a tadpole. Although I didn't appreciate it at the time, Gurdon didn't just clone a tadpole; he removed the nucleus of a fertilized egg cell and replaced it with the nucleus of a cell taken from a tadpole's intestine. The subsequent growth of that tadpole proved that the mature cell still contained the genetic information needed to form all types of cells. Now he's been knighted and is Sir John Gurdon, and has shared the 2012 Nobel Prize in Physiology or Medicine with Shinya Yamanaka. I read online that Gurdon received the post doc offer from George W. Beadle (former Chair of the Caltech Biology Department and a Nobel Prize winner himself). He taught the mandatory course in biology for sophomore physics majors, that I took in the 1957-1958 school year. It's interesting how all these people become interlinked at Caltech.

ZIERLER: Neil, what were some of the most important NASA missions for your research? I'm thinking, of course, of Voyager, but perhaps there were others.

SHEELEY: I was never a co-investigator on the Voyager missions. The closest that I got to those data was when someone like Len Burlaga (a Co-I on the Voyager magnetometer experiments at Goddard) wanted to use wind speeds deduced from our WSA model as a substitute for in situ speeds after Voyager 1 lost its plasma detector in 1980 when it was only 10 AU from the Sun. This resulted in several joint papers.

But there were lots of NASA missions for which I was a Co-I and had direct access to the data. Skylab was the first that I was really involved with. Next, was the SOHO Mission - the Solar and Heliospheric Observatory. It contained a coronagraph package called LASCO (Large Angle Spectrometric Coronagraph), but it contained some other solar experiments too. After 28 years, two of its three coronagraphs are still observing mass ejections, coronal streamers, and sungrazing comets. Next, were STEREO-A and -B. These two spacecraft are very close to Earth orbit. One is a little closer to the Sun, and the other is a little farther away. The one that's closer goes a little faster around its orbit, and the one that's farther away goes a little slower. As a result, they alternately separate until they are on opposite sides of the Sun and then come together again. They were launched on October 26, 2006, and on February 6, 2011, they were 180 degrees apart, seeing both sides of the Sun. Now, in 2023, they're close together again at a location near Earth. Sometimes, they can do stereoscopic observations of the sun, and sometimes they can see what's on the back side of the Sun relative to the Earth. So they can provide a more complete view of the entire Sun. The fact that they have "heliospheric imagers" that look at progressively greater angular distances from the Sun allows them to track ejections from Sun to Earth and beyond. I think that one of the coronagraphs on STEREO-B has degraded slightly, but in general both spacecraft are still doing a pretty good job.

More recent missions include the Parker Solar Probe and the Solar Orbiter. I think I was originally a Co-I of the NRL experiments on those spacecraft, but I haven't participated in those missions since I retired in November 2016. Nevertheless, I'm still interested to know what other people are learning from those observations. I enjoy following the planetary missions too, including the Martian landers that search for evidence of water on ancient Mars, and the Jupiter orbiter that is probing the giant planet's magnetic field. As a Caltech graduate, I was allowed access to (Caltech professor) Mike Brown's online course in planetary science. Do you know Mike Brown?

ZIERLER: Of course.

SHEELEY: He said that his greatest claim to fame was saying that Pluto is not a planet, but instead is an asteroid or dwarf planet. Cassandra Horii helped with the organization of the class, especially as an interface with the Caltech alumni that were taking it online. (Cassandra Horii was the founding director of the Caltech Center for Teaching, Learning, and Outreach, and later became an Assistant Vice Provost before moving to Stanford in 2022.) I took that course very seriously, attending all the lectures, taking notes, and taking all the exams, including the final exam, which I passed. In addition to learning a lot about planetary science, I was able to interact with other online "students". There were other Caltech graduates. Also, the mother of one of the Caltech students was taking it because she wanted to know what her son was learning in his geology course. A 95 year old man was taking it, which I thought was very impressive. Then there were paying customers, that had nothing to do with Caltech. When it was over, and I had passed the course, I received a message from the company, saying, "For $50, they'd give me a diploma." [laugh] I said I didn't need a diploma. I've already received two Caltech diplomas, and that's enough. Then Mike Brown and Cassandra Horii sent me a nice certificate of their own, thanking me "for participating in the first-ever Caltech Massive Open Online Course (MOOC) for alumni, parents, and friends". [laugh] One of my favorite certificates is the one for Mike Brown's course.

ZIERLER: Neil, I wonder in all your research on solar flares if you ever butted up against climate change research and the supposed connection between solar flares and climate change?

SHEELEY: No, I never did any serious work trying to relate solar flares to climate change. I have thought about the effect of calcium emission, which is a measure of solar magnetic activity. The calcium intensity varies from cycle to cycle, much like the sunspot number. During the first 50 years of the 20th century, the sunspot cycles got bigger and bigger, peaking around 1958. Since then, they've been getting mostly smaller. The last two sunspot cycles have been as small as the ones back around 1900, very low. Thus, if solar activity, as indicated by the calcium emission, were to heat the Earth, then the Earth should have heated up during the first half of the 20th century, and then cooled down during the second half of the 20th century and now it should be the lowest since 1900. However, the global temperature seems to have gone steadily upward, perhaps as reflected in the amount of atmospheric carbon dioxide.

Of course, this sunspot cycle variation of the calcium emission correlates with electron density variations in the ionosphere and the associated radio propagation conditions, as E. O. Hulburt found in his research during the 1930s. When I lived in Blacker House during 1956-1960, the sunspot cycle had the greatest maximum ever recorded. In fact, during 1957-1959, the propagation conditions on the 10-m amateur band were so good that on Sunday mornings I often received phone calls from Whitey Jepsen, W6HRA, in Rosemead CA, saying that he had my father, W2FLC, in Ridgewood NJ on a phone patch.

But global warming is another subject entirely. It is interesting to look at the plots of carbon dioxide versus time. One measurement is obtained in Hawaii, and another is obtained in Chile or someplace in the southern hemisphere. What you see is a long-term increase in both hemispheres, but with a weak annular variation superimposed. The annular variations are 180 degrees out of phase in such a way that the enhancements of CO2 occur during the growing season in each hemisphere. This is because the vegetation removes carbon dioxide from the atmosphere. However, on the average, there has been a steady increase of about 30% in the amount of CO2 since Charles Keeling began those observations on Mauna Loa in 1958. I suppose that this increase of CO2 has an anthropogenic origin and is a greenhouse-like source of global warming in much the way that Venus's atmosphere has blocked the escape of infrared radiation and caused the surface and lower atmosphere to become extremely hot.

ZIERLER: Neil, do you see your research contributing at all to preparations in safeguarding our electric grid from solar flares?

SHEELEY: Yes, I think that's very likely.

ZIERLER: How so? What are the connections?

SHEELEY: You can start with the knowledge that coronal mass ejections (and sometimes coronal holes) can produce disturbances of Earth's magnetic field. Then you proceed to the fact that fluctuations of Earth's magnetic field can induce transient currents in the electric power lines, and that these transient currents may burn out a transformer. Evidently, transformers take a long time to build and replace. So, if you don't want the power grid to be down for a long time, you should shut it down for a day or two when your solar observations suggest that a big geomagnetic storm will occur. You need to protect the transformers. Suppose it's summertime, and it's really hot and humid, and you lose your air conditioning, you can put up with it for a day or two, but you don't want to put up with it for the rest of the summer or longer. That also applies to other electronic devices. We wouldn't be having this Zoom session if we didn't have electric power. People who have electric toothbrushes would be [laugh] having to go back to the older manual toothbrushes. Most of what we do involves electricity (computers, iPhones, radios, TVs, heating, air conditioning, automobiles, gas pumps, air transportation, freezers, microwave ovens, medical equipment, cash registers, food transportation and grocery stores, elevators). It is a very long list. So, if we lost our electricity for a long time, we'd be in big trouble. So it's important to know when a big geomagnetic storm is going to occur, and prepare for it. The bottom line is that our research contributed to the WSA-ENLIL forecasting model on the National Weather Service website, which can help with this preparation.

ZIERLER: Neil, when the Voyager spacecraft reached interstellar space, what did that mean for you in all of your study in solar wind?

SHEELEY: It didn't mean too much to me personally because, by that time, I had retired. But my colleagues at the Lunar Lab - I'm a Visiting Scientist in the heliospheric physics group at the Lunar and Planetary Sciences Laboratory at the University of Arizona - are studying the heliosphere, and its outer boundaries, and cosmic rays, how they come into the heliosphere, and so on. For me, it's interesting to learn what's beyond the edge of the heliosphere. I find it difficult to keep track of the names of the various boundaries, like bow shock, magnetopause, and so forth. As far as I can tell, they move (expand and contract) as if the heliosphere were breathing. At one time, there was a lot of debate about whether Voyager 1 and 2 had really left the heliosphere and entered interstellar space. At least some of the confusion may have been due to possible inward and outward motions of these boundaries relative to the position of the spacecraft. I discussed some of these problems with Randy Jokipii, a Caltech grad and one of our colleagues in the heliophysics group, who had been working on them prior to his death in January 2022. Voyagers 1 and 2 have contributed to space science for many years since their launch in the mid-1970s and are the first spacecraft to have entered interstellar space, but I can't keep up with everything, especially in my retirement.

Sourcing Coronal Mass Ejections

ZIERLER: Neil, when it came time for you to retire, just surveying the field, how much closer had you gotten to understanding really the source of coronal mass ejections, why they happen?

SHEELEY: NRL made the first spacecraft observation of a coronal transient in 1971 from the white-light coronagraph on the NASA spacecraft, OSO-7. After the High Altitude Observatory (HAO) observed many additional coronal transients in 1973 from its white-light coronagraph on Skylab, these events were renamed coronal mass ejections (CMEs). So our knowledge of CMEs began in 1971. We learned a lot from NRL's SOLWIND coronagraph on the P78-1 USAF spacecraft, and we learned more from the LASCO coronagraphs on SOHO. Ironically, SOHO was launched near sunspot minimum in December 1995 and we thought that it would be a quiet sun mission. But SOHO has now obtained observations of CMEs during two consecutive sunspot maxima, making it a prime source of knowledge about CMEs. Although there are different kinds of mass ejections, most of them fall into two main classes - fast and slow. Some mass ejections detach from the tops of helmet streamers and slowly accelerate as they move outward, eventually acquiring the same speed as the solar wind. Another kind of CME starts abruptly in an explosion, and decelerates as it forms a shock wave and plows up material while moving outward into the heliosphere.

I think the origin of the explosive mass ejections is still a subject of research. You might assume that the rapid start is related to the reconnection of magnetic fields at the surface of the sun or close to the surface. The rising arcades of closed loops that occur afterwards appear to result from the reconnection of large coronal loops that have been torn open by the explosion. We have learned a lot about these things, but I think we still don't know how to predict the explosion. That could be something to write about 50 years from now [laugh].

ZIERLER: Neil, in retirement life, in what ways have you remained connected with the field, staying on top of the literature, things like that?

SHEELEY: I was thinking about that. In some ways, I like the work that I'm doing in retirement better than the work that I did when I was working because I don't have to make it relevant and I can keep my own time schedule. I can just follow something because I'm interested in it. The first thing I did after taking Mike Brown's course was to write a book—finish a book, I should say. I'd been working on it in my spare time over several years. It is about the way magnetic fields change with time in vacuum, not necessarily in plasmas like the solar corona. I wanted to keep it simple. During my career, I often went to talks in which somebody would describe a complicated process. In many cases, I could understand that process in terms of a much simpler analogous process - one that I could use to decide whether the person's making sense or not. For example, I could understand some kinds of oscillations in terms of a pendulum, or a spring. But for coronal mass ejections and various wave motions, I needed an example, a model for how magnetic fields change with time.

So I decided long ago, in between two missions - it was just prior to SOHO - I decided I would solve a transient problem exactly. For simplicity, I would imagine a wire wrapped around a sphere so that there would be a circular current on the sphere. Somehow, I would make the current turn on suddenly everywhere, and then ask the question: what happens? The answer is that two waves are expelled from the current source on the sphere: one goes in toward the center of the sphere, and one goes outward away from the sphere. Now, when the ingoing wave gets to the center, it turns around, and it comes back. It breaks through the surface of the sphere and moves outward, following the first outgoing wave by a distance of 2 radii. So you've got these two waves moving outward, leaving a uniform field in the sphere, and a dipole field outside the sphere behind them. I worked that problem exactly for a current that was turned on suddenly, and then used the solution to obtain the field for a linear current ramp. I approximated the linear ramp by a sequence of little steps, like the many small steps at the US Capitol. [laugh], and then added the miniature waves from those current steps to approximate the field of a continuous linear ramp. After that, I tried many other examples of currents - on or off, sudden or gradual, curved profiles, exponentials, and so on. Eventually, I turned the currents on suddenly and simultaneously in two neighboring spheres, and watched their waves pass through each other, leaving the spheres either connected or pushing apart, depending on the relative directions of the original currents. I published the results in 2020 as a book called "Transient Magnetic Fields". Then the next thing that happened was we got COVID - not me but -

ZIERLER: The world.

SHEELEY: - the world got COVID. In various newspapers, I began to see plots of the current number of cases versus time, and their rate of change. I thought, "I ought to be able to calculate that," and I took some time off from my physics research to work that problem. I developed a model in which the people in society consist of three types. I called them molecules. A blue molecule has never encountered the virus. A red molecule not only has the virus but can pass it on. So, if a red molecule hits a blue molecule, the blue molecule turns red. Now, there are two red ones, and they go off, and they hit two more blue ones, and turn them red. But, to slow down the spread, I added another condition. A red molecule can remain contagious for only a certain amount of time, let's say two weeks (which would be pretty long compared to a 1-day time between collisions; in general, I worked the problem as a function of the ratio of these times). After that time, the red molecule turns green; it can hit blue molecules, but they won't change. Now you've got three kinds of molecules in your society: red molecules that are contagious and can pass it on; blue molecules that have never been hit by red ones; and green ones that were once red, but are no longer contagious. I programmed the process and ran it on my computer. I was really solving difference equations, rather than differential equations. So the solution was more like a random walk than a continuous diffusion. It was analogous to the way that flux spreads on the Sun by moving from the boundary of one convection cell to the next as the convective cells come and go, rather than by the continuous spreading that is described by Leighton's diffusion equation. And I was learning about difference equations, which I'd never really studied before.

I was able to solve for the numbers of blue, green, and red molecules (and their time derivatives, which are just differences) as a function of time, ending up with 50 pages of text and many-colored figures. Then, when people started getting vaccinations, I included vaccinations in the model. The vaccination of blue molecules turned them green directly without having to become red. This meant that an extra supply of green molecules could shield the remaining blue molecules from red ones. It also added 20 more pages to the manuscript.

I was really pleased with it, and I wanted to publish it somewhere. I thought that at least I can put it on an arXiv at the Cornell web site. But what arXiv? Not astro-ph, which is where we put our astrophysics papers. But there's a place called q-Bio: Populations and Evolution. (q-Bio stands for quantitative biology.) In order to publish there, I had to be "endorsed." This means that I had to find somebody who has already published in that biological area who is willing to say that I'm not a quack - not necessarily that my calculations are correct, but that I have at least some credibility. I found a person, who received her PhD in physics from Cornell, and was doing research in environmental sciences, I think, at William & Mary College in Virginia. I sent her my manuscript and received a reply, saying that she had endorsed me. Also, she asked if I knew that she had been a postdoc at NRL, working in the Plasma Physics Division in 2004-2007 while I was there. I didn't. [laugh] She said there was (and still is) a small group in the Plasma Physics Division modeling epidemics. So thanks to her, the manuscript now resides on arXiv q-bio: Populations and Evolution, as "A Mathematical Model for the Spread of a Virus".

ZIERLER: That must have given you great satisfaction.

SHEELEY: It did because it was an area outside astrophysics. And it taught me something else. I had called my model the RGB model after the red, green, and blue molecules. That's a subtle pun on the RGB in electronics and the RGB color codes that you encounter when you're using computers to plot figures in color. After doing those calculations, I learned that in 1927, W. O. Kermack and A. G. McKendrick published a very similar model ("A Contribution to the Mathematical Theory of Epidemics," The Royal Society (1927)). They described it in terms of continuous variables that were related by differential equations. In addition, they used a contagiousness that decayed exponentially on a given time scale, whereas I used a contagiousness that ended suddenly after a fixed time. Their model is called SIR instead of RGB, but the letters, S (susceptible), I (infected), and R (recovered) correspond to the same types of individuals as my B, R, and G, respectively. I was pleased because I had done the modern digital version of what had been done in 1927 [laugh] as a continuous spreading.

Then I went back and did more solar research - this time on the Sun's mean line-of-sight field —the sun-as-a-star field. It's the field you get if you measure the line-of-sight field in unfocused sunlight. It's essentially the average field over the solar disc. You take the line-of-sight average of all of the positive and negative flux on the solar disk. If there is a lot of positive flux on one side of the sun, and a lot of negative flux on the other side, then you're going to get a big signal that will weaken and strengthen as the Sun rotates with its 27-day period.

I did the associated math using spherical harmonic components, and then applied the results to data obtained daily since 1975 from the Wilcox Solar Observatory, located on a hill behind the Stanford campus. When I submitted the results for publication in the Astrophysical Journal, the referee asked about limb darkening, which I hadn't considered. So I added an appendix about limb darkening. The referee also asked me to emphasize that these results apply not only to the Sun, but also to other solar-type stars, which might be useful for astronomers doing seismology of other stars, that is, to astro-seismologists. This got me thinking that people doing research on exoplanets might also find the mean-field results useful. So I put that in too.

When I did the study, I found that there were several rotational components in the time series of observations. There was a 27-day period, which corresponds to the Sun's equatorial rotation rate. Also, there was a 28.5-day period, which corresponds to a mid-latitude rate, and a 30-day period, which corresponds to a slightly higher latitude. In effect, we were sampling the Sun's differential rotation rate at latitudes where large regions of unipolar flux happened to be located, by observing a point source of light.

When I described my preliminary results at a heliophysics zoom session at the Lunar and Planetary Science Lab, Jack Harvey asked me if I knew when the 30-day rotation originated? I found out by restricting the Fourier transforms to specific times. For example, if you leave out a certain date or a certain interval of time, and then repeat the Fourier transform, does the 30-day period go away or not? The 30-day rotation turned out to be associated with a big active region whose coronal mass ejections were responsible for the 1989 Canadian power failure that I mentioned earlier.

After I described these final results at a later zoom session of our heliophysics group, Kristopher Klein asked if I had considered applying wavelet transforms instead of Fourier transforms to the entire data set. Wavelet transforms are designed to reveal where specific frequencies originate. They do that by limiting the sampling frequency to a finite range and then shifting that wavelet to eventually sample the entire data set. That is, the wavelet transforms allow you to display power as a two-dimensional function of time and frequency, whereas a single Fourier transform allows you to display power as a one-dimensional function of frequency. So I'm working on that now.

I could go on and on with this, but I think it would be more appropriate to tell you a story that applies to such ramblings. It is another story that you ought to listen to too, I think.

ZIERLER: Please.

SHEELEY: This is one of my favorite stories—I used to have a Scottish Terrier. My wife and I would take the Scottish Terrier over to Fort Hunt Park for a walk on Sunday afternoon. At Fort Hunt Park, there are horses, a corral, and park rangers use those horses for crowd control on the Mall in Washington. One time when we were there, a park ranger said to me, "We don't have enough horses. Would you be willing to help us with crowd control on holidays and weekends?" I said, "I'd like to help, but I have no experience in crowd control." He said, "Don't worry, just act yourself, say what you always say, and when you walk into the crowd, you will see that it just melts away. But don't use too many square roots or logarithms because you could cause a stampede, and we don't want anyone to be hurt." [laugh]

ZIERLER: [laugh]

SHEELEY: [laugh] So I have to watch out when I'm talking about some of these technical subjects, even to you because—

ZIERLER: Oh, that's great.

SHEELEY: —I'll look up, and you'll be gone. [laugh]

ZIERLER: [laugh] Neil, now that we've covered right up to the present day, for the last part of our talk, I'd like to ask a few retrospective questions about your career, and then we'll end looking to the future. First and most obviously, what has stayed with you from your Caltech education, your approach to science, collaboration with peers? What's been most important and long-lasting in all of your years?

SHEELEY: At Caltech, I learned a lot of technical things, especially in undergraduate school. The ones that have stayed with me are things that I used in my solar physics career, like Maxwell's Equations in mks units, which is how I learned them from Feynman. I think that my approach to science was acquired during my career as I had to solve problems. Perhaps an exception occurred in my thesis research when I decided to make my polar faculae counts objective by shuffling the decks of photographic plates. Leighton liked that approach and asked for copies of my ApJ paper to use in his undergraduate physics lab as an illustration of how objectivity can be introduced in a scientific study. Over the years, I tended to solve problems by getting the facts, making a figure to display them, finding an equation to describe them if I could, and then solving the equation. From Feynman's 1957-1958 section of sophomore physics, I learned that it is important to check your result by finding alternate ways to solve a problem because, as he would say, "you are the easiest one to fool". From my long-term NRL collaborator, Yi-Ming Wang, I learned that the problem isn't solved until the solution can be described physically in clearly written text. And from my wife, Marybeth, to whom I often rehearse my talks, I learned that the problem isn't solved until the solution can be expressed verbally so that an easily bored audience will understand it and remember it.

ZIERLER: What have been the most important technological or even computational advances that have allowed your work to progress over the decades?

SHEELEY: The computer. I didn't do FORTRAN computing very much when we had those big main-frame computers. It was not until we got the PCs that I started getting into computing more seriously. Math, I've always done. But I'd say that the computer has to be very important. But I should add that I made great progress when I collaborated with others like Jay Boris and Rick DeVore who specialized in serious calculations on large computers, and later when I collaborated with Yi-Ming Wang. So bringing diverse skills to a problem may yield a solution where other efforts have failed. This was also true in my observational studies of coronal observations with Russ Howard and other instrument oriented researchers. NRL was large enough that these collaborations could be done internally. At a smaller organization, it might be necessary to collaborate with researchers at other organizations. And at Caltech, the collaborations would probably be with people in other Departments or Divisions, which, I think, was one of Tom Rosenbaum's points in his 2022 year-end essay.

The Profound Legacy of George Ellery Hale

ZIERLER: Neil, when you were awarded the George Ellery Hale Prize from the AAS in 2009, I wonder if that offered a moment of reflection for who Hale was, what he represented, and how far the field has progressed in solar physics.

SHEELEY: I worked that into my talk. I can think of several examples. I was also present at the meeting of the Solar Physics Division when they created the prize. There was a lot of argument about what the prize should be about. But one thing that they concluded was that the prize should be an opportunity for someone in the Solar Physics Division to give a talk about the latest discoveries in solar physics, and be able to express them to the greater astronomy community. Therefore, I wanted to make sure it was a good talk, and I wanted to link in Hale. During the year in which I received the Prize, the AAS and the Solar Physics Division did not meet together. The SPD met in Boulder a week after the AAS met in Pasadena. So I gave the talk twice.

One of the things that I did in preparation was to look around for a picture of the mountains as seen from the Caltech campus. I couldn't find a good picture at first, but eventually someone referred me to Pietro Perona (the Allen E. Puckett Professor of Electrical Engineering), who specializes in vision research. I was told that whenever an opportunity presented itself, he would go to the roof of his building, and photograph the mountains or the surrounding scenery. He obtained a really beautiful photograph of the San Gabriel mountains that he let me use in my talk. With the help of Harry Warren at NRL, I turned the image into a movie that shows the whole field at first, and then zooms across the rooftops and up to the mountains, panning east past the radio and TV towers, and stopping at the two solar telescopes that Hale built. During the talk, I mentioned that we could see trees, red-tile roofs, a church steeple, and towers. Some of the towers were for radio and TV, but we don't receive all of our communications from those towers anymore. We have other sources, such as geostationary satellites, lined in a row along the Earth's equator (and about 6 Earth radii from the center of the Earth). A friend of mine from Kitt Peak, Bill Livingston, took many pictures of those geostationary satellites. To do this, he used long exposures of the night sky, lasting the entire night. The stars made circular tracks, but the geostationary satellites were visible as a row of bright dots at the equator. I showed that picture next in my talk. Then I went a step further outward to the spacecraft STEREO-A and STEREO-B because they were our most recent source of solar data, and were aligned with Earth in a much bigger orbit around the Sun. This led to the scientific details of my talk, which included waves moving out from the Sun and sweeping past Earth. Finally, I read a poem that I had written, summarizing the talk. At the same time, I showed a density wave sweeping past Earth, and then I showed the Earth, getting larger and larger as I zoomed in; next, we were suddenly looking at Pietro Perona's picture and zooming toward the mountains:

ZIERLER: Oh, wow. [laugh]

SHEELEY: [laugh] At the start of the poem, you're looking at the heliosphere from one of the spacecraft, and you can see these waves sweeping past this little unresolved point of light, this little dot, which is the Earth. Then you're zooming in on that dot. It's getting bigger and bigger and, of course, is just an out-of-focus dot. And then, suddenly, poof, you're looking from the Caltech campus up at Mount Wilson. So it was a flashback to the start of the talk, showing that with these new STEREO spacecraft, we could see giant waves moving out from the Sun and sweeping past Earth with its red tile roofs, trees, church steeples, and astronomers observing the Sun from towers that had been built by Hale.

I invited George Ellery Hale's grandson, Sam Hale, and his wife, Sylvia. She attended the talk, but he was on business in Texas at the time, so we met the next morning. I also invited Hal Zirin, Caltech Professor of Astrophysics, Emeritus, and his wife, Mary Zirin, who taught Russian at Caltech (long after I took it in 1959-1961). Hal founded the Big Bear Solar Observatory, which is similar to what Hale did when he founded the Mount Wilson Solar Observatory (a name that was shortened to the Mount Wilson Observatory when nighttime facilities, like the 60 inch and 100 inch telescopes were installed).

A more scientific link, that I forgot to mention, occurred when I moved to Tucson and used the spectrograph with the 80 cm solar image on Kitt Peak to confirm my measurements of strong magnetic fields outside of sunspots. When I told Bob Leighton about that experiment, he pointed out that Hale had used essentially the same technique 45 years earlier in his observations of "invisible sunspots". (My technique was to drive a sequence of alternating analyzers for right and left circular polarization back and forth along the entrance slit while slowly moving the image across that slit. The strong fields were indicated by telltale flickerings of the Zeeman-components of the magnetically-sensitive Fe I 5250 A line, as seen in the exit port. My first summer student, Mike Morrill, designed and built the device that drove the analyzer back and forth along the slit.) Those are some of my links to George Ellery Hale.

ZIERLER: Finally, Neil, last question, looking to the future, as you look over the course of your career in the field, what do you see as the big open questions in solar physics, and what's it going to take to answer them?

SHEELEY: The one that people always talk about is, "what causes coronal heating?" The coronal temperature is millions of degrees, whereas the temperature of the photosphere is only 6,000 degrees. The question is, how do you heat the corona to such a high temperature? That's always been regarded as an important unsolved problem. My colleague Yi-Ming Wang is working on some things that may help to solve that problem. It's a little hard to tell right now. He enjoys working on problems that are very difficult to solve, but sometimes he finds the solution, as he did when he confirmed the inverse correlation between solar wind speed and coronal flux-tube expansion. Another important problem in solar physics, I suppose, is "what's inside the Sun?" Helio-seismology has provided information about the outer layers of the Sun, but helio-seismologists are always trying to go deeper. There are g modes that penetrate deeply into the Sun, but people debate whether or not they've really been observed. I don't know.

One thing that puzzles me. We have this new telescope in Hawaii, the DKIST. That stands for Daniel K. Inouye Solar Telescope, and it's managed by the National Solar Observatory in Boulder, Colorado. That telescope is producing really nice pictures with very high resolution. The ones at Big Bear now are pretty good too. But it takes me back to when I was beginning at Kitt Peak, getting the world's best image of this or that, and wondering, "This is the best [laugh] observation of such-and-such that I've ever obtained, but what do I do with it?" We have this great telescope in Hawaii, producing really high-resolution images, but what do they tell us about the Sun that we don't already know? What really interesting puzzles can we solve with that telescope? I don't know, but I am sure that someone will discover something really interesting and important that we can not even imagine right now, and that the proponents of that telescope could not imagine when they proposed to build it.

Shortly after the Skylab mission, I attended one of the joint SacPeak/HAO meetings at Santa Fe. Somebody, Grant Athay, I think, said, "What we really should be working on is not high resolution. We should be working on global properties of the Sun, how it's affecting the Earth, and its relation to other stars." That was around the time that I stopped pursuing the holy grail of high-resolution observations and began to place more emphasis on coronal holes and their terrestrial effects. You just never know what is going to turn out to be the best thing to do.

ZIERLER: That's what makes it always exciting.

SHEELEY: That's right.

ZIERLER: Neil, I want to thank you for spending this time with me. It's been a wonderful conversation, so good to get your insight and perspective from Caltech and your career. Thank you so much.

SHEELEY: You're very welcome, David.

[END]