The road to helioseismology
As told by Roger K. Ulrich
I was born in 1942 – a very bad year for the United States and for the world. My parents often told me how lucky I was. I didn’t appreciate how true that was until I learned a bit of history. My family has a strong background of learning, teaching and research. My father, Albert, worked for the University of California Extension Service in the Agricultural Extension Station as a plant nutrition specialist. His work to improve crop yields was important to the national war effort and that is why he stayed in California.
My father was born in N.Y. City just after his parents came over from Alsace, Lorraine/Germany. They eventually settled in El Segundo, California, where they opened a bakeshop. The photo shows dad (Albert), his brother Louis and mother Stephanie in their bakeshop. Dad helped by delivering loaves of bread on his bicycle.
My mother, Jane, continued the tradition started by her father by working toward a Ph.D. in forestry. Her father, Harvey Murdock, was a professor of agricultural engineering at the Montana State University in Bozeman, Montana. His story is quite colorful having earned a teaching credential in Indiana. He then modified a bicycle so he could ride it on railroad tracks, which he did to get to Colorado. He taught in New Mexico in 1901 in a single room schoolhouse. The photo is his New Mexico class, is an example of his work.
My family lived in El Cerrito, a town in Northern California in the San Francisco Bay Area, until the early 1950s. We left from El Cerrito and lived in Pasadena, California for several years while Dad worked at Caltech in a facility called the Earhart Plant Research Laboratory, subsequently designated as the Phytotron. This laboratory was the first of its kind at Caltech and was organized by Fritz Went. By 1972 the Phytotron ceased operations and we later returned to the Bay Area and purchased a house again in El Cerrito and my Dad began working at UC Berkeley in a lab studying. Travel was always part of our family activities. We went back and forth between Berkeley and Bozeman regularly, often camping along the way.
Crummy planetarium projector
I developed interest in astronomy and chemistry during my years in high school. I participated in the science club at school and tried to grind and polish a six-inch reflector mirror in our basement workshop. I got a figure into it but then could not polish and test it. I spent a year watching the sky and identifying the constellations as they came up using the Norton Star Atlas. I photocopied the pages from the Norton Star Atlas and used those to poke holes with a pin into black construction paper. I was able to tape these sheets together into a crude sphere, put a planetarium light in the center and create a crummy planetarium projector that won 3rd place in the San Francisco Science Fair. I guess that was easier than making a telescope.
“I was able to tape these sheets together into a crude sphere, put a planetarium light in the center and create a crummy planetarium projector that won 3rd place in the San Francisco Science Fair.”
When it came time to go to a university, the choice of UC Berkeley seemed obvious to me having spent time there in my father’s lab. I was conflicted about the choice of major to enroll in, and picked the practical subject of chemistry. Enrollment on the first day was memorable to me because of it being stormy with the enrollment lines extending outside. I lived at home the first year and commuted with my father. I was missing the campus life and moved into a student run cooperative dorm for the rest of my undergraduate time at UC. My duties at the co-op included cleaning pots at the central kitchen, but I eventually got assigned to the bag lunch crew. I joined the UC Hiking Club and that provided a social focus. I met my wife at one of the UCHC events, and we did many trips together visiting most of the national parks in the west. I had a big decision to make after I graduated: stay with the practical chemistry studies or go with my hobby of astronomy. I chose my hobby and got into the graduate program at UC Berkeley. My wife and I lived in apartments near the Berkeley/Oakland boundary, and she enrolled in an educational psychology program.
Pursuing my “hobby”
Initially I worked with the radio astronomy group under Harold Weaver. I used a spectrograph in the physics department to measure the frequency of the CH radical. I was never sure if I got it right. I transferred to the stellar evolution group under Louis Henyey, and stayed with them until I got my Ph.D. thesis done. They had a big code using assembly language, which was hard to work with. Everything was done on IBM cards, and the jobs were submitted by carrying three big boxes down to the computer center and handing them over to be read in. The results came back on paper printout. Graphing things was done by hand on graph paper – finding the right variable on the printed pages and putting points on the graph paper grid. A bit tedious, but the only way to see a summary result. My Ph.D. thesis topic was a bad choice by me – I was going to try to improve the Mixing Length Theory of convection. I did find assumptions I could improve on, but in the end the result was not convincing.
Two things happened while I was with Henyey’ s group: 1) Ed Frazier, a fellow member of Henyey’ s group, returned from Kitt Peak Observatory with a long series of Dopplergrams at pretty high resolution; and 2) John Bahcall visited with our group for an afternoon and asked us if we had a model of the sun. From Ed’s results and his interpretation that the convection seemed to briefly disrupt the continuing oscillations, it struck me that the standard view that the oscillations were sound waves in the atmosphere needed to be extended into the interior below the photosphere. From John Bahcall’s comments, I concluded that we needed to do a model of the sun.
Model of the sun
I did a model of the sun and sent it to John. It got quite a reaction – much of our physics needed to be updated. John offered me a job at the Kellogg Laboratory at Caltech, which I accepted. My wife and I moved to Pasadena, eventually raising three boys, Scott, Chris and Marcus – all of whom are involved in some aspect of technology and innovation.
As soon as I got to Caltech, two things occurred: 1) the task of fixing the Berkeley solar model fell to my fellow grad students, Sylvia Torres-Peimbert and Eric Simpson, who got the physics corrected and the paper written and published and 2) John immersed me in calculating solar models using a code developed by Giora Shaviv. It was what is called a shooting approach, wherein integrations from the center to a mid-point are matched up with integrations from the surface to the mid-point. The starting boundary parameters are iterated until a match of all parameters at the mid-point is achieved.
The solar evolution starts with an initial model, which has a uniform composition that is then updated by the results of nuclear evolution. It is a much less sophisticated approach than used at Berkeley, but I did not feel I could have managed the big UC Berkeley code on my own. I modified the Shaviv code to use backward time differences in the nuclear evolution. The approach to steady state where there are short-lived intermediate species was necessary to avoid numerical instabilities. The species 3He in particular requires this approach and is important in the production of solar neutrinos. I also brought with me the atmosphere code from Berkeley that had been written by M. S. Vardya then substantially revised by me for my convection studies.
During this period I attended a conference at the Western White House, organized by the UC Irvine Department of Physics. A very prominent group including Ray Davis, a driving force in the neutrino detection experiment; Maurice Goldhaber, head of the Brookhaven National Lab; A. G. W. Cameron, a Harvard professor and one of the founders of the field of nuclear astrophysics; William A. Fowler, head of the Kellogg Lab and leader in the measurement of light nuclei reaction rates; Icko Iben, an early leader in the computation of stellar evolution; Dave Schramm, an energetic young researcher on nuclear astrophysics and cosmology; John Bahcall, a neutrino theorist and astrophysics spokesman; and Richard Sears who computed one of the earliest solar models. John and I collaborated until 1985, when I accepted the responsibility of managing the 150-foot tower telescope at the Mt. Wilson Observatory.
The story about my work on the solar surface oscillations, which led to helioseismology, is rather different from what was happening with the solar neutrino problem. Everyone was aware of the neutrino problem, and recognized it as a major issue. The solar oscillations were known, and people wondered a bit about them, but they were not at the top of anyone’s list of things that had to get understood. They seemed interesting to me, but I worked on them strictly out of curiosity. The effort started at the end of my time at Berkeley, and I did not discuss the work with anyone in that group. As evidenced by the Neutrino72 conference, the solar neutrino problem was on everyone’s mind.
I did finish the calculation of the properties of sound waves in the outer solar envelope. The important result was a prediction of where the oscillatory power should be found on what can be called the modal diagram. The parameters on the modal diagram are the horizontal wave number as the abscissa and the frequency as the ordinate. This plot is often called the k-omega diagram. I found that the power in this modal diagram should be along diagonal dispersion lines. The calculation of k requires the measurement of both horizontal directions. I also tried to calculate the energetics of the modes, and concluded that they are self-excited producing energy that can be dissipated in the layers above the photosphere and chromosphere. This secondary conclusion is hard to verify and has not been generally accepted. I discussed the results of the calculation with Robert Christy, who thought my energy calculation could be correct. Peter Goldreich was interested, but did not really comment. I approached Hal Zirin thinking he might have an employment opportunity for me after the year and a half appointment with the Kellogg Lab ended. His response was that he didn’t know what he would do with someone like me – still I persevered.
I gave a paper at an American Astronomical Society meeting in Pasadena, and got an enthusiastic response from people from Sacramento Peak Observatory. I visited Sunspot where Sac Peak personnel live and was kindly hosted by George Simon and others. They wanted me to come work there, but I didn’t really want to leave the urban area. Fortunately, through recommendations by John Bahcall, I was offered a faculty position at UCLA, which I accepted. I have been well employed at UCLA since that time and enjoy the working with students which comes with an academic appointment. The list of Ph.D.s I have supervised is long.
“The list of Ph.D.s I have supervised is long.”
Test my prediction
One of my faculty colleagues at UCLA, Kurt Riegel, recommended that I add a section at the end of the paper I presented at the AAS, explaining what I thought would be needed to test my prediction of the diagonal lines on the modal diagram. I did that and thought that there would be a rush to check out this prediction. I waited and waited and nothing happened. After about 18 months of waiting, I decided I better make a local effort to do this if no one else was going to come forward. I recruited Edward Rhodes, Jr. to work with me on this. We tried a couple local facilities and found that the equipment was not up to the task. I have learned subsequently that at various observatories around the world, the signal of the oscillations was quite clear. What was difficult was the need to measure, in a resolved fashion, the Doppler signal in a moderately large area with a cadence of once per 45 seconds or so for a duration of hours. The data management for that was a bit of a stumbling block.
There were investigations in the early 1970s, particularly by Franz-Ludvig Deubner, which had good spatial and temporal resolution as well as temporal duration, but which lacked the second spatial dimension. Deubner was able to rule out excitation of the oscillations by means of piston-like perturbations by granulation cells but in his earlier works did not find the diagonal power concentration in the diagnostic diagram. In the meantime, Ed Rhodes and I tried a number of options without much in the way of good results until Ed went to Sacramento Peak Observatory. With the assistance of George Simon, Ed was able to get good data from the diode array system. Ed was able produce diagnostic diagrams showing the diagonal ridges and was well into writing the work up for publication when we learned the Franz Deubner had announced at a meeting in Europe that he had found evidence for the diagonal ridges also using data from Sac Peak. Ed and I with George Simon were able to demonstrate the utility of the method for the study of the structure of the solar envelope – a first step toward what became helioseismology. I initially called the method solar seismology until my better educated European colleagues corrected me – you can’t combine a Latin prefix with a Greek suffix.
“I initially called the method solar seismology until my better educated European colleagues corrected me – you can’t combine a Latin prefix with a Greek suffix.”
By the early 1980s it was clear to the community that pursuit of helioseismology was an important task. There were many limitations to the spectrographic approach we used for the early work. The top two issues were that continuous observation was possible for only about 10 hours at a time and regular observatory facilities were only available for limited observing runs. Jack Harvey, Tom Duvall and Marty Pomerantz took continuous data from the South Pole in 1981/82, which took until 1986 to get reduced and published. The side lobe issue was resolved as expected, but the duration and number of observing of sequences was limited – the South Pole is not a vacation spot nor a regular work environment. So, Ed Rhodes and I worked with Robert Howard at the Mt. Wilson Observatory to modify the spectrograph in the 60-foot tower telescope so that regular helioseismic observations could be acquired. Our objective was to identify variations in the frequencies over time, possibly caused by the solar cycle. That system was implemented successfully in 1984.
Invited to NASA
Ed Rhodes and I felt the best long-term solution was to get NASA to launch a mission that could observe from away from the Earth’s shadow. Others suggested a ground-based network. Ed and I proposed adding an instrument call a Magneto-Optical Filter (MOF) to a mission called the Out-of-Ecliptic mission. NASA doesn’t work well with unexpected instruments to be added to an already defined mission, and our proposal was rejected. The idea did pique their interest, and Ed Rhodes and I got invited to several planning sessions at NASA Headquarters.
One important meeting for the two of us was with the European Space Agency (ESA) in Noordwijk, Holland at the ESTEC Test Centre. That meeting was to coordinate NASA and ESA missions so costs could be shared. It featured a bit of intrapersonal fireworks as various instruments were proposed or eliminated for inclusion. I did manage to get what was called the Dynamics Program included for the helioseismology instrument. They were not going to have any high data-rate periods for the helioseismology. “What do you need that for?” was the question I had to answer. Eventually our instrument was accepted to be part of the Solar Dynamics Observatory.
Ed Rhodes and I continued our efforts at Mt. Wilson, until observatory closed in 1986. For me, that was the end of my helioseismology research. The magnetic and Doppler series from the 150-foot program at Mt. Wilson has been challenging and enjoyable, and I am grateful for the experience.
Total solar eclipse
The American Astronomical Society held their meeting for Fall 2017 in Portland OR, and included an excursion to the total solar eclipse nearby on August 21. I let my family know about this event and got a surprising number of takers. All three of my sons came, their sons except for one, my wife’s father, the brother of my son’s wife and his two boys. Altogether our group numbered 13, one of the largest at the meeting. Everyone enjoyed the minutes of extra darkness at totality.