Solar and Stellar Studies at CfA: The Rosner Years. Leon Golub Harvard-Smithsonian CfA

Transcription

1 Solar and Stellar Studies at CfA: The Rosner Years Leon Golub Harvard-Smithsonian CfA

2 G. S. Vaiana, the S-054 X-ray Spectrographic Telescope and the evolution of a coronal hole in the soft X-ray corona.

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4 Skylab Apollo Telescope Mount (ATM)

5 The Solar Outer Atmosphere: Structure is Fundamental

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7 The Solar Outer Atmosphere: Structure is Fundamental

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9 The Solar Outer Atmosphere: Structure is Fundamental

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11 The Solar Outer Atmosphere: Structure is Fundamental

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13 Solar Magnetism: Formation of Coronae

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15 Solar Magnetism: Formation of Coronae

16 HEAO-B HEAO-2 The Einstein Observatory

17 Late Stars: The Sun in Context

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19 Late Stars: The Sun in Context

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21 Late Stars: The Sun in Context

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23 Magnetic Fields in Astrophysics

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28 Current sheath thickness classical (Spitzer) resistivity: Anomalous:

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34 Golub presentation for Rosnerfest. I am deeply honored to have been asked to speak at this special event. Bob and I worked closely together for some years, mainly from about 1975 until the late 80 s when he moved to Chicago. But our connection goes back much farther than that. We were both born in the same part of Germany shortly after the end of World War II. We both eventually ended up in New York, each going to one of the special high schools in New York City, and I am fully convinced that we faced each other in the math team competitions during those years. We both ended up going to grad school in Cambridge, though again divided between the two great competitors there. Bob first came to my attention indirectly. I was at AS&E in the space research group founded by Riccardo Giacconi, working on the Skylab solar X-ray mission (see slide). It was led by a dynamic person named Pippo (short for Giussepe) Vaiana (see slide), who was spoken of as someone who always left behind himself a turbulent wake wherever he went. Vaiana left AS&E and moved to the Harvard College Observatory. I stayed behind for a while, and one of my duties was to head the computer users committee. In those pre-pc days most places had a large mainframe (see slide) and people submitted jobs then waited for them to be run. Vaiana called one day and asked whether a new postdoc working for him could run some jobs on our computer, and of course I said yes. At the next meeting of the users committee, the operator of the mainframe asked me Do you know this Rosner person? He s now the single biggest user of CPU time on our computer. So I had to contact Bob ask him to scale back a bit. The Skylab mission was a major one for solar studies. Observations from space had been available since the late 40s, but at first they were only snapshots obtained via sounding rockets. After Sputnik was launched, and the U.S. space

35 program got going, small satellites notably the OSO series allowed us to access these new wavelength regimes over long periods of time and with increasingly more sophisticated instruments. The Skylab Apollo Telescope Mount (slide), or ATM, flew the highest quality instruments launched into orbit up to that time, and also provided adequate data analysis support to make good use of the observations. In those days, 1973 to 1974, before large photodetector arrays were available we used film, retrieved and returned to the ground by the Skylab astronauts. In 1976 I moved over to Harvard to join Vaiana s group, and later we all moved over to the Smithsonian side. I got to know and work with Bob and, as the line goes, it was the start of a beautiful friendship. So what I would like to do is to survey those years, point out the major areas that were addressed and illustrate them with some of the key papers published. I ll only provide a sampling, a small taste, because if I tried to be comprehensive I d still be talking after this fest ends tomorrow. The separation into topics is somewhat arbitrary as the work at the time was all of a piece, a whole with many parts. As you ll see, it was a time of remarkable progress in understanding the Sun and the Sun s place among the stars, as well as the role of magnetic fields throughout all of physics. We traversed one revolution in our understanding during those years, and ended at the brink of another. To begin: what we realized from Skylab was that the structuring of the corona into closed loops and open streamers is fundamental (slide), that in effect the structure is the corona. In addition, the structures that make up the corona are highly variable, typically appearing and disappearing as fast as they can. Both of those statements require some explanation.

36 An initial step toward this understanding was a study of a closed, plasma-filled, magnetic structure treated in what we might call thermodynamic fashion the famous RTV paper (slide). This line of work examined some of the basic properties of magnetically-closed loops, in effect treating a loop as a relatively isolated loop atmosphere, many small coronae as the paper noted. What you can derive, with some assumptions such as hydrostatic equilibrium and steady heating input from both loop footpoints, are scaling laws relating the loop pressure, temperature and length; the most famous was the 1/3 power relation between T and pl, although people tend to forget that it applied to the special case of a constant heating function throughout the loop volume. Also, you can conclude that the maximum temperature must be at the top of the loop and that heating functions with short deposition scale heights will yield unstable loops. Nearly simultaneous with RTV was work done to explain the high temperature of the X-ray emitting loops based largely on laboratory plasma physics work, first in collaboration with Bruno Coppi of MIT and then with Albert Galeev of the Space Research Institute of Moscow. Galeev visited us for a year and his big luxury seems to have been the purchase of a season pass to the Harvard Square movie theater. I recall that after seeing the first Star Wars movie he commented It s a very good Western. This work concentrated on the energetics of loop heating via currents, showing that, in Coppi s words It all hangs together. (slide)the first paper became known for emphasizing the need for anomalous resistivity in thin dissipation sheaths as a way to dissipate in the corona the stresses induced at the footpoints by photospheric motions. This analysis was further developed in the Galeev 1981 study which explored possible mechanisms for dissipating the energy stored in magnetic shear, finding among other things an RTV-type scaling law having an additional dependence on the available magnetic free energy represented by the poloidal plasma β J. [β is the ratio of the plasma pressure to the magnetic pressure]

37 It soon became clear that this type of analysis had to be supplemented by numerical codes, developed by our colleagues in Palermo, especially Salvatore Serio (Serio pic) the reason he s pointing to an old globe is a story in itself, but I think I ll leave it to Bob to tell that story and by Giovanni Peres (Peres et al slide). These 1-D codes provided a way to analyze how the loop would respond to changes in the heating function, and included a way to deal with the problem of the enormous change in values of key parameters in the transition region. I ll note that this type of modelling has developed into a subfield of its own, and such modelling continues to this day. One of my personal favorite bits of work was the one we called B vs. X, in which we analyzed a large assortment of compact X-ray emitting regions on the Sun ranging from the small X-ray bright points to so-called ephemeral active regions to larger and longer-lived active regions looking for a correlation between properties of the magnetic field and the coronal response in the emitting region. At first I was quite pleased to find a strong relationship, a 3/2 power law, between the thermal energy content (nkt integrated over the volume) and the total magnetic flux (B integrated over the base area) in the region. Then Bob pointed out that volume is related to area as the 3/2 power, so this result was not terribly surprising. So I had to dig deeper, and found a different relation, this one between the average coronal pressure and the average longitudinal magnetic field strength, a power law with slope 1.6. Bob then produced a model in which an azimuthal component of the field is produced by a shearing motion at the loop footpoint and available magnetic energy is generated, with the rate dependent on the magnetic field strength and on the strength of the shear. The predicted law fit very well with the observations, and the shearing velocity was the only free parameter. A very reasonable value of a half km/sec came out of the analysis.

38 The work to that point in time was summarized in the Vaiana & Rosner, Annual Reviews article of 1978, which ended with a most timely preview of what was to come in stellar studies(text box). While all of this solar work was going on, HEAO-B (slide), the second High Energy Astrophysical Observatory was launched, in November of 1978, and renamed HEAO-2, then the Einstein Observatory. HEAO-1 had been launched a bit over a year earlier and provided a major step in the maturation of X-ray astronomy. But it detected only a small number of stellar sources, and those tended to be rather unusual ones such as RS CVn stars, close binaries with short orbital periods and strong chromospheric and X-ray emission (~10 31 erg/s). Einstein, with hundreds of times higher sensitivity and the ability to point at and image objects with arcsecond capability turned out to detect huge numbers of stars -- a nuisance for those studying, say, quasars or AGNs, but a gold mine for stellar studies. In conjunction with the International Ultraviolet Explorer (IUE), these new observations initiated the new field of cool star studies. We immediately set about analyzing as many stars of all spectral types as possible, both via our own observing programs and in collaboration with a large number of other researchers. The first results were published in a large stellar survey paper (slide) which announced that X-ray emission was detected from stars of nearly every spectral type (slide) early, late, dwarf, giant, T Tauri, RS CVn, and on and on. So the problem then became one of explaining how this emission arises, and it turned out that there were different answers for different types of stars. For stars like the Sun, the first problem was to explain the large spread in luminosities, with the Sun falling near the bottom for main sequence stars of late spectral type. Early-type stars, the O and B main sequence high-luminosity types,

39 also showed X-ray emission, but of a different character. For the early-type, it turned out to depend on their luminosity (slide). The key parameter for the solartype stars, discovered simultaneously by a number of different methods, turned out to be the rotation rate, producing universally higher levels of X-ray emission than the minimal ratio extrapolated from the early-type data. As summarized in an Annual Reviews article, for early stars the coronae seem to come from luminosity-driven winds and their resultant instabilities, whereas for late-type stars it is the internal magnetic dynamo coupled with an outer convection zone that feeds energy into the corona and produces the hot X-ray emitting plasma. You will be hearing more about this from the next speaker, Salvatore Sciortino (slide). From the start, going back to work with Galeev on accretion disks, Bob used the solar and stellar work as a basis for more general and more exotic astrophysical applications work on astrophysical jets with Attilio Ferrari (slide) and with Kanaris Tsinganos (slide), for example, and with Gianluigi Bodo (slide) on rotating magnetic jets. You ll be hearing more from both Attilio and Kanaris this morning. And in one of a series of papers on the diffuse soft X-ray background, Bob worked with a young Vinay Kashyap (slide) on the stellar contribution. And with a graduate student, Bill Jeffrey, he worked on inversion methods for the burgeoning new field of helioseismology. (You ll have to ask Bill what he worked on after graduating; he couldn t tell any of us.) Around the time that Bob left the CfA and eventually stopped working on coronal physics, there was a big change in our understanding of how to envisage coronal dynamics. The view developed that we need to analyze the topological structure of the magnetic atmosphere, looking for the places where the field vanishes (null points) or places where there is a large divergence between regions of simplyconnected fields (separatrices). These tend to be where strong currents form or where major field changes occur, where large topological changes release energy.

40 It took several years for this view to penetrate solar studies, but Bob was onto it at the start (slide). It is unfortunate that he did not continue to work on the implications of this view in the general astrophysical context. The work has had a huge impact within solar physics but is still to this day not well known outside of the field. Had Bob stayed, I'm sure that things would have been different, and I want to take this opportunity to chastise him for leaving the job unfinished. There was much work to be done in bringing this new understanding to bear on more general astrophysical problems -- and there still is.