Report on "Cosmic Winds and the Heliosphere" This meeting was one of the review meetings organized around the preparation of books in the University of Arizona series (cf "The Solar Interior and Atmosphere" and "The Ancient Sun" already published in this series). In such a meeting the authors come together and present fairly detailed reviews (45-minute talks) on their individual subjects. In this case there were several communities involved - stellar astronomers, heliospheric physicists, cosmic-ray physicists, and solar people. I attended partly because the organization was not strong on the solar side, partly because I wanted to learn about the solar wind with an eye towards Yohkoh, and partly to make contacts with the heliospheric community (especially the Ulysses mission) for the sake of Yohkoh scientific operations. This field is not strongly represented in the Yohkoh team but of course it's an area that is rapidly becoming more important as the flares die away. Many of the big names on the author list did not show up - Axford, Scudder, Sheeley, Parker - but there were many excellent talks and I found it to be one of the most rewarding meetings I'd been to recently. It was quite international in scope. We need badly to coordinate with Ulysses. Many people from that project approached me and suggested joint workshops and/or joint data analysis projects. There are several concrete ideas for the latter that I'll work into the comments below. As for the former, the sooner we have a workshop of any sort, the better - the first Ulysses polar passage is coming quite soon. Miriam Forman gave me a plot of the solar "sub-Ulysses point", the track of the intersection of the Sun-Ulysses line with the photosphere. I'll copy this around as requested. Following are some comments derived from the presentations on the solar wind, which of course is one of the key areas of Yohkoh/Ulysses collaborations. I will say at the outset that there is a tendency among some solar-wind theoretical people to oversimplify the Sun grossly: they would like to take our beautiful SXT images and reduce them to two parameters (the temperature and density of the "base of the corona"). Classical Parker theory starts from these parameters, and the pressure at infinity, to make a model of the radial variation of density, temperature, etc. This theory is not complete, because the mass flux is determined ad hoc by these boundary conditions, which are maintained by the unknown coronal heat input. 0. Wind Theory & Basic Stuff >From the viewpoint of Parker theory (E. Leer), there is a big mystery in the regulation of the mass flux, which is observed not to vary by much over the cycle, whereas in the mathematical model the mass flux is very strongly dependent on the temperature of the base of the corona. Wang (NRL) pointed out that the requirement of consistency between the downward conductive flux and the coronal base pressure reduces this model dependency quite a bit. Hundhausen, based on work by Jackson and Webb, claims that the CMEs cannot contribute much of a fraction of the solar wind (it would be the slow-speed "interstream" component). I am a little bit unsure of this conclusion because of the huge selection effects in CME event selection and the gross systematic uncertainties in CME mass estimation. The relevance to Yohkoh here, of course, is the possible identification of Uchida's expanding active regions with the slow end of the CME velocity spectrum and the slow component of the solar wind. Progress here is possible but, as I often maintain, it will depend on getting a systematic correlation analysis going. J. Schmitt presented a rather amazing review of ROSAT and other observations of stellar X-ray emission, with a rather pessimistic conclusion as regards wind studies: in the presence of bright coronal X-ray sources, the X-ray flux from a stellar wind is probably undetectable. In the Yohkoh context, of course, this is consistent with the fact that the "power law corona" Lemen has discovered, which would of course contain any direct contributions from the solar-wind plasma, only contributes weakly at least during active times. Can we measure the fractional contribution of the power-law corona from the image histograms? The Hyades stellar X-ray sources do not show much variability, compared with what one would predict from Yohkoh total flux measurement or from GOES. Why is this? Attend the ISAS seminar to be given by Stern next month and find out. 1. Coronal Temperature Coronal temperatures are the first important thing that the solar wind looks to Yohkoh for. Of course there are many other ways to determine coronal temperatures, but the soft X-ray emission is one of the most direct. One can determine temperatures also from within the solar wind itself, incidentally. One way is through the "frozen-in" level of ionization. I was interested to learn that even in the high-speed streams, which originate in coronal holes, these ionization temperatures require a multithermal plasma. Judging from Hara's results, so does SXT. I also heard a rumor that the Harvard group may have announced a related result at the Elba meeting that just took place (from their SPARTAN data). Ulysses is now beyond 30 degrees S heliographic latitude, and the nature of the wind flow that it detects is changing radically. The slow solar wind has disappeared, and high-speed streams reach speeds of up to 900 km/sec. The interstream regions also have higher and higher speeds as the latitude increases. The presence of speeds like this was expected from IPS data, but this is much more comprehsensive information, of course, including all kinds of parameters of the flows and the particle populations. 2. Distribution Functions There is a wealth of information in these modern solar wind measurements from the distribution functions of the particles. These tell us about the magnetic connectivity, for example, and there is a powerful argument from the observed distribution functions that true plasmoids (locally closed field lines) do not exist; instead the non-open structures must be either tongues or flux ropes instead. This is of course consistent with the distinct impression I get from Yohkoh filament-eruption events, ie that they are three-dimensional rather than two-dimensional. Interestingly enough, the distribution functions become more non-thermal the closer one approaches the Sun (data, for example, from the Helios spacecraft that went in to 0.3 A.U.). The existence of non-thermal distribution functions is expected from the requirement for coronal heating, either by waves or by "nanoflares". (No, not "microflares"!) I get the impression that Alfven waves are still very much in the running as a coronal heating mechanism. Axford is talking about solar wind "furnaces" located in the network boundaries, in which the waves would be generated by reconnection (I think - his talk was presented by proxy since he couldn't come). I strongly criticized this suggestion because, of course, you don't see the network in soft X-rays. [But why don't you? It seems like this is quite an interesting question]. The presence of non-thermal distributions may be an alternative to straightforward DEM thinking in reconciling SXT and other measures of "the" coronal temperature. The distribution functions can be very complicated, but one typical simplification that is often used is the so-called "kappa distribution", which is a three-parameter representation of a Maxwellian with a power-law tail. I'm writing an IDL function for this in case anybody's interested. 3. Coronal Mass Ejections Art Hundhausen was at the meeting and presented three Yohkoh events (24-Jan-92, 4-Mar-92 (? - this event was discussed by Sime at the Kofu meeting, 30-Apr-93). These are all of the polar crown streamer blowout type. Art was as usual not particularly interested in flare-associated events, although he rather cited one huge one to underscore the requirement for very large energies associated with CMEs (>10^33 ergs for the X6 flare in this case), and he did comment that he knows of about 15 examples of flare-associated events. For a long time he didn't seem to want to know about any, so this is a big improvement! Ulysses experimenters are interested in searching out Yohkoh data for specific transients. I volunteered myself to help with quick searches and pointed out Klimchuk and Nakagawa as the persons most actively engaged in studying solar counterparts. 4. Coronal Hole Morphology There were interesting discussions (e.g. with Ruth Esser, who is to make an extended visit to Toyokawa this winter) regarding the appearance of coronal holes. There seem to be at least two complicating features: the plumes one can see in polar holes at the limb, and the veiling structures that we often see forming over holes. Do the latter confuse the proper definition of coronal hole areas? Probably this is an active subject of research in Yohkoh now. For example, the analysis that Hara_san has been doing of the histograms might bear on this. As for the plumes, how much of a filling factor do they present? Esser thinks it's established that the plumes arise in XBP's, but Wang and Widing of NRL don't seem to concur with this. Is there something in the work Strong and Harvey are doing that is relevant here? Is anybody in the Yohkoh team studying polar plumes? The plumes and BP's seem to me to be the key to our contribution to the understanding of the high-speed streams, because the most plausible scenario seems to be that they originate in the network boundary regions in coronal holes. The paradox is that SXT doesn't see the network in any steady way - is it conceivable that it is just too variable in time for its pattern to be visible, and that the network really is there in some time-averaged sense? 5. Outer Boundary of the Heliosphere I won't say much about this, because it can't possibly be very relevant to the Yohkoh observations, but it is extremely interesting. The four probes (two Pioneers, two Voyagers) now approaching the edge of the solar universe are definitely seeing interesting new things. 6. Summary Sorry to have been so verbose here, but you can tell that I was fascinated by this stuff. There should definitely be Yohkoh representation at the "Solar Wind Eight" conference, which will take place next summer. I'll just say in conclusion that Frank Shu, an eminent outsider in this community, commented privately that whereas the solar wind itself seemed to be in relatively good shape theoretically (modulo the big unknown of coronal heating), the same could not really be said for CMEs. This conference did not deal much with CMEs but it's a major area of interest for Yohkoh, of course.