SXT Status Report 4 December 1995 - 17 December (Weeks 49 and 50) --------- Hugh Hudson, Serge Savy 17-Dec-95 SXT operations during the past two weeks have been uneventful. We have carried out some special operations aimed at filament eruptions and the quiet corona. The next major joint observational campaign will come at the end of the year with the FlareGenesis balloon flight. SOLAR ACTIVITY Solar activity could only be classified as "low", with (believe it or not) only two B flares during the interval. As NOAA active region 7930 rotated past the W limb on 16 Dec, we could only wonder if this might have been the last active region of the old cycle! In spite of the general feebleness of activity, however, the normal variety of microflares and other phenomena was occurring during the interval, only not quite so spectacularly. SXT INSTRUMENT STATUS The SXT instrument has continued to work well. There were no significant SXT errors during the last four weeks. SXT CALIBRATION ACTIVITIES The main effort for SXT calibration continues to be the "terminator" program, whereby correction images are obtained by acquiring pictures when the X-ray sun has set but the visible sun is unaffected by atmospheric absorption. These images still show the scattered optical light, and we have found that excellent corrections for this effect can be made by proper use of the terminator data. We continue to work on ways of getting such correction images efficiently and in the right filters. Starting next week (pendiing approval from the weekly operations meeting), we plan to begin routinely updating the "normal" pointing of Yohkoh to keep the solar image nearly fixed on the CCD. This was not done previously because of worries about CCD overexposure (at the limbs, especially, since the active regions spend so much time foreshortened there). However activity is so low that this does not appear to be a concern now. The Yohkoh pointing variations have been analyzed by Acton. The current "secular" drift rate of pointing error is about five full-resolution SXT pixels per month, so that a one-month update interval would be about right. At other times seasonal effects reduce the drift. We think the drift must be due to thermal distortions of the spacecraft structure, but they are complicated and it is also possible that structural creep is involved. PASS CONFLICTS In week 49, 9 KSC passes were lost due to conflicts with ASCA. In week 50, 6 KSC passes were lost due to conflicts with ASCA. CAMPAIGNS There have been no official SXT campaigns during the past two weeks but a number of special observations were carried out. * Long-duration PFI exposures of the quiet corona near disk center * Long-duration PFI exposures of a nice-looking filament The objective of the latter is to catch a filament in the process of eruption, with good cadence in SXT images so that the development pattern can be studied. Ideally such observations would be made with H-alpha Doppler imaging, which can be provided by the MCCD instrument at Mees Solar Observatory. Again in week 50 we came close, having picked the right filament by luck and having it erupt or otherwise disappear in an observing gap sometime between the file KPNO helium images of Dec. 14 and Dec. 16. This is about our fourth near-miss, but we will keep trying. The forthcoming FlareGenesis balloon flight from the Antarctic will be our next major effort. Unfortunately, conditions for launch are likely to be good at the beginning of the Yohkoh New Year's holiday, during which limited data arrive and no uplink is possible. We hope that we can coordinate effectively starting Jan. 4, and there is good communication via e-mail and the Web to the optical observers as well as to the local penguins. SCIENCE During these two weeks the only visitors at ISAS have been Bob Bentley (MSSL), and W.Q.Gan (at ISAS for three months from Purple Mountain Observatory; see the seminar comments below). The regular solar people at ISAS seemed to be rather busy writing up conference-proceedings papers. N. Nitta, not exactly a "visitor", was in residence working on superhot events in preparation for the AGU meeting in San Francisco. The bottom-line view of the "superhot" phenomenon is that it is distinctly worth investigation, and may provide clues to a minority of flares that might be heated without strong non-thermal effects. This subject seems to spark hot debates. SEMINARS 6 December. K. Yaji (GUAS, Mitaka): "Microwave and X-ray sources at the looptop in solar flares" The observations from Nobeyama presented here were really quite spectacular, and in a sense this work represents one of the late Keizo Kai's dreams for this facility: delayed non-thermal sources have been found at the tops of large loops in flares, directly revealing the trapped non-thermal (MeV) particles there and opening the door for real diagnostic work on these hard-to-observe sites that Yohkoh has shown to be so interesting. 13 December. W.Q. Gan (Purple Mountain Observatory): "Solar flares from low to high temperatures" In this presentation, Gan-san reviewed modeling work, including both the semi-empirical modeling needed to explain the radiation signatures, and the hydrodynamic modeling needed to explain the dynamical effects. From his point of view the latter is not in good shape and cannot at present be thought of as successful. He is looking at BCS spectra, which combined with careful analysis of simultaneous SXT data, can in principle offer decisive choices among models. PERSONNEL Nariaki Nitta visited. Week 49 Tohbans: SSOC: H. Nakajima and J. I. Khan KSC: Y. Suematsu and S. Ueno SXT Chief Observer: S. Savy SXT Systems and Data Engineer: Week 50 Tohbans: SSOC: H. Sekiguchi and Y. Nishino KSC: S. Ueno and S. Morita SXT Chief Observer: H. Hudson SXT Systems and Data Engineer: