SXT Status Report 2 September 1996 - 23 September 1996 (Weeks 36, 37, 38) H. Hudson, S. Savy, J.-P. Wuelser SUMMARY Solar activity subsided. Operations were generally smooth, with considerable coordinated observation activity with SOHO. The HXA sensors, used to calibrate the Yohkoh gyros for fine pointing reference, have degraded (very slowly) to the point that corrective measures are being taken. We had some interesting visitors and seminars. SOLAR ACTIVITY The fourth (and final?) rotation of AR 7978 brought few surprises as it rotated onto the disk on about 20 September. It is still surprisingly large, but much fainter except for a still-active core. It appears to have finished with major flare activity, not surprising for an old-cycle region this late in the cycle. As noted in the last SXT status report, the global coronal structure changed profoundly with this region's appearance, and the coronal hemisphere containing the active region is still notably bright. We believe that SOHO made many observations of the nice large-scale loops in this region. SXT INSTRUMENT STATUS Software recovery from the filter rupture last month is complete, and stray-light corrections look very good now. Other than the fact that there were surprisingly few SEU's or other onboard software errors in SXT during this period, there is nothing to report. SXT CALIBRATION ACTIVITIES The terminator image program is now complete for the thin filters and the S/C 5'E and 5'W offpoints used in support of SOHO JOP044. We are now proceeding to the less-used filters. Our plan for periodically updating the Yohkoh "normal pointing" position, in order to minimize the number of stray-light references, has not yet been implemented. A gradual degradation of the signal levels of both HXA sensors (X slightly faster than Y) has caused software problems for SXT image registration over the last few months. Yohkoh currently uses these HXA sensor outputs (HXA is part of the HXT instrument) as its limb reference for pointing jitter, although there are alternatives that are a bit more complicated. Jean-Pierre Wuelser is currently updating the software to allow for the newly-discovered problem, and the HXT group will increase the gain on the HXA-X output this week. It appears from our analysis that these measures will suffice until 1998 at a minimum. The discovery of the problem will require re-generating some of the recent data reformatting. A bakeout of the SXT CCD is now tentatively planned for the first week of October, since it has been February since it was last done. PASS CONFLICTS Week 36: 4 passes lost to ASUKA. Week 37: 3 passes lost to ASUKA. Week 38: 3 passes lost to ASUKA. CAMPAIGNS Weeks 36-38 saw the end of support observations for the global coronal campaign (Joint Observing Program (JOP) 44, with SoHO). In Week 38 Yohkoh cooperated with SOHO observations of polar bright-point fluctuations, probably of interest in connection with coronal-hole boundary motions. Week 39 will see an extensive program of coordinated observations with ground-based observations at Tenerife, and there will be additional SOHO-focused observations during October. The New Brunswick rocket may fly in September, SERTS in October. For Yohkoh operations, the SXT weekly observing plan is available on the Web at http://www.space.lockheed.com/SXT/html2/First_Light.html or with "finger campaign@isass0.solar.isas.ac.jp | more" See http://sohowww.nascom.nasa.gov/operations/targets/today" for SoHO target planning. VISITORS Don Melrose (U. Sydney), Jagdev Singh (IIA Bangalore), and Ta. Watanabe (Ibaraki U.) visited during this period. They presented the following seminars: SEMINARS Sep 5 D. Melrose, "Solar Flares---the role of large-scale currents" Sep 19 Ta. Watanabe, "An interdisciplinary study of the eruptive prominence on 28 August 1992" Sep 19 J. Singh, "Detection of intensity oscillations in the solar corona during the total solar eclipse of Oct. 24, 1995" These were all quite interesting. The event Watanabe described is a beautiful example of "coronal dimming", and the interplanetary scintillation observations from that period suggest that a CME did indeed happen - in any case, Watanabe showed the Norikura observations of a rising H-alpha filament, which fitted neatly within the filamentary structures seen in soft X-rays by Yohkoh SXT. The overall structure looks much like a rising flux rope with hot helical filaments enclosing a cool core as the eruption proceeds. To align the Norikura and Yohkoh images, a two-degree empirical roll correction had to be carried out, and we confirmed that in fact this period was one of the (few!) times in which Yohkoh had lost roll lock from its Canopus sensor. Melrose discussed the role of currents in solar activity. There is a hot debate going on in the pages of Ap. J. between him and E.N. Parker, who also visited Japan during this period. Both Melrose and Parker, from their individual points of view, were extremely influential in encouraging the supporters of their somewhat different ideas. SCIENCE The idea of joint SOHO/Yohkoh workshops was proposed by Piet Martens from the SOHO side, and he and Hudson have been exchanging concepts. Martens presented the idea to the SOHO planning organization and e-mail discussion is underway. One possible model would be a series of small, focused workshops of the Hawaii CDAW type, centered on real workstation comparisons of data. Another possibility, raised by Acton, is that a two-meeting format be adopted for each subject. This would have the advantage of enforcing more data preparation. In any case we might want to start with topics for which SOHO calibrations need not be perfect, since so close to launch there is bound to be some continuing calibration work under way. Another possibility might be something like a Yohkoh "mid-point review of flare physics", now that we have almost reached half an 11-year cycle. Such an activity, whether based on a conference or not, might be extremely effective in archiving the new knowledge of solar flares that Yohkoh has produced. PERSONNEL Jean-Pierre Wuelser arrived to visit until early October. Top items - Yohkoh pointing, Mercury, flares, rehabilitation as Chief Observer, joint campaign observations with SOHO. Hudson took a week off to visit Washington, including SOHO. Week 36 Tohbans SSOC: M. Sawa and Dhani H. KSC: H. Hara and N. Saita SXT CO: S. Savy Week 37 Tohbans SSOC: A. Sterling & J. Shin KSC: N. Saita & S. Ueno SXT CO: S. Savy Week 38 Tohbans SSOC: J. Koyama and H. Nakajima KSC: S. Ueno, Y. Hanaoka SXT CO: H. Hudson, J.-P. Wuelser