SXT Status Report 15 April 1996 - 28 April 1996 (Weeks 16 - 17) ----------- Serge Savy 30-Apr-96 Happy Beltane! Last week saw a resurgence in solar activity, with several flares, including an M4 event. On 17 April we observed a partial eclipse, otherwise SXT operations have been straightforward the last two weeks. The eclipse observations (which used the AlMg filter to continue checking coronal hole brightness as a function of solar cycle phase, and Al.1 to learn more about the stray light) were quite good, even though the Sun was as near a minimum activity level as we have seen. For details see http::/isasxa.solar.isas.ac.jp/~hudson/apr96.eclipse/eclipse_page.html. SOLAR ACTIVITY Several flares occurred in week on 22 April. There were three C-class events between 02:00 and 04:00 UT, and an M4 flare at 04:40 UT that day. SXT observed the M class flare which turned out to be a rather compact event with a flat hard-X-ray spectrum. It had a type II burst (Culgoora), but unlike the events in October 1995, there were no obviously visible soft X-ray ejecta. The M4 and several lesser events were characterize by extremely impulsive time profiles, even in GOES. On 24 April solar activity returned to a very low level. SXT INSTRUMENT STATUS The SXT instrument has continued to work well. There were no significant SXT errors. 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. In particular, to cover the use of the filters in the SoHO campaigns, we have been acquiring additional terminator images. On 19 April a special table was loaded for one orbit to carry out an SXT gain check, utilizing the stray-light from the last entrance filter failure. These data are being analyzed by Barry LaBonte (Hawaii). The objective is to measure DN/photon (the gain) more precisely, but there is an ulterior motive: the fluctuations in the counting rate in a given analysis filter depend upon the mean photon energy. This in principle can be determined from the data, possibly offering a one-filter method for temperature determination. We wish Barry good luck. PASS CONFLICTS Pass conflicts with ASCA: Week 16: 7 KSC pass conflicts. Week 17: 4 KSC pass conflicts. CAMPAIGNS Special Yohkoh/SXT operations were carried out to observe two partial eclipse episodes on 17 April. No off-points were carried out, and the observations were successful and without error. Further information and images can be found on the WWW at http://isasxa.solar.isas.ac.jp/~hudson/apr96.eclipse/eclipse_page.html VISITORS Scientific visitors to ISAS during this period included F. Farnik (Czech Republic), K. Harvey (USA), and J. Harvey (USA). The Harveys left on 26 April after a fortnight stay at ISAS. SCIENCE The flat-spectrum events being studied by Franta Farnik continue to provide surprises. An overlay of the HXT images with KPNO high-resolution magneto-grams (made easier by the presence of the Harveys) showed that the brighter hard X-ray foot-point was very much the stronger field region. SEMINARS The seminar for week 15 was by K. Shibasaki (Nobeyama Radio Observatory) on "Radio Sources above a Flare Loop". Shibasaki_san discussed a flare with clear evidence for transient compact structures above the main flare loop, reminiscent of a Masuda source. He also could clearly see something very much like evaporation. The presentation excited an intense discussion with Shibata-san in particular, owing to perceived differences between the microwave observations and what might be expected from the SXT observations. This is an important research area. All concluded that more flares of these kinds should be studied. The seminar for week 17 was by K. Harvey (SPRC) on "Solar Magnetic Fields: A key to understanding Solar Irradiance". Again there was an extensive discussion, to the extent that the suggestion was made for Karen simply to bundle the questions together in affinity groups and submit them to the Team BB directly. PERSONNEL Hudson, Slater, and Savy still here. Week 16 Tohbans: SSOC: Y. Suematsu, M. Ohyama KSC: S. Masuda, T. Shimizu SXT Chief Observer: S Savy SXT Systems and Data Engineer: G Slater Week 17 Tohbans: SSOC: M. Irie, R. Kano KSC: T. Shimizu, A. Ohkubo SXT Chief Observer: S Savy SXT Systems and Data Engineer: G Slater