SXT REPORT FOR WEEKS 44-45 1 Nov 1993 - 14 Nov 1993 14 November 1993 Hugh Hudson GENERAL STATUS The SXT and the Yohkoh spacecraft are working well. PERSONNEL Jim Lemen left for one week, leaving Nariaki Nitta, Sam Freeland, and Hugh Hudson as the SXT personnel at ISAS, with Hudson serving as Chief Observer. Other visitors at ISAS included John Mariska and Franta Farnik. Bob Bentley returned to the U.K. and K.D. Leka returned to Hawaii. SOLAR ACTIVITY The solar activity was extremely low in week 44 but picked up considerably at the end of week 45 with several flares crossing the M threshold. At the time of the Mercury transit (see below) the Sun was as quiet as it has been during the Yohkoh mission. Nevertheless the images still show much that is interesting. SXT graduate student Hara_san, in his analysis of the image histograms, has found occasions when the "quiet Sun" flux is approximately 70% of the total, but the remaining 30% still shows typically fascinating structure and dynamics. SCIENCE PROGRESS Farnik is working with Lemen on limb events and precursor flares (including flares associated with the 23-June-1993 CME found in one of our standard offpoints); Nitta and Leka worked mainly on preparations for the Hawaii CDAW meeting at the end of the month; Freeland and Hudson worked on special operations (see below). The Yohkoh seminars were as follows: Nov 4 F. Farnik Stereoscopic Observation of the Sun by GOES and Phobos Spacecraft Coronas and Interball Projects J. Mariska The BCS Blueshift Survey Nov 11 R. Stern Was der Sonnenphysiker vom Stern lernen konnen SXT OPERATIONS - SPECIAL These two weeks featured complex operations centered on two unusual celestial phenomena - a transit of Mercury (Saturday, 6 November) and a partial eclipse (Saturday, 13 November). In both cases special operations were mounted; at the time of writing we have not seen the eclipse data, which are in DSN playback, but the Mercury results (as most people know already) were successful. Transits of Mercury only occur about every eight years, so this presumably will have been Yohkoh's only shot at one. We were extremely fortunate in that all of the limb contacts occurred during Yohkoh day intervals. However we were even more fortunate that we were able to observe the Mercury shadow long before first contact and long after fourth contact. In addition we succeeded in getting data as Mercury crossed a narrow but deep coronal hole channel. To carry out these observations properly required two new procedures, which the Yohkoh team developed and tested in the week preceding the phenomenon. These were a major south offpoint, to put the south limb of the Sun in the center of the CCD and thus to avoid the "pinhole" stray light problem in the thin Al filter, and an offpoint commanded not in real time, but in the OP stored command system. Needless to say, these novel operations features plus the need for special observing sequences in SXT made it a very complicated affair. The partial eclipse observation was the second such opportunity, the first having been 4 January 1992. In that case data were taken but were lost irrevocably due to an operations error. The present eclipse was much less favorable - one orbit with a grazing occultation of the corona, and one orbit with a rather deep but brief partial eclipse. Again OP-commanded offpoints (two in this case) and special SXT observing tables were set up. The advance warning for this operation was minimal because we erroneously concluded months ago that the circumstances were not favorable. These operations were extremely difficult. The SXT team and coronal researchers, the main scientific beneficiaries of these rare data, owe a great debt especially to Yohkoh deputy project managers Drs. Kosugi and Watanabe for their energetic and enthusiastic participation. Kosugi_san notes that in 1994 we will have two much more favorable eclipse opportunities, and that this time we should get ready for them slightly earlier than the last minute! We should also note finally that DSN coverage was essential for both of these special operations, and the remarkable reliability of the DSN services was a real blessing. Special thanks here are due to Joe Espinueva. SXT OPERATIONS - ROUTINE A bakeout of the CCD was performed, unfortunately after the Mercury transit rather than before because of operations difficulties. The normal EW weekly offpoints were executed approximately on schedule. Jim Lemen's new automated system for taking special terminator images has worked admirably, and we are now cycling rapidly through the different filter combinations to build up a data base of stray-light correction images.