SXT Status Report Feb. 1, 1998 - Feb. 16, 1998 (Weeks 6-7) N. Nitta, D. McKenzie, H. Hudson SUMMARY Yohkoh and SXT continue to operate well. SXT had suffered from another loss of piece of its entrance filter, but the software and databases have almost recovered. Solar activity was low, but as usual not without interesting phenomena. SOLAR ACTIVITY There were no C-class flares during this interval, except that the end of the two-week period saw an upturn that raised the background level into the high B range. A varying level of B-flare activity was present, increasing in Week 7. The McIntosh "solar status" reports continued to be helpful. SXT INSTRUMENT STATUS AND CALIBRATION ACTIVITIES A hiccup happened on Jan. 24, when SXT lost another piece of its entrance filter. Starting in late 1992 we have seen now four or five instances of this. By now the software and database system are quick off the mark. We did suffer from low-grade image reduction for about two weeks, but only in the near-real-time images that we distribute for forecasting and monitoring purposes. The same data have now been re-reduced to the same high quality that we had before. It is now becoming clear that Acton had succeeded in a breakthrough in our understanding of a puzzling variability in the stray-light corrections for the Al.1 filter (the thinnest analysis filter). Our routine reductions had been plagued by under- or over-correction of the highly structured stray-light pattern. The breakthrough was to recognize that the apparent diameter of the Sun varies seasonally, so that the stray-light correction image needs to be selected by epoch as well as by pointing coordinates. This fix is in place and good reports are arriving, for example from the Whole Sun Month campaign database. Jean-Pierre Wuelser installed an improvement in the attitude calibration, necessary because of the gradually lengthening time series of data and the existence of (slow and small) secular trends in the relationship between the spacecraft and telescope attitude references. Finally, yet another improvement in the data correction system was implemented by Tom Metcalf and Sam Freeland, namely the incorporation of the orbital phase dependence of the dark signal level. The HXA sensor (two linear CCDs with lenses mounted in the HXT instrument) has been very gradually losing sensitivity. This is an important element of the SXT coalignment, because the data are used to calibrate the Yohkoh gyros. The loss of sensitivity has been determined, via LED stimulation on board, to be an electronics problem, not an optical problem. In Week 7 Sawa_san adjusted the gains of HXA upwards to provide more signal. This should have no effect on anything at present, but in a few years we may have trouble with signal levels. Data archiving is complete through Week 51 (ending Dec. 20, 1997). WEB-BASED SCIENCE REPORTS The scheme of weekly reporting by the SXT Chief Observer on Web pages has continued. The current "science nugget" is normally at ftp://isass0.solar.isas.ac.jp/pub/sxt_co/SXTweekly.html which links to this report and other pages, the previous science nuggets reported by SXT chief observers. See index.html for access to the following list: 971024.html dimming 971107.html Nov 6 flare 971114.html Nov 14 LDE 971121.html Movies of ejecta! 971128.html X2.9 ejective flare 971205.html Solar cycle 971215.html Amazing arcade 971221.html AR development 971228.html Interconnection 980110.html Global time constant? 970117.html Contrasting emergence 970124.html Data request from Ooty 970131.html XDT rocket flies! 980207.html Diffuse eruptive events 980214.html Expanding coronal loops The latter (also currently called SXTweekly.html) also contains links to pages describing the forthcoming eclipse (Feb. 26). PASS CONFLICTS AT KSC (Akebono, HALCA, ASCA) Week 6: No passes lost Week 7: One pass lost to HALCA. CAMPAIGNS The big excitement in joint observations this week came from an ad hoc coordination with Soho CDS. Maltby's group (Oslo) has been looking for spectroscopic evidence of expanding active-region loops, something quite important for Yohkoh because of the initial discovery (reported in 1992) by Uchida et al. For two days, accordingly, SXT set to NOAA AR 8151, a weak decayed sigmoid active region, as chosen by CDS. Then a miracle happened - sure enough, a clear episode of expanding loops appeared in one elbow of the sigmoid. This is described, with a movie, in the current SXTweekly Web pages. We do not yet know about the quality of the Soho data but are hoping that the simultaneous Doppler measurements and plane-of-the-sky feature motions recorded by the SXT PFI's will clarify the nature of the expansions. For details of campaign planning, see http://www.space.lockheed.com/SXT/html2/First_Light.html or do "finger campaign@isass0.solar.isas.ac.jp | more" from your Unix command line. See http://sohowww.nascom.nasa.gov/operations/targets/today for SoHO target planning. VISITORS AND PERSONNEL Nariaki Nitta spent a week at ISAS, and Hugh Hudson was absent for about six days. David McKenzie has arrived and will be the SXT chief observer for most of February. SEMINARS No seminars during these two weeks. TOHBANS Tohbans for Week 6 SSOC : R. Kano, Te. Watanabe KSC : T. Ishii, M. Shimojo SXT_CO: N. Nitta SXT_SW: - Tohbans for Week 7 SSOC: M. Takahashi KSC : M. Shimojo, S. Akiyama, S. Yashiro SXT_CO: H. Hudson, D. McKenzie SXT_SW: -