SXT Status Report 7 Oct. 1997 - 20 Oct. 1997 (Weeks 41-42) D. McKenzie, H. Hudson, J. Lemen, S. Savy SUMMARY Solar activity diminished, but not to quiet-Sun levels; SXT and Yohkoh continued to operate well and there were several coordinated observing campaigns. An analysis of the "false flare" problem has been offered. SOLAR ACTIVITY During two weeks of reduced solar activity, we only saw one C flare but there was the usual stir of excitement with CME events, including a major filament eruption at the SW limb noted by the KSC tohbans and reported in GBO mail. The largest sunspot group during the interval had an area of only 90 millionths. SXT INSTRUMENT STATUS AND CALIBRATION ACTIVITIES SXT continues to operate well. There was one interesting Yohkoh phenomenon, namely a "false flare" trigger, again at the usual spot in the Yohkoh orbit - the Pacific at the N extreme of the Yohkoh orbit. The difference with this one was that some research got done on the causes of such events, as described below. Data archiving is complete through Week 37, 1997. ORIGINS OF FALSE FLARE TRIGGERS Flare mode triggered on 10-oct-97, at 19:38 UT; the flare mode lasted for ten minutes. There was no solar flare at the time. However, the count rate from SXS PC1 showed an increase in counts to the trigger level at that precise time. Yohkoh thus responded correctly to enhanced count rates, but what the source of the enhancement? Yohkoh was far from SAA, but the period of time 19-20 UT was notable for enhanced auroral activity and elevated intensities of high-energy electrons at the lower boundary of the inner radiation belts. The intensity of >30 keV electrons is routinely measured by NOAA-12, which is in a polar orbit at an altitude below 1,000 km. Plots of NOAA-12 data made on our behalf by Dave Evans and SEL showed fluxes "at least 10 times the last year's average." These electrons were seen mainly as NOAA-12 was southbound from the northeast to the soutwest, east of Japan. Along this track, at about 19:06 UT, NOAA-12 was observing intensities of >30 keV electrons that were "about 30-40 times higher than the long term average intensities." The >100 keV channel similarly had intensities more than a factor of 3 above nominal. Although NOAA-12 flies higher than Yohkoh (820 km, compared to Yohkoh's 600 km), we suspect that bremsstrahlung from these electrons on the spacecraft could have been intense enough to trigger the flare mode. According to Dave Evans of the NOAA Space Environment Lab, these kinds of transient low-latitude electron flux enhancements are not too common, though Evans feels they are (i) most probable in the Pacific, and (ii) most probable in the dawn (local time) sector. Our false flare event has both these characteristics. This finding has a good chance of explaining many of the Yohkoh false flare effects, although the explanation apparently doesn't go all the way - in other words, the mechanism for putting these electrons at this location, which is not a stable trapping region, is apparently not known. PASS CONFLICTS AT KSC Week 41: One pass lost to Akebono. Week 42: No passes lost! CAMPAIGNS During Oct. 6 - Oct. 12 we participated in a coordinated observing plan centered on Advanced Stokes Polarimeter observations at Sac Peak (Pevtsov and Metcalf) trying to delineate separator structure. The EIT calibration rocket flew on October 16, we believe, but close to the time of launch we decided not to do anything non-standard (after correspondence with Joe Gurman). We were unable in fact to locate the CDS target region during the rocket flight using Web tools, not a network problem but apparently a lack of timely updates on SOHO's part. For Yohkoh target planning, 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 AND PERSONNEL Jim Lemen departed. Kano, Khan, and Sterling departed for various destinations but all of them including the second SOHO/Yohkoh CDAW on their itineraries. SEMINARS October 9: McKenzie discussed "Periodic Oscillations in SXT Light Curves." The probable detection of periodic modulations in X-ray emission has at least a couple of potential implications: resonant wave heating of active-region loops and dynamical effects caused by MHD perturbations. October 16: Hudson discussed "Coronal Changes associated with Halo CMEs," based on a paper submitted to GRL by (Hudson et al et Sterling et al). (a) Even tiny almost undetectable low-coronal counterparts may be associated with major halo events; (b) even these tiny features may be eruptive flares; and (c) a curiously high fraction of reported halo events (should be 50%) seems to arise on the visible hemisphere. OTHER SCIENCE TOPICS As reported in the previous report, Sam Freeland had noted a rather remarkable trans-equatorial loop system that appeared suddenly on the East limb, and captured it on a Web movie: http://www.space.lockheed.com/SXT/movies/sfd_72hours.html. We speculated that as they crossed central meridian we would see a complicated pattern of reconnected loops crossing the solar equator. This proved not to be the case, and in fact the north and south active regions were not on similar longitudes. As an alternative speculation, the first having been shot down, we now suggest that the appearance of loops was illusory and that these structures were in fact of the "quadrupolar" streamer structure often seen by LASCO C1. Anybody interested can check this out in the LASCO synoptic data! SXT does not see such structures often, so this must have been an unusual circumstance and perhaps worth following up. TOHBANS Tohbans for Week 41: SSOC : H. Nakajima & T. Magara KSC : Y. Saitoh & Y. Ohyama SXT_CO: J. Lemen & D. McKenzie SXT_SW: G. Slater Tohbans for Week 42: SSOC : Y.Nishino & M. Yamamoto KSC : Y. Ohyama & A. Siozawa SXT_CO: D. McKenzie SXT_SW: G. Slater