SXT Status Report March 22 through April 4, 1999 (Weeks 13-14) N. Nitta and H. Hudson SUMMARY Solar activity plummeted to an astonishing low during this period, pushing even below the GOES B level before rising at the very end. SXT presented no technical difficulties. Reverberations still continue from the NASA press release on "S marks the spot" (sigmoids leading to CMEs). SOLAR ACTIVITY The low level of solar activity was unprecedented in the current cycle. There were three M-class events at the end, but in the meanwhile, for a a period of ten days, only one C-class flare happened! Of course this is good news, it means a bit of a postponement of Yohkoh's final re-entry, and more of an overlap with HESSI (to be launched July 4, 2000). OBSERVING CAMPAIGNS There were no official campaigns during this period. SXT INSTRUMENT STATUS AND CALIBRATION ACTIVITIES We have begun anticipating a co-observing campaign, scheduled for June, that will require offpointing Yohkoh in order to get the best possible overlap with UVCS. Scientifically the objective is to learn more about emission-measure distributions and non-Maxwellian plasma behavior in the low-density regions at the limit of SXT's effective viewing field. So, offpoint experiments have been probing the scattered-light environment, homing in on a location 10' W, 2' S from normal pointing. The analysis of the 20-Jan-99 flare (for a preliminary report, see http://www.solar.isas.ac.jp/sxt_co/990122.html ) aroused interesting discussion about the SXT off-axis response. Work is under way to see if appropriate corrections are provided by the program ``SXT_VIGNETTE'', as called in ``SXT_PREP.'' We may need substantial refinement to the program. The decreasing signal level of HXA resulted in substantial uncertainty in the determination of the Yohkoh pointing as derived from HXA limb address data. Jean-Pierre Wuelser has developed new software to make use of HXA scan data, and the result has been very encouraging. There is another problem applicable only to temporary database (including the pointing information) that is generated after each KSC contact, because of occasional reversals of time tags. This has been a major obstacle for those who try to obtain quantitative results before the official reformatting, which normally happens 3-6 weeks later. Greg Slater is fixing this problem. After the latter problem is completely resolved, the whole pointing database from mid-1997 will be regenerated using HXA scan data. In the meantime, we hope that a warning will be sent to the entire Yohkoh team, clearly explaining what the user should be aware of when registering SXT images. SXT OBSERVING SEQUENCE TABLES -------------------------------------------------------- JST Day UT Time Pass Table ID ======================================================= 22-MAR-99 4 990322 P4 ARS1 DAR 23-MAR-99 4 990323 P4 ARS2 DIF 24-MAR-99 3 990324 P3 ARS1 STD 25-MAR-99 None 26-MAR-99 1 990326 P1 ARS1 W-OFF 2 990326 P2 ARS1 STD 27-Mar-99 3 990327 P3 ARS1 W-OFF 4 990327 P4 ARS1 STD 29-MAR-99 2 990329 P2 ARS1 DIF 30-MAR-99 4 990330 P4 ARS1 DARCAL 31-MAR-99 1 990331 P1 ARS1 E-OFF 2 990331 P2 ARS1 STD 1-APR-99 1 990401 P1 ARS1 W-OFF 3 990401 P3 ARS1 STD 2-APR-99 None 3-APR-99 4 990403 P4 ARS1 STD ===================================================== SCIENCE Science nuggets were: 2-Apr-99: Flare Eruptivity as Observed by SXT 26-Mar-99: Cusp and CME 19-Mar-99: A magnificent cusp The full list of nuggets is kept on http://www.solar.isas.ac.jp/sxt_co/index.html, and the current week's nugget is also normally known as http://www.solar.isas.ac.jp/sxt_co/SXTweekly.html. SEMINARS April 1 S. Akiyama (NAO/GUAS) "The Occurrence Probability of X-ray Plasma Ejections by Solar Flares" No, not a joke - SXT sees ejecta so frequently that we can do statistics! See Shibata et al., 1995, and this week's science nugget. YOHKOH OPERATIONS ISSUES HXT continues in the "cal mode" of observation. In this mode HXT gets 64 energy channels at 8-sec binning, as opposed to 4 channels at 1/2-sec binning. Because we have now accumulated many M-class flares in this mode, we may consider returning to the usual 4-channel mode. We regret that there still is no general user software available yet for these HXT data. VISITORS AND PERSONNEL Alisdair Davey departed, leaving many system changes in his wake. Nariaki Nitta arrived and has been serving as SXT chief observer. TOHBANS (spacecraft operators) Tohbans for week 13 SSOC : M. Takahashi KSC : S. Nagata and K. Shinoda SXT_CO: H. Hudson, N. Nitta SXT_SW: A. Davey, P. Shirts Tohbans for week 14 SSOC: T. Kosugi, R. Haga KSC: K. Shinoda, H. Kozu SXT_CO: N. Nitta SXT_SW: P. Shirts