August 16, 1999 H. Hudson CHIEF OBSERVER'S REPORT for Week 33 9-Aug-99 to 15-Aug-99 GENERAL The level of solar activity plummeted, as though the Sun were exhausted following the previous week's X-class flare. The major event was the eclipse, which Yohkoh observed with almost full success. This week we enter into Whole Sun Month, a major international campaign with a "target of opportunity" plan for observing sigmoid active regions. There is a set of notes attached below concerning the suggestion to increase the level of the Yohkoh flare threshold. SXT TECHNICAL No news. SXT SCIENCE The eclipse "quasi-real-time" Web pages (http://isass1.solar.isas.ac.jp/sxt_co/eclipse/qrt.html in English; see link on www.solar.isas.ac.jp for Japanese) worked well, especially for the first episode needed for the 10:00 pm news program. This week's science nugget basically follows up on this affair. For Web material see Current Week: http://isass1.solar.isas.ac.jp/sxt_co/SXTweekly.html Current Week: http://isass1.solar.isas.ac.jp/sxt_co/990813.html Index: http://isass1.solar.isas.ac.jp/sxt_co/index.html CAMPAIGNS For SXT plans, see: http://isass1.solar.isas.ac.jp/sxt_co/sxt_future.html For SXT completed campaigns, see: http://isass1.solar.isas.ac.jp/sxt_co/sxt_catalog.html PERSONNEL No changes. SEMINARS August 12: T. Sakurai (NAO), "Fifty years of green-line observations at Norikura" SXT TABLE PLAN, WEEK 34 (16-Aug to 22-Aug-99) This week begins "Whole Sun Month," which we intend to support as fully as possible. The tables listed below with "SIG" in their name are intended as place-holders for direct observations of any selected sigmoid region that should appear. ________________________________#_______________________________ JST Day UT Start Time Pass Table ID Monday 16-AUG-99 12:42 3 990816 P3 ARS1 STD Tuesday 17-AUG-99 09:32 1 990817 P1 ARS1 DARK Tuesday 17-AUG-99 12:56 3 990817 P3 ARS1 DIFF Wednesday 18-AUG-99 09:46 1 990818 P1 ARS2 SIG Thursday 19-AUG-99 08:19 1 990819 P1 ARS2 SIG Friday 20-AUG-99 13:39 4 990820 P4 ARS2 SIG Saturday 21-AUG-99 12:11 2 990821 P2 ARS1 STD This plan is subject to change. Continuing requests: KSC & SSOC Tohbans: If any KSC passes are likely to be cancelled (for example due to a typhoon) or an SXT table cannot be uploaded for some reason, please telephone and e-mail the Chief Observer immediately. (sxt_co@isass0.solar.isas.ac.jp, copy to sxt_st@flare2.solar.isas.ac.jp). SSOC Tohban: On the weekly visible diagram, please mark the official five passes first and then cross out those passes that will be given to other satellites. That way we can keep the numbering straight in the Yohkoh software system. SSOC Tohban: SXT operations do not currently include terminator off- pointing. It is unnecessary this week to ask the SXT Chief Observer for terminator off-pointing information. KSC Tohbans: In general flare mode supersedes table upload, i.e. if a flare is in progress please do not upload a new table, and contact the chief observer. Normally it is OK to go ahead with the planned table without change on the next pass, if it is available. Chief observers should keep an eye on this. Notes on the Yohkoh flare threshold Essentially since the beginning of the mission, the flare threshold has been set near C2. This is not an exact level since it really comes from an SXS counting rate that differs physically from the GOES rate. The threshold has been quite stable. There is some feeling that Yohkoh has now accumulated enough data on small flares, and that we may make more scientific progress by not triggering at such a low level. These notes examine the pro's and con's that have surfaced over the last few weeks as this issue has been discussed. Because of the nature of the flare distribution function, doubling the threshold count would approximately halve the trigger rate. Reasons to change * Flare mode eliminates FFI data, often for long periods of time, during which we cannot see global evolution. * Flares often have global effects, which SXT cannot see well during flare mode. * Excessive flare-mode operation unnecessarily uses the SXT mechanisms * HXT does not normally get full spectral coverage on the weaker events Reasons not to change * HXT will get less 4-channel data. It should be possible for the flare triggers on weak events to be used for a composite HXT background catalog, parametrized in (B, L), which would improve HXT imaging in general. * There is a long record at an almost constant threshold. Statistical studies may suffer if we change. * The "movie" FFIs during faint flares will saturate a bit. It would not be desirable to change the exposure times. Commentary (HSH opinions). 1. The gains may be fairly substantial, since we'll double the number of "big" C2-C4 events that may be associated with global effects. 2. The idea of using weak flares for HXT background studies sounds good, but there is no identifiable manpower to carry out the study. There is also a huge database already. 3. The statistics of the flare trigger are pretty meaningless, since the threshold is quite arbitrary from a physical point of view; anybody interested in this question can look directly at the SXS counting rates. 4. Another way to study global effects is to reduce the number of "movie" exposures and replace them with high-cadence Q-resolution data. With this 32-sec cadence, we might see lots of counterparts of EIT waves, which would be quite important.