SXT REPORT FOR WEEK 4: 17-23 January 1993 ========================================= General Status -------------- Yohkoh continues to function well, as does SXT. It has been a quiet week because solar activity has been low but we have used the opportunity to create some test tables for calibration and complete the CCD bakeout that was started a couple of weeks ago. We continue to lose some passes to Akebono which complicates operations. The NRL rocket was delayed by at least a week but we will continue to support it by sending daily images to give warning of a likely target region. SXT has taken a total of over 865,000 images (143,000+ FFIs and 722,000+ PFIs) of the Sun. It is snowing and many Camellias are in flower. Personnel --------- There have been no changes of SXT personnel at ISAS this week. Keith Strong leaves for a one-week visit to San Francisco next Sunday, Nariaki Nitta has agreed to take over as Chief Observer for the time that Keith is away. Karen Harvey is scheduled to come to ISAS to work on XBPs with Keith Strong in February. Greg Slater has agreed to stay here to help put together some new time sequences of full disk and partial-frame images on the optical disk. He is also going to work on some of the synoptic representations of the data with Gary Lindford and Keith Strong. The KSC tobans were Shimizu-san and Shoji-san. The SSOC tobans were Shibata-san, Fukushima-san, and Dick Canfield. We had a very interesting seminar by Lidia van Driel-Gesztelyi. To break up the long working hours and help keep ourselves fit Greg has built a table-tennis table in D-toh and we have started a ping-pong ladder, modesty forbids me from saying who is at its head at the moment but it looks as if Professor Uchida will be the eventual winner if anyone dares play him! Solar Activitiy and Interesting Events -------------------------------------- Solar activity has been low this week, with the GOES background declining from about B7 to below B2. There have been 12 C flares recorded on GOES but we will have to wait for all the data to be reformatted to see if Yohkoh saw any of them. The Sun is dominated by a large trans-equatorial coronal hole at the moment. It is at central meridian today and has similar characteristics to the coronal hole channel observed for several months last year. There are clearly visible some very high loops (open field lines?) along its southern bondary where it turns from a N-S orientation and sweeps westward, becoming narrow. NOAA active regions 7403 and 7406 mark its western and eastern boundaries, respectively. There is and isolated "anemone" region in it in the northern hemisphere and sevral XBPs have appeared and disappeared within its boundaries. There are a few small active regions that are not producing much in the way of flares. We followed the decay of active region 7401/2 as it crossed the disk, which showed many large-scale interactions even as it went from being a bright, moderately-large region to little more than an X-ray bright point in the course of a week. Similarly, AR 7403 has slowly decayed but it is a large sprawling region and still has a complex set of loops associated with it. Active region 7406 seems to be the most active and has produced several large-scale loop brightenings, one particularly spectacular one occured at 00:37 UT on 23 Jan 93; one end of the structure brightened rapidly and then a 250,000 km arcade appeared. There seems to be a large number of XBPs seen in the images but as we are taking longer exposures now that may be an instrumental effect. We did observe several XBP flares with typical large-scale interactions. Observing active regions when the Sun is this quiet may tell us a great deal about the way in which the corona responds to a decaying region as the images are free from saturation and taken at a regular cadence because there are no iterupts from flare mode. We have also been fortunate in getting a lot of DSN coverage recently. Science Progress ---------------- Nariaki Nitta and Lidia dan Driel-Gesztelyi are working on the intercomparison of Yohkoh and ground-based data from NOAA Active Region 7260 which was observed in August 1992. The object of their work is finding the association of flare activity with flux emergence in this region. They intend to present this work at the February Science Meeting at ISAS and are preparing a paper on this subject. Dick Canfield has continued to work on the 15 November 1991 flare, looking at the preflare eruption of the filament which show interesting dynamics in H-alpha images. Hugh Hudson and Keith Strong are working on quantifying the impulsive bursts seen by SXT at the footpoints of flaring loops, comparing them with the eqivalent hard X-ray images from HXT, from Sakoa-san. They have concentrated on the 26 Jan 92 flare as it was thought to be the most simple example of this effect. It was discovered this week that it is a spotless white-light flare. They are preparing presentations of the results for the February meeting and plus an ambitious series of papers, possibly starting with one to Nature announcing the "discovery" of this phenomenon. Gary Lindford has been working on the revision of the temperature response of the SXT filters and has made good progress on this remarkably tenaceous problem. Greg Slater is currently working on trying to understand the nature scatter pattern that is frequently seen in images taken during intense flares, looking for evidence of highly collimated sources. he is mainly concerating on the development of synoptic X-ray maps as a function of height in the corona from SXT images of the emission above the solar limb. SXT Operations -------------- Several SXT table loads were made this week. The PFI full-resolution images have been complemented with an occasional dark frame image in order to help us reliably remove the dark spikes. We will add some full-frame dark frame to the weekly calibration sequence for comparison. As the Sun became quiet and the NRL rocket was delayed we decided to bring forward the CCD bakeout procedure. We modified the tables to correct the problems that we had in early January, namely the loss of aquisition of the brightest X-ray features by the PFIs. This problem can be dangerous as the exposure time increases to its maximum value (~5s). If the Sun should produce a large flare then the CCD could suffer some damage. However, we had adjusted the flare and quiet patrol intervals so if the Sun brightened rapidly the SXT would go to the new bright region within 64s. Unfortunately the solar X-ray emission dropped by nearly a factor of 3 and the regions were too faint to be detected by the patrol images so again the SXT was unable to find the bright region. This caused some confusion when the KSC tobans alertly noticed the problem during a real-time contact. After much discussion we eventually let SXT run in this mode for 24 hours as we believed that there was little or no danger. We did load up a table on the last pass on Friday to take a series of quarter-resolution images at various exposures and in various filters to determine what combinations will work next time. It looks as though we should build a data base of optimum patrol exposure times as a function of GOES background level or SXT integrated flux in our standard filters. The CCD bakeout ended saturday and inspection of the full-resolution images show no signs of contaminents on the CCD. The KSC tobans deserve special thanks for their perseverance during this difficult operational exercise. The next scheduled CCD bakeout will be done in April. Keith Strong, 24 January 1993