SXT REPORT FOR WEEK 24; 7 JUNE 1993 TO 13 JUNE 1993 GENERAL STATUS Sxt continued to work well this week. Highlights of the week's operations included tests of improved fast (2 to 4 sec) cadence PFI dominant mode observations and a set of off-limb observations. The normal CCD dark current calibration sequence was also run on Wednesday. The fast cadence observations were very rewarding, and have produced some striking observations. Use of quarter resolution FFI's during the periods of PFI dominant observations solved the problem of maintaining a sufficient frequency of intact full frame images for the SXT movie. The off-limb observations yielded good images above both limbs, and a good set of occultation data for the East limb. Timing problems frustrated the occultation data from the West limb. PERSONNEL M. Bruner, H. Hudson are the U.S. SXT team scientists in residence, and M. Morrison is providing software support. B. Labonte from the Univ. of Hawaii will be in residence until the end of July. Tom Metcalf, also of the Univ. of Hawaii, arrived for a short stay. SOLAR ACTIVITY The X-ray sun was dominated by NOAA Active Region 7518, which produced four GOES M class flares (all but one at night) and numerous C-class events. The GOES background remained at a high B level until the AR 7518 passed the West limb, at which time it dropped to around B1. A number of B and C class flares were observed on the West limb. SCIENCE PROGRESS Bruner completed the co-alignment of half-resolution C IV images from the 1982 rocket flight with the SXT full frame images. This work, which was based on co-registration of coronal bright points, determined the roll orientation of the C IV image with respect to the SXT, fixed the relative scales, and found the lateral offsets. Over much of the disk, there is a close correspondence between the footpoints of X-ray loops seen in the SXT images, and bright patches in the chromospheric network as seen in the C IV image. At least one of the coronal bright points spans an obvious pair of bright footpoints in C IV. A dark filament channel seen in the C IV image lies underneath a long arcade of X-ray loops whose footpoints clearly coincide with bright patches of C IV emission. The correspondence is not universal. There are at least two active regions where the magnetic topology is apparently too complex for the correspondence to be identified. Morrison is working on a program to use the results of the aspect determination to extract the portions of the full-resolution C IV image that correspond to the SXT partial frame images so that a more detailed comparison may be made. OPERATIONS AR 7518, while quite active, was compact enough that it could be readily observed with a 1 x 1 ROI. We took advantage of this to test a second version of the high time-resolution sequence that was run last week. This sequence used the Al1 and Dag filters for temperature measurements that would have higher photometric sensitivity than the normal one using the thick Al filter. Substituting Dag for Al 12 permitted us to limit the maximum exposure time to about 0.9 seconds and to cut the filter changing time. The result was a sequence with a basic frame rate of 2 sec in PFI dominant mode, and a pair of exposures each 4 seconds. This is more than an order of magnitude faster than our normal Quiet High PFI mode, which requires 64 seconds to record an image pair. The new sequence also uses quarter resolution in lieu of half resolution images for the FFI's. This substitution was made to ensure that an adequate number of complete FFI's would be recorded to maintain the continuity of the movie. Results of the second test, which was run for three orbits on Tuesday (8-June), were very rewarding. Multiple events were recorded during each orbit, the most interesting of which was a B6 (approx.) flare that maximized at 07:30. The light curve for the brightest part of the event shows a flat-topped peak lasting about 24 seconds. Fine structure just before the maximum is coherent along the loop and appears to be real. The frame repetition rate was high enough for AEC to keep the images unsaturated for almost all of the frames, and none are seriously saturated. Following the main peak, there was a series of rapid bursts that were spatially coherent along only a part of the loop, indicating the probable presence of unresolved fine structure. The duration of the entire event was only about 5 minutes. The post flare bursts lasted around 10 to 15 seconds each; six occurring in a space of two minutes. The coronal offpoints began June 3, and we thus now have two weeks' worth of data already. The operations have been quite successful, not counting a few glitches, and have not required much special effort. In the June 10 operation we succeeded in getting one of the terminators, and it appears that we have good prospects for getting the rest of the necessary terminator data fairly quickly. The Sun was a little too bright on the W limb on June 10, but no ill effects were noted. Labonte and Hudson have made preliminary evaluations of the scattered light function, and find that it appears to have little dependence on the value of the offpoint angle. If this result holds up under further evaluation, then it will be straightforward to prepare algorithms to correct for the scattered light, even at substantial distances above the limb. Marilyn Bruner SXT Chief Observer