SXT REPORT FOR WEEK 50
                         13 Dec. 1993 - 20 Dec. 1993             
                                                         20 December 1993
                                                              Hugh Hudson


GENERAL STATUS
     
     The SXT and the Yohkoh spacecraft are working well.  The solar
activity has been low, sinking to about a GOES background level of about
A4 for a short while. Even at this low ebb, incidentally, there were
still several complexes of activity present. At true solar minimum will
there be no sign of magnetic activity detectable by SXT whatsoever?


PERSONNEL

     Currently, the ISAS SXT personnel are approaching the holiday minimum
level - Freeland will return to the U.S. next week, leaving Hudson and
Nitta to watch after things. Hudson is serving as chief observer through
December, after which Nitta will take over. Long-term visitor Frantisek
Farnik departs 20 December after a visit that was quite successful,
including an ambitious study of precursor events seen at the solar limb
in collaboration with Lemen.  


SOLAR ACTIVITY

    The solar activity was pretty low during weeks 48 and 49.  The largest 
flare was a C2.1 on 14-Dec-93.  There was also a pretty LDE with type IV
emission - C2.0 - on 17-Dec-93. This followed the minimum activity level
of about A4 mentioned above.


SCIENCE PROGRESS

     Last week we heard the very good news that the HXT/SXT coalignment
problem had been solved by HXT graduate student S. Masuda. Essentially he
found that the flexure long noted between the two telescopes is mainly
in the HXT aspect sensor, not the HXT telescope itself. This cleared the
way for a reliable empirical matching of hard and soft X-ray images.
Masuda's initial parameters for this have now been checked by Lemen and
Farnik and quite extensively in his AR7260 study by Nitta, and they look
good. This is a crucial step forward in correctly understanding the 
non-thermal/thermal physics of solar flares. The coalignment accuracy 
now is probably as good as one SXT pixel, pending the completion
of Masuda-san's thesis, and because of the large number of flares observed
there is every reason to think that it can be refined further.

     Masuda's "home run" science, based upon this breakthrough, is the 
discovery of hard X-ray cusps, e.g. in a limb loop flare of 13 Jan. 1992,
as reported last week. This has now been summarized in a beautiful 
illustration that is available wherever the Yohkoh software is installed
under SHOW_PIX. For those not familiar with it, this is a simple program
devised by Sam Freeland to give ready access to memorable graphics, with
captions - different menus in SHOW_PIX already have different views of
the Mercury transit and the solar eclipse observed last month. Readers
of these status reports are urged to log onto a computer with Yohkoh
software and take a look - very often the workstation views of these
images are much better than the hardcopies that get circulated. For
non-Yohkoh persons, the easiest way to use SHOW_PIX from a workstation
is to log onto an LPARL computer, using the guest account, either at 
ISAS or Palo Alto.

     Finally, the weekly Yohkoh seminar at ISAS was conducted by myself,
as a last-minute substitute for the scheduled speaker. The discussion
derived from the stimulating meeting in Hawaii the week before. There were
several important items arising out of this workshop, but the one that
demanded the most attention in the review discussion was the remarkable
distribution pattern of  microflares observed by SXT (Shimizu) in the
context of the new MHD-based calculations of the coronal field (McClymont
and Mikic). These unexpectedly showed a clear pattern of avoidance of
"bald spot" neutral lines, defined as the neutral lines where the
inversion-line field lines are concave up (concave down means simple
crossing loops). The charm of the microflares is that there are enough of
them so that real statistical work can be done, as opposed to big flares
which may be beautiful but lonely.

SXT OPERATIONS 

     SXT operations were normal, with no special campaign activities other
than the usual weekly coronal offpoint. We are well into the program of
"terminator" images, and did an experiment with an OP-driven offpoint to
acquire terminator images at the coronal, eclipse, and Mercury offpoint
positions. This went well and we will proceed next week to get real data.

     ISAS this week conducted a rare external committee review of its
situation, and the review committee visited the data analysis facility in
building D. The astronomically-minded membership of the committee included
L. Fisk, M. Longair, R. Luest, J. Maddux, and M. Oda, and they  seemed
quite pleased by the scientific situation.