SXT REPORT FOR WEEK 23: 29 MAY 1993 TO 5 JUNE 1993


GENERAL STATUS

SXT continued to work well. There were some flares at the M level. Two
experimental operations were carried out, as described below: high time
resolution, and the first routine coronal offpoint. There were no special
coordinated observations during this period.


PERSONNEL

There were no personnel changes this week. Bruner took part of the week off
to visit the Norikura and Hida observatories; Hudson pinch-hit as chief 
observer.  Morrison continues to keep the software and computer systems under 
control. 

SOLAR ACTIVITY

Solar activity picked up a bit, with the quiet GOES background in mid-B
range. There were several M flares and a whole string of minor active
regions, one of which approached 500 millionths sunspot area.

SCIENCE PROGRESS

LaBonte is making substantial progress on analysis of SXT data from post-flare
loop systems, using all of the bells and whistles of the Yohkoh analysis
software plus some tricks of his own. Ichimoto presented half of the regular
Yohkoh seminar on H-alpha and SXT observations of one such system. It is clear
that these large loop systems have properties unexpected from the classical
observations (and from the classical models). Hudson present the other half
of the Yohkoh seminar, reviewing progress on solar radius measurements.
Morrison has been systematically calculating X-ray limb locations using
Metcalf's software, and the results are quite encouraging for future
coalignment calibrations. Hara pointed out that a systematic change in the
SXT white-light radius values coincided nicely with the major decrease of
solar activity in early 1992.
     Bruner successfully coaligned rocket CIV images with SXT images. The
initial comparisons show striking similarities and intriguing discrepancies in
the image morphology.

SXT OPERATIONS

A special observational mode was run for three orbits on May 31 to obtain
high time resolution in a limited observing region. The scientific objectives 
are twofold: to determine physical conditions in tiny flare-like events, and to
define their parameter distributions better. The "tiny flare-like events" are
also called microflares and active-region transient brightenings (Shimizu), and
in normal observations they give the clear appearance of being unresolved
temporally.
     With SXT set to PFI-dominant, in principle we get one exposure every two
seconds in a 1x1 OR. The May 31 experiment was a qualified success, in that
rapid times structures were observed much more clearly. The active region
observed was not bright enough, however, even during the microflares, for the
AEC to move away from its default long exposure limit in the thick Al filter.
Thus the rapidity of the cadence was slowed by these intervals.  Some scientific
assessment of these data is now under way, and  we will probably try to get more
of this type of data, perhaps in conjunction with Kiplinger's H-alpha
observations at high time resolution in Boulder. 
     A second new item was the beginning of a possible routine off-point
maneuver to obtain deep observations of the middle corona. These data will
be used for studies of expanding structures at larger elongations, for
example, and also as an unprecedented synoptic database of the true (X-ray)
corona at a once-per-week cadence. The initial experiment took place
June 3 with 18 arc min offpoints (the same amplitude as used for the
celebrated images Acton put together from the test observations of
8-May-92). The experiment was a qualified success (we did not succeed
with stray-light monitor images), and analysis is under way.
Experiments will continue on a weekly basis in the near future, with
the aim being to establish a simple operational pattern that will be no
more complicated than the routine observations. The steering committee
for the deep coronal observations consists at present of Bruner, Hara,
Hudson, Kano, and Labonte.


 
                                             Marilyn Bruner and Hugh Hudson
                                             SXT Chief Observers