SXT Status Report
                 20 August 1996 - 1 September 1996
                         (Weeks 34 - 35)

                        S. Savy, H. Hudson


INTRODUCTION

The third rotation of the large old-cycle active region appeared, producing
a large LDE flare before it got to the east limb.  Otherwise solar activity
looked normal - extremely quiet.  Operations were generally smooth.  SXT
suffered from its third hit on its aperture filter, with an accompanying
change in the stray light configuration. 

SOLAR ACTIVITY

The third rotation of AR 7978 (now called AR 7986) brought new surprises,
including a major LDE flare event (22 August) that reached the C4.5 level
even though the active region was 15-20 degrees beyond the limb
(occultation height >20,000 km).  Although the flaring activity died out,
the coronal structure was profoundly changed with this region's appearance
- a much brighter streamer/arcade structure, and a coronal hole channel
advancing downwards from the north polar hole. 

SXT INSTRUMENT STATUS

SXT suffered from its third aperture-filter rupture.  This increased the
stray-light level about 40% and caused some near-real-time data reductions
to look funny.  The software and database system responded nobly to this
change, and within a week terminator images had been acquired and
high-quality daily images were appearing again.  No further problems are
anticipated, except that further segments of the aperture filter could
rupture at any future time - the only theory for these events currently
being discussed is the space debris theory. 

SXT CALIBRATION ACTIVITIES

The filter rupture ended the terminator image stray-light mapping activity
that's always under way for SXT, and started a new sequence.  We want to
examine this week the possibility of a more hands-on spacecraft attitude
control so as to minimize the free range of attitude variation and thereby
to keep the mapping activity simpler.  As of the time of writing
terminators for normal pointing had already been acquired, and offpoint
terminators for 5'E and 5'W are in the plan as a part of support for SOHO
JOP044. 

PASS CONFLICTS

Week 34: 5 passes lost to ASUKA.
Week 35: 4 passes lost to ASUKA.

CAMPAIGNS

Weeks 34-35 saw continued support observations for the global coronal
campaign (Joint Observing Program (JOP) 44, with SoHO).  The New Brunswick
rocket and SERTS may fly in September, and there are other SOHO
mini-campaigns in the works. 

For Yohkoh operations, the SXT weekly observing plan is available on the
Web at http://www.space.lockheed.com/SXT/html2/First_Light.html or with
"finger campaign@isass0.solar.isas.ac.jp | more"

See http://sohowww.nascom.nasa.gov/operations/targets/today" for SoHO
target planning. 

VISITORS

There were no visitors and no seminars these two weeks.  Summer is ending
soon, though. 

SCIENCE

With this status message we'd like to suggest a scientific theme for the
November international meeting in honor of Yohkoh's 5th anniversary.  The
solar-minimum active region born as NOAA #7978 was born on the disk and
quickly grew to more than 400 millionths sunspot area.  It produced an X
flare as well as a major LDE event when still around the limb just arriving
for its third disk passage.  Why not make studies of this region, which is
unique in its size and significance during solar quiet, the focus of
several studies? The November meeting would be an ideal time to exchange
ideas about interpretations of this unprecedented phenomenon.  Some
possibilities are:

  * Variation of coronal temperature with AR growth
  * Variation of flare frequency distributions during its evolution
  * Height structure as revealed by limb occultation
  * Global coronal restructuring induced by reconnection of AR field
  * Coronal-hole boundary evolution during the coronal changes
  * Total solar irradiance variations and buoyant energy (see HSH)
  * CMEs observed both by Yohkoh and by SOHO
  * Coronal loop structure defined by Yohkoh and by SOHO
  * Wavelength calibration of the BCS orbital-phase-dependent pointing
  * Three-dimensional structure of coronal loops via Loughhead's
      method and/or tomography
  * Morphology of expansion/shrinking in the context of emerging
      flux

There are of course probably many other ideas, in particular only four of
the above really involve SOHO data, and yet there must be a lot of good
material available. 

SEMINARS

None.

PERSONNEL

Savy, Hudson at ISAS. Khan returned from Chapman conference in Bozeman.


Week 34 Tohbans

SSOC: H. Hudson,  K. Nakakubo
KSC:  M. Shimojo, K. Kumagai
SXT Chief Observer: S. Savy

Week 35 Tohbans

SSOC: S. Tsuneta, S. Akiyama
KSC:  K. Kumagai, H. Hara
SXT Chief Observer: S. Savy