SXT Status Report
                    April 19 through May 3, 1999
                            (Weeks 17-18)
 
                       D. McKenzie and H. Hudson 


SUMMARY

Solar activity was extremely low for a week or so, but recovered to
the extent of an M flare. SXT presented some difficulties with stray
corrections, but that has been worked out. Joint observations
with SOHO and TRACE concluded a two-week "stare" at AR 8518.

SOLAR ACTIVITY

Only one M flare, from AR 8524, but an extremely interesting one in a
kind of negative sense. Although this was reported by LASCO as a halo
CME, the flare in fact did not appear ejective in the SXT data, did
not form an arcade (nor a cusp), and was not an LDE.  This must have
been "the exception that (dis)proves the rule"!  The maximum sunspot
area during these two weeks was about 300 millionths (same region that
flared, about the time of the flare).

OBSERVING CAMPAIGNS

We concluded the long "stare" observation, ARS2, on region 8518. It
was a dull region during a dull time, but no doubt the systematic
coverage by TRACE, SOHO, and Yohkoh will provide an excellent data
set for studies of active-region evolution and of microflaring.
Towards the end of the observations we were rewarded by a sudden
appearance of "moss" in the TRACE data, as noted by Jake Wolfson;
this coincided with the formation of a weak arcade to the S of the
active region. So one byproduct of this campaign could wind up being
a first study of moss in the context of physical parameters determined
from simultaneous SXT observations.

SXT INSTRUMENT STATUS AND CALIBRATION ACTIVITIES

Stray-light variations took place during this interval, and Sam Freeland
implemented a new algorithm for adjustment of the stray correction 
images to allow for the new conditions. Since about April 20, things
have been stable, so tentatively this is a new epoch for terminator
acquisition.

Also, we are near the saturation limit of the (NaBan/Open + Dark)
straylight monitor images, and so we will experiment with different filter
combinations to find an alternative straylight monitor configuration. 

An update of Yohkoh normal pointing took place April 28.

We are getting excellent telemetry support from the Santiago station,
which really helps to minimize gaps in Yohkoh coverage.

SXT OBSERVING SEQUENCE TABLES

     --------------------------------------------------------
        JST Day   UT Time  Pass   Table ID
      =======================================================

        18-APR-99          2      990418 P2 ARS2 STARE
        19-APR-99          2      990419 P2 ARS2 STADA
        20-APR-99          2      990420 P2 ARS2 STADI
        20-APR-99          4      990420 P4 ARS2 STARE
        23-APR-99          3      990423 P3 ARS2 STARE

        25-APR-99          2      990425 P2 ARS2 STARE*
        26-APR-99          2      990426 P2 ARS2 STARK*
        27-APR-99          2      990427 P2 ARS2 STIFV2
        27-APR-99          5      990427 P5 ARS2 STARE*
        28-APR-99          2      990428 P2 ARS1 STARE*
        29-APR-99          2      990429 P2 ARS1 STD*
        30-APR-99          2      990429 P2 ARS1 STD*
        01-MAY-99          2      990430 P2 ARS1 STD

       =====================================================


SCIENCE

Weekly science nuggets were:

            30-Apr-99: "Striking Similarities and Intriguing Discrepancies":
                        bright rays above an active region
            23-Apr-99: What's in a coronal notch?
            16-Apr-99: A CME with no LDE

The full list of nuggets is kept on
  http://www.solar.isas.ac.jp/sxt_co/index.html, 
and the current week's nugget is also normally known as 
  http://www.solar.isas.ac.jp/sxt_co/SXTweekly.html.

SEMINARS

April 22 

H. Nakajima (Nobeyama): "Radio and Hard X-Ray Observations of Flares
          with Sub-Second Time Structures"

April 28 (unofficial; Thursday was the beginning of Golden Week)

D. McKenzie: "Downward motions above a flare arcade" 
H. Hudson: "Interplanetary particles and 3D reconnection (journal review)"

YOHKOH OPERATIONS ISSUES

HXT continues in the "cal mode" of observation. In this mode HXT 
gets 64 energy channels at 8-sec binning, as opposed to 4 channels 
at 1/2-sec binning. We regret that there still is no general user
software available for these HXT data.

VISITORS AND PERSONNEL

David McKenzie arrived; visitor Barry Labonte departed; visitor Dick
Canfield arrived; regulars Yashiro and Akiyama enjoyed a workshop
at NASA Goddard.

TOHBANS (spacecraft operators)

Tohbans for week 17

  SSOC : K. Matsuzaki and H. Nakajima
  KSC  : M. Hagino and K.Kobayashi/S. Masuda
  SXT_CO: H. Hudson/D. McKenzie
  SXT_SW: - 

Tohbans for week 18

  SSOC : M. Nishio and R. Canfield
  KSC  : S. Masuda and M. Shimojo
  SXT_CO: D. McKenzie 
  SXT_SW: -