SXT Status Report
                    April 5 through April 18, 1999
                            (Weeks 15-16)
 
                        N. Nitta and H. Hudson 


SUMMARY

Solar activity remained low. SXT presented no technical difficulties, but
we did a routine bakeout of the CCD. We were busy with joint observations
(SOHO, TRACE).

SOLAR ACTIVITY

Only one M flare... the small surge in activity two weeks ago did not
lead anywhere. The Sun may be saving its strength, or we may have had an
exceptionally fast and brief solar maximum. We've had a month now (both
sides of the Sun, two of these biweekly reports) in which solar activity
has been quite low. The maximum sunspot area during these two weeks
was 290 millionths (AR 8506, April 6).

OBSERVING CAMPAIGNS

We cooperated with VLA observations aimed at characterizing the
differential emission measure distribution of active-region loops, and at
TRACE and EIT observations of loop structure.

SXT INSTRUMENT STATUS AND CALIBRATION ACTIVITIES

Barry LaBonte suggested a "principal components" analysis to help with
the identification of terminator reference images for SXT. We have all
been rather impressed by the ability of the software and database system
to make corrections for the (now very bright) pinhole leak signatures in
the thin Aluminum filter. But the corrections are not satisfactory all the
time - why not? Optimism abounds, and the longer Yohkoh works, the longer
we will have to get this problem straightened out in spite of
changes in the stray-light pattern. The most recent change (a few-percent
increase) appeared on March 12, 1999.

Two updates of Yohkoh normal pointing took place (April 12 and 17).

The long-awaited and twice scrubbed CCD bakeout took place on Friday.
This was successful and should be repeated in about June or July.
The dark current dropped very slightly and we await some analysis
of the image quality.

The Week 16 SSOC tohbans noted that the BDR optimization feature
of OP_FIRST_GUESS could be improved. First, it may allow short intervals
between the computed HI time and an immediately following SAA, which can
result in partial image; second, the optimization algorithm assumes
a complete set of BDR banks, and we've lost one. 

SXT OBSERVING SEQUENCE TABLES

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

         5-APR-99          2      990405 P2 ARS1 DIF
         6-APR-99          2      990406 P2 ARS1 DAR
         6-APR-99          1      990406 P1 ARS2 LOOP
         7-APR-99          2      990406 P1 ARS1 STD
         8-APR-99          1      990407 P1 ARS2 LOOP
         8-APR-99          2      990407 P2 ARS1 STD
         8-APR-99          1      990408 P1 ARS2 LOOP
         9-APR-99          3      990408 P3 ARS1 STD
         9-APR-99          1      990409 P1 ARS2 LOOP
        10-APR-99          2      990409 P2 ARS1 STD
        12-APR-99          1      990411 P1 ARS2 LOOP
        12-APR-99          2      990411 P2 ARS1 STD
        13-APR-99          1      990412 P1 ARS2 VLA
        13-APR-99          3      990412 P3 ARS1 STD
        14-APR-99          1      990413 P1 ARS0 BAKEOUT
        14-APR-99          4      990413 P4 ARS1 DIF
        15-APR-99          1      990414 P1 ARS0 BAKEOUT
        16-APR-99          3      990415 P3 ARS1 DAR
        17-APR-99          2      990416 P2 ARS2 TRA*
        17-APR-99          3      990416 P3 ARS2 TRA*
        17-APR-99          4      990416 P4 ARS2 TRA*

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

This list shows 21 SXT tables in 12 working days, quite remarkable in
that we are getting such reduced KSC coverage now.

SCIENCE

            16-Apr-99: A CME with no LDE
             9-Apr-99: Trans-equatorial Loops
             2-Apr-99: Flare Eruptivity as Observed by SXT 

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.
The science nugget page was advertised in the most recent AAS newsletter.
We need a volunteer to organize a topic index and roadmap for these pages.

SEMINARS

A busy time for seminars, thanks to visitors, tohbans, and locals:

April 8

A.C. Sterling (CPI/NRL/ISAS) "'S' Marks the Spot - the NASA Video"  
K. Shibasaki (NRO) "Radio Detection of Umbral Oscillations"

April 15

B.J. LaBonte (U. Hawaii): "Vector Magnetic Fields in AR8210"
N. Nitta (LMSAL): "The Geometry of Flare Loops"

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  yet for these HXT data.

VISITORS AND PERSONNEL

Nariaki Nitta departed, having set an SXT_CO record in number of
observing tables launched, having found bugs here and there, and
having made real progress on identifying the purpose of BCS. Barry 
LaBonte arrived to serve as tohban and to make his usual profound
contributions to SXT in many directions.

TOHBANS (spacecraft operators)

Tohbans for week 15

  SSOC  : K. Shibasaki and M. Hata
  KSC   : H. Kozu and K. Kobayashi
  SXT_CO: N. Nitta
  SXT_SW: P. Shirts

Tohbans for week 16

  SSOC  : S. Akiyama and B. LaBonte
  KSC   : M. Hagino and K. Kobayashi
  SXT_CO: N. Nitta, H. Hudson
  SXT_SW: -