SXT Status Report
                     July 19 through August 2, 1999
                            (Weeks 30-31)
 
                        B. Handy, H. Hudson


SUMMARY

Once again, we report high solar activity but no X-class flares.
There were 9 M-class events from 4+ regions, including the beauty
on 25-July that's the subject of the current science nugget. SXT
experienced no technical difficulties and we had minimal coordinated
observing activity (but see the nugget please).

SOLAR ACTIVITY

Over the last month or so, the background solar X-ray flux has remained
above or near the C1 level, and innumerable flares have occurred.
In the on-line database, we found no less than 34 intervals of
Yohkoh flare mode. At the time of writing, a major sigmoid active
region (8645, 620 millionths spot area on 31-Jul) was at central
meridian and was confidently being predicted to provide major flares.
Presumably the Sun is saving up its ergs and, in exchange for the
ten or so X-class flares we should have had this year, it will give
us a couple of X10's soon.

OBSERVING CAMPAIGNS

This was a quiet period from a campaign standpoint, with no active
campaigns taking place.  Serendipitous flare observations with
TRACE continue to be extremely interesting (see below).  On August 11
we will have eclipses, and then the Whole Sun Month campaign begins.

SXT INSTRUMENT STATUS AND CALIBRATION ACTIVITIES

A routine CCD bakeout was carried out successfully on July 22,
but not without communications problems (first, the power commands
themselves had been omitted from the command lists; second, a
flare occurred during the table switch following the bakeout, and
the KSC tohbans had an ambiguous set of instructions). Because
of this we intend to formalize the command requests a bit further
with a special fax message, which we will try at the next bakeout
opportunity. The schedule would call for one in about mid-October,
depending upon orbit status.

A routine update of the Yohkoh normal pointing (s/c 10" N) took place
July 31.

SXT OBSERVING SEQUENCE TABLES

     --------------------------------------------------------
       JST Day/UT Day   Pass   Table ID
     ========================================================
    Tue   20-JUL-99     2      990719 P2 ARS1 DARK
    Wed   21-JUL-99     3      990721 P3 ARS1 DIFFUSER
    Thu   22-JUL-99     1      990722 P1 ARS0 SPECIAL 
    Fri   23-JUL-99     4      990723 P4 ARS1 STANDARD
    Sat   24-JUL-99     3      990724 P3 ARS1 STANDARD
    Mon   26-JUL-99     2      990726 P2 ARS1 STANDARD
    Tue   27-Jul-99     5      990727 P2 ARS1 DARK
    Wed   28-Jul-99     3      990728 P3 ARS1 DIFFUSE
    Thu   29-Jul-99     3      990729 P3 ARS1 STD
    Fri   30-Jul-99     3      990730 P3 ARS1 STD
    Sat   31-Jul-99     2      990731 P2 ARS1 STANDARD
    Sun    1-Aug-99     4      990801 P4 ARS1 STD
     =======================================================


SCIENCE

Weekly science nuggets were:
  
           30-Jul-99:  A Lyman-alpha "Rosetta Stone"
           23-Jul-99:  An SXT bake-out induces major flares!
           16-Jul-99:  X-ray eclipse preview

The full list of nuggets is kept on
  http://isass1.solar.isas.ac.jp/sxt_co/index.html , 

and the current week's nugget also normally resides on
  http://isass1.solar.isas.ac.jp/sxt_co/SXTweekly.html .

The reference to the "Rosetta Stone" has a couple of implications.
This is the 25-July joint observation with TRACE mentioned above.
It was studied by Handy and Sato at the Nobeyama workshop last week,
and it's the object of intense interest among TRACE persons because
it has a clear FeXXIV signature in the 195 channel, as well as a
clear hot continuum signature in the 171 channel. We need to calibrate
these against HXT, SXT, and Nobeyama free-free views of the same
material. Another slab of the "Rosetta Stone" has to do with clean
Lyman-alpha observations, which TRACE got nicely in this limb flare.
Quite beautiful, not clear yet whether there is anyting really new
there, but this is a major new potential window on flare physics.

SEMINARS

There were no seminars during the reporting period. It's much too hot!

YOHKOH OPERATIONS ISSUES

See above regarding missed communications.

Is the flare-mode threshold set too low for solar maximum?

1. arguments in favor of raising it to (say) C7):

  * We'e seen many C flares, who needs more?
  * Flare mode is harder on the SXT mechanisms
  * Flare mode intervals reduce our coverage of global properties
  * Flare ARS sometimes misses (probably fixable by parameters)
  * TBD

2. arguments against

  * TRACE gets great data on C flares, we should be there to help
  * The SXT FFIs will saturate, requiring a change in the standard
    SFD movie exposures (or a software fix)
  * TBD

VISITORS AND PERSONNEL

Brian Handy departed.

TOHBANS (spacecraft operators)

Tohbans for week 30

  SSOC:   J. Sato,    H.Ishizaki
  KSC:    T.Magara,   S.Kato
  SXT_CO: B. Handy
  SXT_SW: -

Tohbans for week 31

  SSOC:  T.Sakao/T.Watanabe, N.Shinohara
  KSC:   S.Kato, S.Kubo@ssoc
  SXT_CO: H. Hudson
  SXT_SW: -