SXT Status Report
                     June 7 through June 21, 1999
                            (Weeks 24-25)
 
                        P. Martens, H. Hudson 


SUMMARY

Solar activity remained at a high background level, but with only two
M-class flares.  SXT conducted an observing campaigning requiring
off-pointing, and is currently supporting the first Max Millenium observing
campaign.  A new dark-current artifact appeared at the NW limb, so we are
planning (after 5 years!) to vary the Yohkoh "normal pointing" position.

SOLAR ACTIVITY

NOAA listed a great many active regions, as many as 13 on June 17!!
But the resulting flare activity did not live up to the expectations
of the predicting pundits.  No doubt the invisible hemisphere had a high
level of activity, judging from Type II bursts, CMEs, and flickering X-ray
emission around the north limb.  The C/M ratio of GOES flare magnitudes was
54/2 up through June 19, implying a steeper distribution function than the
canonical ratio of less than 10/1.  This would be explained most likely by
the statistics of small numbers.  But it's unusual to have so many C flares
with so few M flares.

The science nugget for Week 25 concerns a giant polar-crown filament
eruption.

OBSERVING CAMPAIGNS

The Week 23 streamer observations, conducted jointly with UVCS especially,
went well.  In Week 25 we began a coordination with the Max Millenium first
campaign, which incorporates a large number of space and ground-based
observatories as a practice run for HESSI.  Before the campaign began, it
seemed that activity detected from the invisible hemisphere would lead to
major flares, but as of the time of writing this has not transpired.

On June 24 Yohkoh will support the SERTS rocket observations.

SXT INSTRUMENT STATUS AND CALIBRATION ACTIVITIES

The terminator image acquisition of the current cycle, beginning April 18,
is now complete and we are planning to redefine the normal pointing 
coordinates.  The major reason for this is the persistent appearance of
dark-current artifacts originating in the heavily overexposed region at
the NW limb.  It's not so clear just why there is no symmetrical region 
at the NE, incidentally.  This re-pointing will be on the order of one
arc min and should be imperceptible to most users of the Yohkoh data.

SXT OBSERVING SEQUENCE TABLES

     --------------------------------------------------------
    JST Day/UT Day  Pass     Table ID
     ========================================================
    Mon  07-Jun-99     2      990606 P2 ARS1 UVCS *
    Tue  08-Jun-99     2      990607 P2 ARS1 UVCS *
    Wed  09-Jun-99     2      990608 P2 ARS1 DIUVCS *
    Wed  09-Jun-99     5      990608 P5 ARS1 DAUVCS *
    Thu  10-Jun-99     3      990609 P3 ARS1 DAUVCS *
    Fri  11-Jun-99     3      990610 P3 ARS1 STD *
    Sat  12-Jun-99     1      990611 P1 ARS1 STD *
    Mon  14-Jun-99     2      990613 P2 ARS1 STMAX01-1*
    Tue  15-Jun-99     2      990614 P2 ARS1 DAMAX01-1*
    Wed  16-Jun-99     3      990615 P3 ARS2 DIMAX01-2*
    Thu  17-Jun-99     2      990616 P2 ARS2 STMAX01-2*
    Fri  18-Jun-99     2      990618 P2 ARS1 STANDARD
     =======================================================

SCIENCE

Weekly science nuggets were:
  
            18-Jun-99: "Yohkoh sees a beautiful arcade in the aftermath
                       of the eruption of a giant polar crown prominence"
            11-Jun-99: "Yohkoh dares to offpoint!"
             4-Jun-99: "Two solar cannons. Two clean misses"
           
The full list of nuggets is kept on
  http://www.solar.isas.ac.jp/sxt_co/index.html , 
and the current week's nugget also normally resides on
  http://www.solar.isas.ac.jp/sxt_co/SXTweekly.html .

The large-scale polar-crown eruption reported in the June 18 nugget reminds us
of the original "global coronal restructuring" noted by Tsuneta et al in 1991.

SEMINARS

June 17:  "The Nature of Moss" (P. Martens, Montana State U.)

The moss, for those not familiar with the term, is the granular structure
(not obviously loops) seen in NIXT, TRACE, and EIT observations in the EUV.
It turns out that these "mossy" regions match one-to-one with the footpoint
regions of high-pressure coronal loops visibile in SXT images.

YOHKOH OPERATIONS ISSUES

One redefinition of the Yohkoh normal pointing was performed. On June 9 we
lost half a day's SXT data due to an SEU.

VISITORS AND PERSONNEL

P. Martens arrived to work as SXT CO and to enjoy rain again.

TOHBANS (spacecraft operators)

Tohbans for week 24 
                     
  SSOC:  J. Khan,     Y. Liu
  KSC:   Y. Nakagawa, D. Sasaki
  SXT_CO: H. Hudson, P. Martens 
  SXT_SW: G. Slater

Tohbans for week 25

  SSOC: T. Sakao, Y. Kato
  KSC:  D. Sasaki, Y. Hanaoka
  SXT_CO: P. Martens
  SXT_SW: G. Slater