SXT REPORT FROM LPARL   12/20/91 (0750 PST)
L. Acton

The interrupt rate in California is much worse that at ISAS so I have
been remiss in reporting SXT progress.  Sorry. 

In my last few hours at ISAS Kosugi and Sakao help me transfer HXT
images of the 15 November flare to my workstation so I could compare
them to SXT observations.  The results are exciting!!!! Yohkoh is doing
just exactly what we had hoped and dreamed.  The HXT images are superb
right up beyond 50 keV and seem to achieve the desired resolution of
around 5 arcsec.  Congratulations to the HXT team who have worked very
hard on a very challenging instrument and appear to have succeeded
brilliantly.  The comparison with SXT is just what we have desired for
flare physics for so very long.  This flare was a white light flare well
observed by SXT and Mees Observatory.  Thanks to Mees we know that there
was a magnetic neutral line tucked right against the north side of a
small sunspot lying south of the main group.  This spot and neutral line
was the location of the primary flare.  The time relation of the hard
x-ray and while light flare indicates good agreement between the rise of
the largest hard x-ray burst and the white light brightening.  The soft
x-rays and the low HXT channels do not show a convincing relation to the
white light location (just north of the neutral line).  However, the
hardest HXT iage appears to break down into two kernels, one in the
small spot and one at the white light location.  It wouldn't be fair to
the HXT team for me to go into much more detail about these results here
but let me tell you, we can anticipate a quantum leap in our
understanding of flare toplolgy, energetics and timing from these and
similar Yohkoh/Mees observations. 

Yesterday, before I rushed off to jury duty, I studied a small loop
flare near the west limb that happened on 17 December.  It clearly shows
the primary x-ray emission coming from the top of the loop and then
extending downward, first to the north footpoint and later to the
southern footpoint.  Surprisingly, the loop appears fatter at the south
than at the north and yet the propagation was latest into the "openest"
part of the loop.  I think I will include this little event in the
kSCIENCE paper as an example of the power of SXT for such studies.  This
small event was observed in quiet mode, it was too small to trigger the
SXT flare flag. 

Yesterday Mons Morrison reduced all of the SXT narrow band image data to
determine the central brightness of the image.  With all of the data
spread out at once it is now evident that the decrease is NOT linear but
the expected exponential fall off.  There is a duperposed modulation
that may help us pin down the cause of the fall-off in response.  Thanks
to John Owens who suggested this would be a good time to have a look for
modulations of the decrease pattern.  Perhaps we can make sense of this
yet. 

Got to go to jury duty.  Merry Christmas to all!

Loren