(Week 52, 1998)
The Soft X-ray Telescope operated well as usual. See the weekly operations report for details. []
In the display above (click to enlarge), the colored lines show the times of SXT images that currently are on-line at ISAS. The purple lines are flare mode. The gaps will be considerably reduced when the NASA telemetry arrives and gets reformatted. For a summary index listing of the weekly science reports, click here.
Yohkoh SXT has not succeeded very well in detecting
Moreton waves (large-scale coronal shocks) in spite of the simple
logic that compressional motion should result in adiabatic heating
and detectability.
Yet we know from meter-wave type II bursts that such waves occur, and
in the meanwhile the EUV observations of SOHO/EIT show a plethora of
waves.
Jim Lemen has analyzed the reasons for this discrepancy, which we
attribute (a) to poor SXT counting statistics in the quiet corona
(expressed differently, the image dynamic range in soft X-rays is
larger than in the EUV); and (b) to differences in temperature
response between EIT and SXT,
However, maybe we've found one in the SXT data!
This difference image (450 Mm wide) shows an arc propagating rapidly
to the north.
The arc appears to start at some distance (> 100 Mm) from the flare
core, and to accelerate outwards, reaching a projected velocity
exceeding 1000 km/s.
The fast ejection seen to the N in this event may be associated with
a Moreton or Type-II-burst wave, or in some other way to a CME ejection.
The plots below show (a) height vs time, and (b) the timing of the
samples relative to the GOES low-channel X-ray flux.
The measurements here refer to the SXT "quarter resolution" field of
view of about 10 arc min.
The plot on the left shows the projected distance from the flare core
as a function of time, while the time profile on the right shows the
times of the images used for these measurements.
We can't say yet whether this is a true wave or something magnetic (loop
or flux-rope) anchored to the photosphere.
But we're working on it!
Note added Jan. 20, 1999 - further information is available from a
presentation at the Yohkoh seminar at ISAS Jan. 21.
Comments to: hudson@isass0.solar.isas.ac.jp, akiyama@flare2.solar.isas.ac.jp