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, with its sharply tuned filter, EIT will show greater modulation than SXT, which has a broader temperature response.

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.