Weekly Notes from the Yohkoh Soft X-Ray Telescope

(Week 51, 1998)

Instrument Status

The Soft X-ray Telescope operated well as usual. See the weekly operations report for details. Solar activity was reduced, but the northern hemisphere - the quiet part - continued to puzzle and amuse (see below).

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.

Science nugget: Homologous Compact Flares

Two nicely almost-homologous compact flares occurred on Dec. 14, and this science nugget describes them. These did not show ejecta. First take a look at the time line, via GOES:

Here the stippling shows the timing of flare mode (the Al12 full-resolution images), and the diagonal lines show as usual the orbit night and SAA regions. Yohkoh and SXT worked perfectly, as we've come to expect, with ARS, AEC, and even with timely flare-mode triggers!

And here are images:

showing the first and last flare-mode images for each event. The homologous tendency is clear, but in the impulsive phases (actually three of them, because the rightmost image shows another compact brightening) one can see that the illuminated structure may or may not be the same physical item - certainly the locations are similar, however.

The associated gradual bursts seem much more similar, however. The overlaid contour diagrams below (06:35:45 UT in yellow; 10:02:27 UT in green) match almost exactly. The overlay corrects for solar rotation, about 8 pixels. The displacement (perhaps one SXT pixel, 2.46") needs to be measured more carefully - coalignment here has been done with the quick-look databases and may improve, and in any case the conclusions depend completely on the coalignment accuracy for a study of this type. But even so, it seems likely that the same field structure brightened up in each of the flares.

What conclusions might we eventually want to draw?

  • Magnetic reconnection probably played no role in restructuring the large-scale field system of the gradual phase, since there was apparently quite minimal structural change. Thus we probably don't need the scenario of loop-loop interactions for energy supply. Of course, a triggering relationship could be possible, but that is always hard to be quantitative about.

  • A possible source of the loop-filling material could be X-ray irradiation of the footpoint near the compact bright regions. So far as we're aware, this mechanism has not been explored recently in the solar-flare context, although there is considerable work in the X-ray astronomy literature on the subject.

    Comments to: hudson@isass0.solar.isas.ac.jp, weber@isass0.solar.isas.ac.jp