(Week 6, 1999)
The Soft X-ray Telescope operated well as usual, with one unfortunate lapse: on Feb. 3 an SEU happened just at the end of KSC real-time contacts, and the recovery could not be completed until Feb. 4. Activity was very low, but just during this interval - sure enough - a C5 flare happened. 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.
Flares often result in arcades of bright loops, seen in X-ray and then later on in chromospheric lines as post-flare loops. Below the loops, there is a void. Why is this? In this "science nugget" we show an example of this in the context of the long search for pre-flare heating, a topic of theoretical and observational significance, especially via soft X-ray images such as those of Yohkoh SXT.
These show an arcade seen end-on at the NE limb, in the process of formation. The plots below show the GOES event, with the timing of the images shown above, plus photometry from SXT - clearly the slow rise leading up to the event should be called a "precursor", since the SXT data show that things appear to be brightening in place.
Yohkoh has seen many arcade events such as this one. They share (by definition) the property of an apparently empty tunnel, seen end-on as a void below the loops (see S. Tsuneta's description of the celebrated "candle flame" flare of Feb. 21, 1992 . Chromospheric lines such as H-alpha notoriously produce "coronal rain" at the surface of this tunnel, in discrete loops, as the hot, dense X-ray loops go thermally unstable. But why don't the loops simply crash, and why do they form around a roughly cylindrical void? We speculate that this is because of magnetic pressure internal to the cavity. The cylindrical configuration points to the existence of a separate domain in the form of a flux rope. We know that such large-scale flux ropes also must exist in quiescent filament channels, which indeed can erupt to form large, weak arcades in quiet-Sun areas away from active regions. One answer to the question "why is this interesting?" would be - what is the behavior of the current sheet that must form to separate the arcade loop domain from the tunnel interior domain?
As frequently happens, the ideas presented in a "science nugget" such as this could be completely wrong, since we're trying to describe what we are seeing long before accurate theories can be developed. So, if you see something controversial or misleading, please contact the author quickly!