(Week 11, 1999)
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.
The first look at this event focused mainly on the comparison between preflare and postflare active-region structure, and on the mass motions observed in coincidence with the launching of a halo CME. Please see those pages for links to some of this material.
Would this flare be detectable as a large-scale coronal wave? The SXT team has become excited about this kind of thing, old hat to EIT and to radio astronomers of course, because of the possible detection of a wave in another May 1998 flare from the same active region, NOAA 8210, when it was near its W limb passage. Please see the NOAA event listings for detailed summaries of both events.
The plot below gives the time line of Yohkoh observations, superposed on the ever-reliable GOES soft X-ray photometry.
The hatched regions show (left and right) a passage of Yohkoh through the South Atlantic Anomaly, and orbit night. The flare onset was interrupted by the Earth's shadow, which produced the sharp drop in the HXT low (13-23 keV) and M1 (23-33 kev) channels shown, the latter with a factor of ten displacement for visibility. No doubt the Neupert effect is working well here! Because of the design of its spectral passbands, one can read off the hard X-ray spectral index from the ratio of M1 to L: equal counts corresponds roughly to a photon number spectral index of 2.0. This very impulsive event has, as expected, a very hard spectrum. Tick marks at the bottom of the plot show soft X-ray image times.
The flare development was eruptive, but with an interesting pattern. The brightest ejection proceeded almost directly towards disk center, from the flare location at S15, W15; this implies a large inclination angle to the local vertical. The ejection included a sinuous jet structure and obvious flow to the northeast, both there and to the north of the jet. The jet seemed to be anchored in bright point somewhat distant from the brightest flare loop. Here's the movie in the half-resolution data, .gif format at 150 kB:
Well, this is an interesting subject, but beyond the scope of a "science nugget" that really should be comprehensible to almost anybody with a technical bent and an interest in things that go whizz-bang on the Sun. We have several views of large-scale waves emanating from flares -
The key to understanding how flares launch such waves is probably in the momentum generated by the process. So at first glance, everyone will immediately understand why this subject is at the frontier of research: people can use the term "magnetic explosion" to describe a flare, which indeed shoots stuff outwards in all directions. So, observationally it's appropriate. But the same people would be certain that flare energy derives from stored magnetic energy, and to convert it into visible form clearly requires a "magnetic implosion"! From this juxtaposition it is clear that there may be some uncertainty about what is happening at the core of a flare, so please stay tuned for future science nuggets that will continue this interesting story.
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digital.com.
March 14, 1999: Hugh Hudson (email hudson@isass0.solar.isas.ac.jp)