An anniversary sigmoid/halo CME marks SOHO's return

Sciece Nugget:  Oct 23, 1998

Just about one year ago, the SXT observers began the practice of writing weekly ``science nuggets'' like this one. The list contains 49 of them thus far, from many authors (Alexander, Hara, Hudson, LaBonte, McKenzie, Nakakubo, Nitta, Shimizu - we are always on the lookout for authors!). Two or three of them have gotten bigger circulation as an APOD ("Astronomical Picture of the Day") entry. The first nugget dealt with X-ray coronal dimming during a halo CME launch, a subject we still find fascinating. A year later, the sun has done it again! No doubt, both for the sake of our nugget anniversary, and also to celebrate the return to useful life of SOHO.

So, please refer back to the first nugget, and meanwhile let's see how the current sigmoid differs. First, an array of images - one sees that it's an anti-S, or perhaps a nuoid rather than a sigmoid; this reflects the general hemispheric segregation of the sigmoid symmetries for the northern hemisphere:


Brief apologies here - because of the variable observing conditions in the Yohkoh orbit, for completeness in time-series analysis we preferred to use the standard images with 10-arcsec pixelization, not so beautiful as the ones with higher resolution (the single image below shows the difference). In the bottom image, taken at 03:16 UT, one can find finer structures, including perhaps a view of Barbara Thompson's "feathers" in the NW and SE (upper right, lower left). EIT was working at high time resolution during this event, because LASCO had not yet been fully recommissioned. The hint one has of a filamentary structure (center and lower) aligned along the axis of the arcade will probably look much more interesting in the EIT movie.

Second, the GOES time history:


This shows the soft X-ray light curve of the flare (from the GOES soft channel), together with the fluxes from SXT in a box centered on the arcade. This display is only interesting in the sense that it shows again how insensitive GOES is to these large, slow, cool arcade events. The beautiful light curve seen at the top has no counterpart at the bottom! GOES sees the whole Sun, with all of its other sources of variability, whereas the SXT light curve comes just from the fat part of the arcade. However for a true believer in the Neupert effect (see last week's nugget) there is an intriguing hint of GOES hard channel counts at about the right phase of the SXT light curve. This would be interpretable here as heating, not as non-thermal particles, but both intepretations would be a bit flaky because of the many extraneous background effects seen in the GOES hard-channel at low levels.

Finally, a possibly interesting two-frame movie:


which shows a velocity-field variation especially at the NE (upper left) end of the arcade. This shows two frames from an (absolute) difference movie. There is a combined outward and inward motion. Could this be... an inward motion associated with the long-sought but never discovered reconnection inflow? Or, could we be seeing the signature of something helical escaping to the NE? Either way, it is a rather gradual motion, as judgable from the time interval and the image scale.

A summary of the comparison with the Oct. 23, 1997 event: this one had a more complex time profile, with a two phases of excitation. The dimming regions here were not so well-defined, nor so obviously related to the brightening, as in the earlier event. Please see Crooker et al.'s Coronal Mass Ejections, especially the Hudson and Webb paper there, for a fuller picture - not particularly changed by this beautiful event!

Oct. 23, 1998: H. S. Hudson (email; D. McKenzie (email