Sideways ejections

Science Nugget:  Nov 06, 1998

Flares and CME events seen in soft X-rays often show ejective motions, and we've noticed that the new flare observations appearing in this maximum seem to be more ejective than those of the past, the declining phase of Cycle 22. The ejecta during this week's flares showed strikingly non-radial behavior, another factor noted before but never the subject of a "science nugget" nor a research paper. From basic geometry, it's clear that any radial motion projects onto a line passing through disk center. Thus a cusp that points away from disk center corresponds to a non-radial outward flow. We show three examples of extreme non-radial motions below.

A large-scale cusp left by a major flare

The nearly X-class flare visible in the GOES plot at the top of this page came from a strongly sigmoidal region. This sigmoid incidentally violates the usual pattern - it is a normal "S" but in the north. Pat McIntosh remarked on this in his prediction of the flare, in terms of the vorticity visible in H-alpha. The image below is a one-day difference image so that the flare created the white parts and removed the dark parts. Solar rotation has been corrected.


Note that the cusp points DOWN on this image, almost exactly towards sun center. From projection, a vertical cusp should point UP, away from the center. This structure therefore has a highly non-vertical orientation and must be pressed almost flat against the solar surface.

Streaming motions in an earlier flare

During an earlier flare in the same active region, around 01:00 UT November 5, SXT detected interesting streaming motions. We illustrate these in a short movie, with still excerpts below:


The flow has a westward trajectory, as shown by arrows on the still images Please look at the movie because it shows the effect far better; which suggests a kind of analysis we've never succeeded with in SXT data: correlation tracking. It would be extremely valuable to track the motions in detail, especially since we now have promising means with which to understand the full 3-D geometry of flare loops. See for example an earlier science nugget on this subject, or follow the development of our fourth SOHO/Yohkoh coordinated data analysis workshop as it develops.

Jets in a compact flare

The C3 flare visible in the GOES plot, just following the major flare, produced an unusually well-observed jet. This is unusual because normally jet-related transients do not become bright enough to trigger the Yohkoh flare mode. Here one can see three frames from the movie of the flare in the AlMg filter, with a normal 2.6-arc-min field of view. The first frame shows the patchy flare onset, with a jet already formed and pointing to the west (right); the second frame, with a heliographic grid superposed, shows the compact flare structure (hot loop); and the third frame shows another jet along the same trajectory.


The recurrence of the jet is also interesting, but we're interested here in the trajectory itself. The jet points at almost right angles to the radial direction. Again, this motion must have been close to horizontal, although we cannot solve for the true orientation without more information.


This is a powerful active region with flux emergence, we believe; the pre-flare active region was extremely bright. We therefore suppose that the coronal magnetic field is intense. If so it can collimate these ejecta away from stronger regions. It also seems likely, since a global process must be at work and that the larger-scale structure must be playing a role in the flare energy release, that the strong channeling means that magnetically weak corridors exist within the active region. In the case of a jet, because of jet associations with Type III bursts, we speculate that the corridor may actually consist of open field lines leading out to the solar wind.

Nov. 6, 1998: H. S. Hudson (email, T. Magara, D.E. McKenzie