A coronal mass ejection without a long-lasting X-ray event

Science Nugget: Apr 16, 1999

Introduction

The launching of a coronal mass ejection (CME) most often accompanies an X-ray "long decay event" (LDE) or a giant arcade. These are soft X-ray signatures of a system of large loops forming in the corona under (or near) the location of the mass ejection, and provide some of the strongest evidence for the occurrence of large-scale magnetic reconnection as a part of the process of eruption. The association between CMEs and LDEs is in fact a strong one, but a recent preprint by Davina Innes (submitted to Solar Physics) calls attention to SOHO observations of an impulsive flare association, to wit April 9, 1997. So, this science nugget goes back to look at this relatively unusual event. Note that it happened just a couple of days after the much-celebrated sigmoid -> dimming event of April 7. The Innes et al. preprint stresses that many of the instruments on SOHO, including CDS, made relevant observations, so that fact (as well as its lack of an LDE) make it interesting.

What does SXT see?

In this wonderful time, we are ``calibrating'' the coronal mass ejections - thanks to Yohkoh, SOHO/EIT, and now TRACE, we can see the low corona much more clearly than we could before. In fact, with only a coronagraph (which observes explicitly far above the solar limb) one could never have been sure before about CME antecedents (see, for example, the review of ``dimming'' by Hudson and Webb).
 
 
The GOES plot shows the impulsiveness of the event (here the colored marks show the SXT image times, with dotted lines showing orbit night intervals:   
 

Too bad, Yohkoh missed the beginning of the event, but that happens a lot in our kind of orbit. The brevity of the event (ie, it's not an LDE) is striking. The flare is a C1.5 event in ordinary active region 8026, well-observed by Yohkoh and within a few degrees of the SW limb at the time of the CME.
 
 

  Difference image spanning 3 hours before and after the event (13:50 minus 10:54 UT), showing subtle dimming and prominent flare loops in the C1 void region found by Innes et al. 
 

The difference image above shows clearly that the post-event structure has a multiple loop structure with a bright crown, typical of the loop arcade often seen in a long-decay (LDE) event. Further large-scale brightening above these loops suggests still larger arcade structures. The 3-hour span of this difference implies that the arcade, though faint, persisted for that duration. So! With a different definition of the somewhat elusive ``LDE'' terminology, one could argue that this CME too involved an LDE. Perhaps the small angular span of the mass ejection - 27 degrees according to the authoritative LASCO observations, corresponds to a relatively feeble LDE arcade.

Jets or legs or ablation or streamers?

Another interesting aspect of this event has to do with jet-like structures visible in the soft X-ray images; these are important because one passes through the CDS spectrometer field of view, so that Doppler imaging could be analyzed by Innes et al. There are multiple possible interpretations of such structures. In the classical sense of an X-ray jet, as the magnetic leg of fieldlines opened by a CME, as the upward evaporative flow into an arcade loop, or possibly as a helmet-streamer geometry

Any of these scenarios seem possible at present, but we are planning a joint observing campaign emphasizing Yohkoh and the SUMER EUV instrument on SOHO, in hopes of catching this sort of flare-associated jet event with still more observational power. The observing campaign will take place in about two weeks, so please stay tuned in case we catch something interesting.

A modest conclusion!

The lack of a long-decay soft X-ray event observable by GOES probably says that this coronal mass ejection was a modest one, and does not by itself rule out the idea that all CMEs must have LDEs. On the other hand, the narrow angular range (high collimation) of this CME still offers a puzzle, and it would be very premature for us to imagine that we have any clear theoretical idea here (for example, how can a narrow column of open field lines persist and not reconnect immediately?). As for the jet-like structures, these may provide important clues, which is why we are trying to sharpen up our observations next month.
 

April 19, 1999: Hugh Hudson and Masumi Shimojo (hudson@isass0.solar.isas.ac.jp, shimojo@flare2.solar.isas.ac.jp)