The Slowest Flare
Nugget | |
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Number: | 180 |
1st Author: | Sam Freeland |
2nd Author: | Hugh Hudson |
Published: | 23 July 2012 |
Next Nugget: | TBD |
Previous Nugget: | Flare Nimbus |
Introduction
A remarkable flare, distinguishable by its very slow rise phase, has just occurred (SOL2012-07-17T17:15, M1.7) near the W limb of the Sun. According to the NOAA database from the GOES X-ray photometers, using the standard definitions of start and peak time, it took this flare 5.2 hours to develop. We show an overview from SDO/AIA and RHESSI in Figure 1. Please refer to the extensive movies showing many dynamic features of this event.

A skeptic might want to describe this as a coincidental superposition of smaller events, perhaps on different parts of the Sun but in fact the movies linked above show that the entire flare evolution consisted of a single organic process. A search of the GOES database reveals that in fact this event had the longest start-to-peak time interval of any M or X-class flare in the present Hale cycle link, in fact since1995 (FIgure 2).
What else went with this event?
Because this flare was a major gradual event, we might expect a CME to have occurred. This happens with about half of the M-class flares, and it happened this time as well (see the movies pages). Furthermore, since this flare occurred near the west limb, an associated CME almost always means an SEP - a particle event. This too was observed. Finally, even better, another (more powerful and more impulsive) flare occurred about a day and a half later (SOL2012-07-19, M7.7), in the same active region. In such a case we may see "CME cannibalism", a phenomenon that may occur when a later faster ejection overtakes an earlier slow ejection out in the [2], in this case about halfway between the Sun and the orbit of Earth.
Energetics
Such an event appears to have no "impulsive phase" with hard X-ray and microwave signatures of powerful energy release into non-thermal particle acceleration. The non-impulsive behavior here may or may not reflect the presence of non-thermal particles, simply because the longer time scale of the event may make them less detectable This event, well-observed by both RHESSI and Fermi in hard X-rays, provides an excellent test case for this question: can slow CMEs also be associated with non-thermal energy release? A detailed study of these data might be rewarding. In a sense the answer is obvious, because we can see the SEPs. But do we see hard X-rays and/or microwaves as well? Figure 3 shows time series of GOES, the GOES temperature, and the [3] observation of the He II 304 A line (the line that dominates the AIA image shown in Figure 1). The temperatures show several peaks approaching 10 MK, consistent with this being a normal flare but one with slow modulations of its development; the He II data provide indirect evidence for the existence of non-thermal particles exciting chromospheric emissions.
Conclusion
We can conclude that yes, this was a remarkably slow-rising event, but no, it probably did not break the flare paradigm particularly. The AIA images show that the W ribbon probably was obscured by the limb, and the initial excitation in the E ribbon by RHESSI and the He II line was as expected; the later emission from the corona (in both bands) shows the development of the standard loop prominence system usually associated with a CME. This was an active-region event, which makes its slow rise still more unusual. Often, especially at solar minimum, we see large, slow, and relative cool flare-like events associated with filament eruptions outside the active latitudes. In extreme cases such events may become "stealth CMEs".