The Slowest Flare

From RHESSI Wiki

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.

Figure 1: Three images of the very slow flare SOL2012-07-17T17:15 (M1.7) as seen by AIA 304 A (left) and by RHESSI at two different times during the long rise phase, separated by two hours. These images are not registered well, but by using the limb as a guide one can see that the initial RHESSI 12-25 keV brightenings were in the ribbon region to the lower left, and that later on the coronal sources dominated.

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], ie starting in 1995.

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 [link]. Figure 1 shows images from the flare and the CME. [probably a panel from the AIA three-color movie, plus a frame from the C2 movie] Furthermore, since this flare occurred near the west limb, an associated CME almost always means an SEP - a solar particle event [link]. 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 occurs when a later faster ejection overtakes an earlier slow ejection out in the heliosphere [link], in this case about halfway to the distance of the Earth.

Energetics

Such an event appears to have no "impulsive phase" [link] with hard X-ray and microwave signatures of particle energy release. That 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 [link] in hard X-rays, provides an excellent test case for this question: can slow CMEs also be associated with non-thermal energy release? 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?

Conclusion