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, in fact since1995 (FIgure 2).

Figure 2: Histogram of the rise times (start to peak) of all of the tabulated GOES M-class and X-class flares since 1995. The red asterisk shows the location of our flare, somewhat above the general trend of the histogram.

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