The Flare that Time Forgot

From RHESSI Wiki

Introduction

RHESSI, over its lifetime in space thus far, has detected many, many flares. Often a single event inspires a single paper, and then a flood of literature even if it's not very typical The Masuda flare [1] would be a case in point, and we've had Nuggets on this topic before: "An alternative view...", "Acceleration without heating", "Acceleration-region densities", etc. In spite of this it is generally held that this particular event was of quite a rare type. Thus we were amazed when a retrospective view of the RHESSI flares disclosed that the very first M-class flare observed by RHESSI looks very much like a Masuda flare! Somehow nobody picked up on this one, in spite of its newsworthiness. See Browser for a concise overview.

How do we know nobody studied this one?

The RHESSI book [2] has an extensive index, which makes use of the now-standard IAU target designation scheme. A flare, in this scheme, has an identifier such as SOL2002-02-20T10:04 (with the option of an extension for location). This makes it easy to search any database, and (for example) a Google search just brought up an entire page of SOL2002-07-23 - but none of the "Flare that Time Forgot". The RHESSI index has one entry, for a study of the albedo patch, but nothing concerning the interesting coronal developments. By contrast everybody's favorite SOL2002-07-23 has no fewer than 79 entries, and featured in Bob Lin's final chapter of the book specifically because of its Masuda-like properties. The "flare that time forgot" of course had not made the top-10 list of RHESSI flares.

The flare

A couple of images show some of the attributes of this flare. Figure 1 shows soft, medium, and hard X-ray images via two imaging algorithms. They show the familiar cusp to be present at 25-50 keV, and above it a significant 50-keV source. In other words, it is possibly a much-sought-for Masuda event, and there it was on RHESSI's plate 11 years ago!

Fig. 1: RHESSI images from the flare using two inversion techniques: CLEAN at the top, and MEM at the bottom. From left to right, increasing energies (6-12, 25-50, and 50-100 keV). the simple structure seen in soft X-rays becomes more complicated with time, revealing a Masuda-type "above the loop top" source at high energies. Note that this appearance is in the early phase of the flare, as it was for Masuda's original event SOL1992-01-13 ([http://sprg.ssl.berkeley.edu/~tohban/browser/?show=qlpcr+qli02+qli03+qli04+qli05+grwa&date=20020220&time=095921&bar=1 Browser

A conclusion

We discovered this event accidentally, while doing systematic pipeline processing. This is recommended - a throwaway paper describing a "gee whiz" event may or may not be worthwhile; in cases where the phenomenon discussed is not typical, it may still be important, but in the case of solar flares we have major systematic uncertainties in how things work. Thus typical patterns are interesting, and routine analyses of many events together are certainly worthwhile. It is especially nice when they serendipitiously suggest the identification of the Masuda phenomenon, for which we have few good examples.

References

[1] "A loop-top hard X-ray source in a compact solar flare as evidence for magnetic reconnection"

[2] "Preface"