An Alternative View of the Masuda Flare

Nugget
Number:	125
1st Author:	Nariaki Nitta
2nd Author:
Published:	2010 April 12
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Previous Nugget:	Particle Acceleration due to a Plasmoid-Looptop Collision

Introduction

The "Masuda flare" [1] has defined flare physics for a generation of solar astronomers, but do we understand its message? Regular readers of the RHESSI Science Nuggets know that the bulk of hard X-ray emission in solar flares comes from the foot-points of a coronal magnetic loop structure. These X-rays result from interaction of energetic electrons, presumably accelerated in the corona, with the denser solar atmosphere. We also have learned about the hard X-ray sources actually in the corona [2], which are usually weaker than the foot-point sources. These take a variety of forms and appear at different times or phases in the flare development. Among these the Masuda flare, shown in Figure 1, remains unique. In this Nugget we question how typical this flare is.

Figure 1: This figure compares Yohkoh hard X-ray images of the 13 January 1992 "Masuda" flare at different energies with soft X-ray data (also from Yohkoh). Above the soft X-ray loop lies a high temperature region and low-energy hard X-ray source. The distance of the hard X-ray source from the loop increases with increasing energy.

The 2D "standard model" of a solar flare

Already in that original Masuda paper, a cartoon described the coronal hard X-ray source as the result of a high-speed reconnection jet colliding with the flare loop seen in soft X-rays. In order to establish the importance of magnetic reconnection in solar flares, the Yohkoh researchers tried to find additional examples: other limb flares that also contained the "Masuda" above-the-loop hard X-ray source. No other good examples were in fact found, but it was nevertheless argued [3] that impulsive flares, traditionally thought not to be eruptive, might commonly have "plasmoid" ejections. This inspired a re-invention of the wheel, applying or slightly modifying the 2D theory cartoon for eruptive flares from the 1960s and 1970s directly to explain all flares. The "standard" model to date assumes that magnetic field in the corona above the flare region somehow gets destabilized, causing magnetic reconnection associated with plasmoid ejection, which then intensifies the inflow to the reconnection region and drives faster reconnection. Particle acceleration is not an official part of the model, but it is often assumed that the outflow from the reconnection region can account for this as a secondary effect due to turbulence or shock waves. Because the images of the Masuda flare appeared to contain several of the building blocks of this model, it has been extensively cited as a prototype. But then how does the model qualify as standard if a vast majority of flares look different from this prototype?

Revisiting the data

We have re-analyzed the old Yohkoh data for this and other limb flares. Concerning the 13 January 1992 "Masuda" event, the flare itself has been analyzed by many and there is not much to add. We confirm the location of the above-the-loop top source, its dependence on energy, its brightness relative to the foot-points, etc. The only thing to mention is an indication of loop shrinkage in the very early phase as captured in low-energy hard X-ray images, probably adding constraints on how energy release proceeds. This may be a key feature, of flare development, as noted in an earlier RHESSI Nugget.

We have (re-)assembled 20 limb flares observed by Yohkoh to determine how unique the Masuda flare is. Most of these flares come from earlier lists compiled by Masuda himself, and also by others, but we have added four more flares with good data coverage and enough counts in the HXT 33-53 keV energy band. Among all of these we conclude that only the original Masuda flare shows a hard X-ray source more than 5000 km above the soft X-ray loop, the key attribute that promoted this flare to its iconic status. So it is definitely unique within the decade-long Yohkoh archive.

Now let us look at large-scale structures around the flare loop. We play with movies of soft X-ray images that cover a 10 arcmin field of view, such as this one. It is very hard to trace any ejection in this example, even though this was cited in [3] as good evidence. We do encounter other flares that look like plasmoids or loop ejections. This one appears to contain both a wave and a loop ejection, and we found two other similar related events.

So the Masuda flare may not be regarded as a good example of a plasmoid ejection either. One noticeable thing about its movie is that the corona to the north looks quite dynamic. Fortunately, SXT took full-disk images (which were usually disabled during flares) just before and after the flare, letting us know what it the global corona was doing. Figure 2 shows a time sequence of images of the west limb on which the Masuda flare occurred. The northern part of the movie actually covered part of the trans-equatorial loops that connected the active region (AR 6994) with another region in the northern hemisphere. See the largest box in (b) that corresponds to the field of view pf the movie. These trans-equatorial loops existed before the flare, although diffuse, and changed shape and brighteness through the flare, suggesting that they are post-flare loops as is the clear loop in Figure 1. This is not predicted in the standad model. It is possible that 3-d quadrupolar reconnection may be needed to explain the observations, although we do not know much about how this works.

Figure 2: SXT images of the west limb before (a) and during/after (b-d) the Masuda flare. We find prominent trans-equatorial loops that may have been an important ingredient of the flare process. The boxes in (b) indicate (smaller to larger) high, medium, and low-resolution soft X-ray images. The arrows in (a) and (b) may indicate shorter connections of some transequatorial loops during the flare. The arrow in (c) shows a structure moving outward.

References

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

[2] Coronal Hard X-ray sources

[3] Hot-Plasma Ejections Associated with Compact-Loop Solar Flares