An Alternative View of the Masuda Flare

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Nugget
Number: 125
1st Author: Nariaki Nitta
2nd Author:
Published: 2010 April 12
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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: The original Masuda flare, showing images from Yohkoh's hard X-ray (contours) and soft X-ray (color) telescopes. The "Masuda source" appears to float in the corona above the loops.

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?


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

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