Particle Acceleration due to a Plasmoid-Looptop Collision
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|1st Author:||Ryan Milligan|
|Published:||2010 March 29|
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A solar eruptive event (flare and associated CME) converts magnetic energy into the heating of plasma, the acceleration of particles and the mass motion of the CME, all through the process of magnetic reconnection. The primary release of energy occurs when magnetic field lines converge and reconnect along the current sheet in the wake of the erupting CME. In some cases more than one reconnection site can form resulting in the formation of plasmoids, or 'magnetic islands' along the current sheet. These are sometimes visible as 'knots' of material in white-light coronagraph images, or above-the-looptop coronal sources in X-rays. The motion of these plasmoids is determined by the relative reconnection rates above and below the source (see Figure 1). As magnetic field strength and electron density (and consequently, the reconnection rate) both decrease with increasing altitude the vast majority of plasmoids observed close to the Sun tend to rise, often in sync with the CME. However, in rare cases conditions can be right for plasmoids to decrease in height and collide with the underlying looptop. According to a recent simulations these collisions can lead to significant episodes of particle acceleration. Only one such case of such an interaction - a Yohkoh observation - had previously been reported . In this Nugget we present the first plasmoid-looptop interaction to be observed with RHESSI.
The eruptive (occulted) event occurred above the eastern limb on 25 January 2007 (see Figure 2), only a few months after the launch of STEREO. Figure 3 shows that the earliest RHESSI image at 06:29 UT revealed a single, high-altitude coronal source. At 06:35 UT a second source began to brighten at a lower altitude. This lower source was observed to lie directly above the post-flare loop arcade later observed by STEREO, suggesting that it was a hot, looptop kernel. We therefore assume the upper source to be a plasmoid that formed in the current sheet above the loops. Between 06:37 UT and 06:43 UT the plasmoid was observed to decrease in height and merge with the underlying looptop. Note that this downward motion is inherently different to that observed in other looptop sources observed by RHESSI .
By tracking the peaks of the two sources we can clearly reveal the downward motion of the plasmoid source relative to the limb as shown in Figure 4c. (Note that here we chose the peaks of the RHESSI sources rather than the centroids to remove the possibility of interpreting the relative change in intensity of the two sources as a motion.) Although the time of the merging with the looptop can only be estimated from the corresponding height-time plot around the same time that there is a noticeable increase in both HXR (mostly 12-15 and 15-18 keV) and radio emission (245 MHz channel of the Learmonth Radio Telescope) as shown in Figures 4a and 4b (marked by the vertical dotted lines). Both are evidence of nonthermal particles. From STEREO data we can also see that this merging and subsequent particle acceleration occurred when the associated CME underwent its phase of greatest acceleration (Figure 4d).
As mentioned in the introduction, to achieve a downward-moving plasmoid the reconnection rate above the source must be greater than that below. The consequence of the greater reconnection rate is that a greater magnetic tension force is applied to the newly connected field lines, which in turn drives the motion of the plasmoid. As a previous Nugget  by Wei Liu showed, the mean photon energy should decrease with increasing distance from the reconnection site. To test if this was case in the 25 January 2007 event we imaged the plasmoid source over 2 keV wide energy bins. From Figure 5 we can see that the plasmoid emitted higher energy emission at higher altitudes. In the case of rising plasmoids (reference ), the reverse was found to be true with photon energy decreasing at higher altitudes. The energy gradient shown in Figure 5 therefore does suggest that the dominant reconnection site was indeed above the plasmoid, rather than below it. The increased nonthermal emission, on the other hand, could bea result of a secondary current sheet forming between the two sources due to pile-up of the anti-parallel magnetic field lines. This in turn leads to a secondary episode of reconnection which is very efficient since it is driven by the same magnetic tension that governs the plasmoid motion.
This Nugget presents evidence for the first plasmoid-looptop interaction observed with RHESSI during an occulted limb event; perhaps only the second ever. The study of X-ray sources in the corona is crucial in gaining an understanding of the magnetic reconnection and particle acceleration process(es) during solar flares. Combining RHESSI observations with those from STEREO also allows us to understand how particle acceleration fits into the broader picture of the flare-CME relationship.
Ryan Milligan is currently a post-doc at Goddard Space Flight Center as part of the RHESSI team. This nugget is based on his recent " publication of work carried out with James McAteer at Trinity College Dublin, and Brian Dennis and Alex Young at Goddard.