Particle Acceleration due to a Plasmoid-Looptop Collision

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Introduction

During a solar erutive event, defined here as a flare and an associated CME, energy is believed to be converted 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 simulation these collisions can lead to significant episodes of particle acceleration. Only one such case is reported in the literature as observed with Yohkoh. In this nugget we present the first plasmoid-looptop interaction to be observed with RHESSI.

Figure 1: A schematic diagram of how varying reconnection rates in the current sheet above a flare loop affect the motion of the resulting plasmoid; The rising plasmoid (left) has a greater rate of reconnection below the source than above (v1B1 < v2B2). The reverse is true for a desending plasmoid (right) (v1B1 >v2B2).

Event Overview

The eruptive (occulted) event occurred above the eastern limb on 25 January 2007 (see left hand panel of Figure 2), only a few months after the launch of STEREO. The right hand panel of Figure 2 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 arcade later observed in EUVI images suggesting that it was a hot, looptop kernel. The upper source was therefore assumed to be a plasmoid that formed in the current sheet above the flare 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 looptop sources by Sui & Holman and others.

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