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).