Solar flares-the most powerful explosions in the solar system-are also efficient particle accelerators, capable of energizing a large number of charged particles to relativistic speeds. A termination shock is often invoked in the standard model of solar flares as a possible driver for particle acceleration, yet its existence and role have remained controversial. We present observations of a solar flare termination shock and trace its morphology and dynamics using high-cadence radio imaging spectroscopy. We show that a disruption of the shock coincides with an abrupt reduction of the energetic electron population. The observed properties of the shock are well-reproduced by simulations. These results strongly suggest that a termination shock is responsible, at least in part, for accelerating energetic electrons in solar flares.One Sentence Summary: A termination shock is captured in action during a solar flare using radio observations, which show that it is a source of energetic electrons. Main Text:The acceleration of charged particles to high energies occurs throughout the Universe. Understanding the physical mechanisms is a fundamental topic in many space, astrophysical, and laboratory contexts that involve magnetized plasma (1). For solar flares and the often associated coronal mass ejections (CMEs), it is generally accepted that fast magnetic reconnection-the sudden reconfiguration of the magnetic field topology and the associated magnetic energy release-serves as the central engine driving these powerful explosions. However, the mechanism for converting the released magnetic energy into the kinetic energy in accelerated particles has remained uncertain (2, 3). Competing mechanisms include acceleration by the reconnection current sheet, turbulence, and shocks (2-5). 2Of possible interest in this regard is the termination shock (TS), produced by super-magnetosonic reconnection outflows impinging upon dense, closed magnetic loops in a cusp-shaped reconnection geometry (6). Although often invoked in the standard picture of solar flares (7,8) and predicted in numerical simulations (6,(9)(10)(11), its presence has yet to be firmly established observationally and, because of the paucity of direct observational evidence, its role as a possible particle accelerator has received limited attention (2, 3). Previous reports of coronal hard X-ray (HXR) sources in some flares have shown convincing evidence of the presence of accelerated electrons at or above the top of flare loops (referred to as the "loop-top" hereafter, or LT) (7,12), where a TS is presumably located. The often cited observational evidence for a solar flare TS has been certain radio sources showing spectroscopic features similar to solar type II radio bursts (radio emission associated with propagating shocks in the outer corona), but with small drifts in their emission frequency as a function of time, which implies a standing shock wave (13-17). However, because of the limited spectral imaging capabilities of the previous observations, none of ...

We perform resistive magnetohydrodynamic simulations to study the internal structure of current sheets that form during solar eruptions. The simulations start with a vertical current sheet in mechanical and thermal equilibrium that separates two regions of the magnetic field with opposite polarity which are line-tied at the lower boundary representing the photosphere. Reconnection commences gradually due to an initially imposed perturbation, but becomes faster when plasmoids form and produce small-scale structures inside the current sheet. These structures include magnetic islands or plasma blobs flowing in both directions along the sheet, and X-points between pairs of adjacent islands. Among these X-points, a principal one exists at which the reconnection rate reaches maximum. A fluid stagnation point (S-point) in the sheet appeared where the reconnection outflow bifurcates. The S-point and the principal X-point (PX-point) are not co-located in space though they are very close to one another. Their relative positions alternate as reconnection progresses and determine the direction of motion of individual magnetic islands. Newly formed islands move upward if the S-point is located above the PX-point, and downward if the S-point is below the PX-point. Merging of magnetic islands was observed occasionally between islands moving in the same direction. Reconnected plasma flow was observed to move faster than blobs nearby.

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The origin of the Moreton wave observed in the chromosphere and the EIT wave observed in the corona during the eruption remains being an active research subject for a while. We investigate numerically in this work the evolutionary features of the magnetic configuration that includes a current-carrying flux rope, which is used to model the filament, after the loss of equilibrium in the system takes place in a catastrophic fashion. Rapid motions of the flux rope following the catastrophe invokes the velocity vortices behind the rope, and may invoke as well slow and fast mode shocks in front of the rope. The velocity vortices at each side of the flux rope propagate roughly horizontally away from the area where it is produced, and both shocks expand toward the flank of the flux rope. The fast one may eventually reach the bottom boundary and produces two echoes moving back into the corona, but the slow one and the vortices totally decay somewhere in the lower corona before arriving the bottom boundary. The interaction of the fast shock with the boundary leads to disturbance that accounts for the Moreton wave observed in Hα, and the disturbance in the corona caused by the slow shock and the velocity vortices should account for the EIT wave whose speed is about 40% that of the Moreton wave. Implication of these results to the observed correlation of the type II radio burst to the fast and the slow mode shocks, and that of EIT waves to CMEs and flares have also been discussed.