Radio emission from solar flares offers a number of unique diagnostic tools to address long-standing questions about energy release, plasma heating, particle acceleration, and particle transport in magnetized plasmas. At millimeter and centimeter wavelengths, incoherent gyrosynchrotron emission from electrons with energies of tens of kilo electron volts to several mega electron volts plays a dominant role. These electrons carry a significant fraction of the energy released during the impulsive phase of flares. At decimeter and meter wavelengths, coherent plasma radiation can play a dominant role. Particularly important are type III and type III-like radio bursts, which are due to upward-and downwarddirected beams of nonthermal electrons, presumed to originate in the energy release site. With the launch of Yohkoh and the Compton Gamma-Ray Observatory, the relationship between radio emission and energetic photon emissions has been clarified. In this review, recent progress on our understanding of radio emission from impulsive flares and its relation to X-ray emission is discussed, as well as energy release in flare-like phenomena (microflares, nanoflares) and their bearing on coronal heating.

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 ...

No abstract

No abstract