Energy release in solar flares

Abstract

We examine observational evidence concerning energy release in solar flares. We propose that different processes may be operative on four different time scales: (a) on the sub-second time scale of 'sub-bursts' which are a prominent feature of mm-wave microwave records; (b) on the few-seconds time scale of 'elementary bursts' which are a prominent feature of hard X-ray records; (c) on the few-minutes time scale of the impulsive phase; and (d) on the tens-of-minutes or longer time scale of the gradual phase.We propose that the concentration of magnetic field into 'magnetic knots' at the photosphere has important consequences for the coronal magnetic-field structure such that the magnetic field in this region may be viewed as an array of 'elementary flux tubes'. The release of the free energy of one such tube may produce an elementary burst. The development of magnetic islands during this process may be responsible for the sub-bursts. The impulsive phase may be simply the composite effect of many elementary bursts.We propose that the gradual phase of energy release, with which flares typically begin and with which many flares end, involves a steady process of reconnection, whereas the impulsive phase involves a more rapid stochastic process of reconnection which is a consequence of mode interaction.In the case of two-ribbon flares, the late part of the gradual phase may be attributed to reconnection of a large current sheet which is being produced as a result of filament eruption. A similar process may be operative in smaller flares.

Current Sheet Evolution in the Aftermath of a CME Event

“…This is probably an upper limit to the thickness of the CS, and what is appropriate at later times cannot be easily predicted: while h increases, M A decreases, and these processes compete in determining the actual size. These values are much larger than those obtained by assuming that the reconnecting region has the typical size of elementary flux tubes (see, e.g., Sturrock et al 1984), and they show that the CS characteristics are still far from being identified.…”

Post-flare evolution of AR 10923 with Hinode/XRT

“…Models of colliding and reconnecting magnetic flux tubes have been developed both for two ribbon flares and compact flares (e.g. Sturrock et al 1984;Jakimiec et al 1998). In these models the increase of twisting of the magnetic flux tubes in the corona accumulates free magnetic energy, which is released when the flux tubes are brought into contact.…”

Solar Radio Bursts with Special Regard to Particle Acceleration and Mass Ejection

“…There are at least three timescale on which acceleration processes operate. An overview which incorporates these timescales was suggested by Sturrock et al [33], who however considered only the high-frequency spikes in discussing the shortest timescale. With minor modifications their overview is as follows.…”