Collisionless Reconnection and High‐Energy Particle Acceleration in Solar Flares

Abstract

Observations with the Hard X-Ray Telescope (HXT) and the Soft X-Ray Telescope (SXT) on board Y ohkoh show that the reconnection process is common to impulsive and gradual Ñares. We apply the collisionless reconnection theoryÈmore exactly, the model of a high-temperature turbulent-current sheet (HTTCS)Èto the coronal conditions derived from the Y ohkoh data on the site and mechanism of magnetic energy transformation into kinetic and thermal energies of "" superhot ÏÏ plasma and accelerated particles. We consider the reconnecting current sheet as the source of Ñare energy and the Ðrst-step mechanism in a two-step acceleration of electrons and ions to high energies. According to our model, reconnected Ðeld lines rapidly move out of the HTTCS, being frozen into superhot plasma, and form magnetic loops on the upstream side of a fast oblique collisionless shock (FOCS) situated above the soft X-rayÈemitting loops of a strong magnetic Ðeld. The electrons and ions energized and preaccelerated by the HTTCS are trapped in magnetic loops. The top of each loop moves with a high speed toward the FOCS, while its feet penetrate through the shock front. For these reasons, two mechanismsÈthe adiabatic heating inside the collapsing trap and acceleration by the shock front at the two feet of the trapÈefficiently increase the particle energy. The lifetime of an individual collapsing trap can be identiÐed with the observed few-second delay to higher energies of hard X-ray and gamma-ray emission. The trap of accelerated electrons can be seen as the coronal hard X-ray "" above-the-loop-top source.ÏÏ Precipitation of accelerated electrons from the trap through the FOCS into the chromosphere is responsible for the hard X-ray "" footpoint sources.ÏÏ The model explains timing, location, and motion of the hard X-ray sources in solar Ñares as well as the observed relative intensity of the coronal and chromospheric hard X-ray sources and other physical properties.

Impulsive and Gradual Nonthermal Emissions in an X‐Class Flare

“…Furthermore, Melrose & Brown (1976) and MacKinnon (1988MacKinnon ( , 1991 implemented trap-plus-precipitation models, which were tested by Aschwanden et al (1997) by applying correlation analysis to hard X-ray observations. Recently, Somov & Kosugi (1997) put forward the concept of a ''collapsing'' magnetic trap, which they consider to provide a second-step acceleration mechanism. They argued that efficiency of the second-step acceleration depends on the plasma properties (such as density) at the energy release site and inside the trap.…”

A Source of Energetic Particles Associated with Solar Flares

“…The literature devoted to the theoretical discussion of relevant acceleration processes may be grouped into several major categories, which include (1) acceleration by a DC electric Ðeld (Holman 1985 ;Holman & Benka 1992 ;Tsuneta 1995 ;Litvinenko 1996), (2) stochastic acceleration (Smith & Brice 1964 ;Miller 1991Miller , 1997Steinacker & Miller 1992 ;Miller & Vinas 1993 ;Miller & Roberts 1995 ;Miller, Larosa, & Moore 1996 ;Roth & Temerin 1998 ;Kocharov et al 1999), and (3) shock-related acceleration processes (Ellison & Ramaty 1985 ;Decker 1988 ;KraussVarban, Burgess, & Wu 1989 ;Zank & Gaisser 1992 ;Somov & Kosugi 1997 ;Tsuneta & Naito 1998 ;Berezhko & Donald 1999). The references cited here are mainly used as examples and are by no means complete.…”

A review of recent studies on coronal dynamics: Streamers, coronal mass ejections, and their interactions

“…The propagation of the shock through the closed streamer field lines gives rise to a magnetic collapsing geometry being an efficient electron accelerator [164][165][166]. This geometry, representative of a shock-enhanced magnetic mirror system, can effectively confine electrons, and return them to the shock for repeated acceleration.…”