First phase acceleration mechanisms in solar flares

Details First phase acceleration mechanisms in solar flares First phase acceleration mechanisms in solar flares

Nov 1979

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1979aipc...56..155s&link_type=abstract

AIP Conference Proceedings, Volume 56, pp. 155-161 (1979).

Physics

Particle Emission, Solar Wind

Scientific paper

Requirements for the number of nonthermal electrons which must be accelerated in the impulsive phase of a flare are reviewed. These are uncertain by two orders of magnitude depending on whether hard X-rays above 25 keV are produced primarily by hot thermal electrons or by nonthermal streaming electrons which contain >~50% of the flare energy. Possible acceleration mechanisms are considered to see to what extent either X-ray production scenario can be considered viable. Direct electric field acceleration is shown to involve significant heating. In addition, candidate mechanisms to convert stored magnetic energy into induced electric fields, steady reconnection and the tearing mode instability, transfer at least half of the stored energy into heat. Acceleration by electron plasma waves requires that the waves be driven to large amplitude by electrons with large streaming velocities or by anisotropic ion-acoustic waves which also require streaming electrons for their production. These in turn must come from direct electric field acceleration or the drift of electrons relative to ions, which can only occur to a sufficient degree in a very small volume. Thus wave acceleration is subject to the same limitations as direct electric field acceleration. It is concluded that less than 10% of the flare energy can be deposited into nonthermal streaming electrons with known energy conservation and acceleration mechanisms, and thus hard X-rays above 25 keV must be produced primarily by hot thermal electrons.

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