Acceleration of Electrons near Solar Flares.
Abstract
Two efficient versions of Fermi acceleration by hydromagnetie shocks are presented. One version is based on the work by Parker (Phys. Rev. 109,1328, 1958) on acceleration between approaching shocks. It is shown that the mechanism is efficient even for moderate shock strengths. Efficient acceleration is also shown to occur ahead of a shock moving into stronger fields. As the particle is accelerated by the shock it is able to penetrate stronger fields, but the shock field also becomes stronger, so that the particle remains trapped even while it gains energy. Doubling of the energy by each passing shock is possible in plausible magnetic fields. Two conditions of acceleration are that the particle moves faster than the shock and that collisional energy losses are negligible. For solar thermal electrons the conditions of acceleration hold only in and above the upper chromosphere in the high-energy tail of the velocity distribution. With Tc=3X105 0K, ne=109 em2, and an estimated shock frequency of 10 sec-', about 10-2 of all electrons are accelerated. The thermal tail is replenished nearly as rapidly as it is depleted by shocks, and in about ~ min the whole electron gas is heated to roughly 30 kev. Further acceleration is inefficient since the electron magnetic moment is then not constant. The electrons emit bremsstrahlung by collisions among themselves and the intensity, risetime, energy, and spectrum of the observed x rays can be adequately explained. Motion of the fast electrons downwards to nc 8 x 1010 cm- may heat the electron gas there to several 1~ 0K and may explain some cm-wave radio bursts with apparently thermal spectra. The hot region is ejected into the corona, where it expands under its own forces.
- Publication:
-
The Astronomical Journal
- Pub Date:
- March 1962
- DOI:
- 10.1086/108830
- Bibcode:
- 1962AJ.....67Q.285W