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Astron. Astrophys. 351, 741-746 (1999)

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Electron Firehose instability and acceleration of electrons in solar flares

Gunnar Paesold 1,2 and Arnold O. Benz 1

1 Institute of Astronomy, ETH-Zentrum, 8092 Zürich, Switzerland
2 Paul Scherrer Institute, Würenlingen und Villigen, 5232 Villigen PSI, Switzerland

Received 9 September 1999 / Accepted 23 September 1999


An electron distribution with a temperature anisotropy [FORMULA] can lead to the Electron Firehose instability (Here [FORMULA] and [FORMULA] denote directions relative to the background magnetic field [FORMULA]). Since possible particle acceleration mechanisms in solar flares exhibit a preference of energizing particles in parallel direction, such an anisotropy is expected during the impulsive phase of a flare. The properties of the excited waves and the thresholds for instability are investigated by using linearized kinetic theory. These thresholds were connected to the pre-flare plasma parameters by assuming an acceleration model acting exclusively in parallel direction. For usually assumed pre-flare plasma conditions the electrons become unstable during the acceleration process and lefthand circularly polarized waves with frequencies of about [FORMULA] are excited at parallel propagation. Indications have been found, that the largest growth rates occur at oblique propagation and the according frequencies lie well above the proton gyrofrequency.

Key words: acceleration of particles – instabilities – Sun: particle emission – Sun: X-rays, gamma rays – Sun: corona – Sun: flares

Send offprint requests to: G. Paesold

Correspondence to: gpaesold@astro.phys.ethz.ch

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999