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

Electron Firehose instability and acceleration of electrons in solar flares

Gunnar Paesold ^1,2 and Arnold O. Benz <sup>1

1</sup> 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

Abstract

An electron distribution with a temperature anisotropy can lead to the Electron Firehose instability (Here and denote directions relative to the background magnetic field ). 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 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

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