Whistler wave excitation by relativistic electrons in coronal loops during solar flares

Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: cvocks@aip.de

Received: 12 August 2005
Accepted: 19 January 2006

Abstract

Whistler wave packets propagating along magnetic loops are the standard model for the generation of the fiber bursts that are observed in dynamical radio spectra during solar flares. The whistlers are believed to be generated by electron-loss cone distributions that form in the loop, if energetic electrons that have been injected by the flare are mirrored at the loop footpoints. A relativistic method for calculating plasma wave growth/damping rates for given particle velocity distribution functions (VDFs) is used to study the wave generation in dependence on the wave frequency and on the wave propagation angle with respect to the background magnetic field. Due to a Taylor expansion of the plasma dispersion function, it yields better accuracy for small growth rates than does a direct solution of the dispersion relation for a complex frequency. The results show that the whistler wave generation is limited to small propagation angles with respect to the background magnetic field. The wave growth rates are found to be high enough to account for the properties of fiber bursts observed in solar radio data.