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Super–Alfvénic Beam–Plasma Instabilities in Solar Flares

Published online by Cambridge University Press:  19 July 2016

Frank Verheest
Frank Verheest*
Affiliation:
Instituut voor theoretische mechanika, Rijksuniversiteit Gent Krijgslaan 281 B–9000 Gent, Belgium

Abstract

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Special type III radio bursts with significantly lower drifts have been attributed to proton beams at the flare site. These low energy beams (some MeV) are nevertheless super-Alfvénic. Recent reports deal with the stability of such beams against resonant scattering by waves below the proton gyrofrequency. Here a different mechanism is proposed, namely nonresonant Alfvén instabilities, which may have higher growth rates and can thus dominate. A selfconsistent multispecies plasma description of such instabilities, developed for solar wind and cometary plasmas, is applied to slowly drifting type III radio bursts. The instability growth rate is fairly insensitive to the beam velocity but decreases with the ratio of beam to ambient plasma densities. Even low beam densities can trigger these nonresonant and hence generic instabilities. Moreover, growth rates are sizeable fractions of the wave frequency, whereas resonant instabilities would give much lower values at low beam densities.

Type
VIII. Solar Flares
Information
Symposium - International Astronomical Union , Volume 142: Basic Plasma Processes on the Sun , 1990 , pp. 383 - 389
Copyright
Copyright © Kluwer 1990 

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