Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 1976
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1976a%26a....47..165m&link_type=abstract
Astronomy and Astrophysics, vol. 47, no. 2, Mar. 1976, p. 165-192.
Astronomy and Astrophysics
Astrophysics
29
Collisionless Plasmas, Electron Beams, Magnetohydrodynamic Stability, Plasma Frequencies, Solar Radio Bursts, Bandwidth, Cold Plasmas, Graphs (Charts), Lines Of Force, Magnetohydrodynamic Waves, Relativistic Particles, Very Low Frequencies, Whistlers
Scientific paper
The stability of a low-density beam of mildly relativistic electrons spiralling along a magnetic field in a cold collisionless turbulent plasma is investigated, and the whistler mode is found to be the most unstable mode for turbulent fluctuations. Calculations are performed which demonstrate that nonlinear coupling of unstable whistler and HF X-mode waves with LF MHD waves can efficiently stabilize the two unstable modes at low beam densities. Quasi-linear relaxation of the O-mode wave instability is analyzed, and it is shown that O-mode waves have characteristics that explain naturally the polarization, narrow bandwidth, and high degree of directivity of type I solar radio bursts. The results are used to construct a theoretical model for type I bursts in which electron beams with nonzero pitch angle and a small momentum dispersion are formed when, at some height in the corona, coronal electrons are accelerated perpendicular to the magnetic field and the ratio of electron plasma frequency to electron cyclotron frequency decreases just above the acceleration region. It is shown that the electron beams will become unstable at some height above the acceleration region and will produce a burst of radiation whose characteristics agree very well with the observed characteristics of type I bursts.
Mangeney Andre
Veltri Pierluigi
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