Radio Emission from Anisotropic Electron Distributions

Abstract

The effect of pitch-angle anisotropy of fast electrons on generation of nonthermal radio emission is studied. Incoherent gyrosynchrotron radiation is shown to depend strongly on the anisotropy. In particular, the spectral index of gyrosynchrotron radiation increases up to a factor of 3-4 compared with the isotropic case. The degree of polarization (X-mode) increases for loss-cone distributions, while decreases for beam-like distributions. Ample evidence of the pitch-angle anisotropy effect on (1) spatial distribution of the radio brightness, (2) spatially resolved light curves of the intensity and polarization, (3) spectral hardness of microwave bursts, is found by exploring observations performed with Nobeyama Radioheliograph and Owens Valley Solar Array. Above some threshold in the angular gradient, the electron cyclotron maser instability (coherent gyrosynchrotron emission) can develop, provided that standard gyrosynchrotron emission is accompanied by a lower-frequency intense coherent emission. The coherent emission is studied in detail for a realistic distribution of fast electrons over the energy and pitch-angle. In agreement with observations of narrowband radio spikes, it is found that (1) hard (power-law) distributions over energy are preferable to produce the coherent emission, (2) the threshold of the instability corresponds to quite an anisotropic electron distribution, thus, the pitch-angle anisotropy derived from the properties of the continuum gyrosynchrotron radiation will not necessarily give rise to coherent emission (either enhanced isotropization implied by quasilnear saturation of the instability).