On the effects of electron-cyclotron masers during flares

First recognized by Wu and Lee (Ap. J. 230, 621, 1979), electron-cyclotron masers can be activated under very mild conditions. Large growth rates can occur even for relatively mild anisotropies in the electron velocity distribution, e.g., the one-sided loss cones that commonly occur when electrons with small pitch angles precipitate into high density regions at the footpoints of flaring loops while others are reflected in the converging field in the corona. Maser action can plausibly occur at the second harmonic of the local gyrofrequency and so explain certain very bright (? 10¹⁰ K) microwave bursts from the sun and other stars. However, the preponderance of the energy is at the first harmonic.We suggest that masers operating at the local gyrofrequency in a flaring loop generate radiation at decimeter wavelengths that is a significant fraction of the total energy of the flare, in fact (and not coincidentally) comparable with the energy in electrons associated with hard X-ray bursts. Essentially all of the radio energy is trapped in the corona and serves to produce localized heating in a volume large compared with the energy release region. Thus it can transfer energy by radiation from one magnetic loop to another, possibly inducing further instabilities, and spreading the course of the flare. Eventually the energy probably escapes the corona as soft X-rays. The electron-cyclotron maser saturates by extracting the perpendicular energy of the electrons, thereby diffusing them into the loss cone at the maximum possible rate; the enhanced precipitation into the footpoints can produce bright emission in hard X-rays, EUV and Hα and remove any necessity for directive acceleration in the energy release region.Details of the proposed mechanism and effects are contained in two papers by Melrose and Dulk (Ap. J. 259, 1982).This work was sponsored by NASA under grants NAGW-91 and NSG-7287 to the University of Colorado.