PROPAGATION EFFECTS ON THE CYCLOTRON MASER MECHANISM FOR SOLAR MICROWAVE SPIKE BURSTS

The loss-cone-driven electron cyclotron maser instability is widely be lieved to be responsible for millisecond bursts of intense microwave e mission often observed during solar flares. However, the maser radiati on is strongly absorbed as it propagates outward from the corona and e xisting analytical models predict that this absorption should be suffi ciently strong to prevent observable levels of the radiation from esca ping, except under highly restrictive conditions. In order to address the problem of how microwave spike bursts can be observed at all, we p resent a numerical ray-tracing analysis which incorporates emission. p ropagation and absorption of fundamental cyclotron maser radiation in a realistic model of a coronal flux loop. It is found that the radiati on can escape to a potential observer and that the physical conditions under which escape occurs are more restrictive for fundamental emissi on in the extraordinary (x)-mode than in the ordinary mode. Escaping r adiation in the x-mode is found to be highly directional and chiefly o bservable toward the center of the solar disk, while escaping o-mode r adiation is found to emerge from the corona over a much wider range of directions, with some cases corresponding to observable radiation nea r the solar limb.