PROPAGATION AND ABSORPTION OF CYCLOTRON MASER RADIATION IN SOLAR MICROWAVE SPIKE BURSTS

It has been argued that the loss-cone-driven electron cyclotron maser instability can account for the properties of millisecond microwave sp ike bursts observed during some solar flares. However, as it propagate s outward from the corona, maser radiation undergoes gyroresonance abs orption when its frequency is a harmonic of the local electron-cyclotr on frequency. Existing analytical models using slab geometries predict that this absorption should be sufficiently strong to prevent the rad iation from being seen at the observed levels, except under highly res trictive conditions or for unrealistic plasma parameters. A more compr ehensive analysis is presented here to determine if and when maser rad iation can escape to produce microwave spike bursts. This analysis emp loys numerical raytracing and incorporates propagation and absorption of fundamental maser emission in a realistic model of a coronal flux l oop. It is found that ranges of physical conditions do exist under whi ch maser radiation can escape to an observer and that these conditions are much more limiting for fundamental emission in the extraordinary (x)-mode than in the ordinary (o)-mode. Detailed investigation implies that escaping radiation in the x-mode is highly directional and chief ly observable toward the center of the solar disk, while escaping o-mo de radiation is found to emerge from the corona over a much wider rang e of directions, with some cases corresponding to radiation observable near the solar limb.