Propagation of Cyclotron Maser Emissions in the Finite AKR Source Cavity

Observations in the AKR source region by the Fast Auroral SnapshoT (FAST) satellite have demonstrated that the primary auroral electron distribution has the form of a ``horseshoe'' or ``shell'' structure, that the source region is nerly void of low-energy electrons, and that the radiation exists as a perpendicularly propagating (to within a few degrees) X mode wave. These observations are used to construct a 2-1/2D particle-in-cell simulation model to study the propagation of cyclotron maser emissions within a longitudinal plane in the bounded AKR source region. These simulations indicate that the maser radiation component incident normally on the cavity boundary (with across track E polarization) tends to leak out of the cavity via conversion to the Z mode and absorption by the ambient cold plasma; the reflected flux is very small. These processes destroy the coherence of this component. In contrast, the component propagating in the longitudinal direction (with along track E polarization) is free to grow along an extended path. FAST observations of the perpendicular E polarization in the source region indicate that it varies between being roughly isotropic for weaker AKR intervals to having a substantial (as much as a factor of 100) enhancement of the along track polarization for the most intense AKR emissions. No evidence was found that the across track polarization was ever dominant. The implications of these results will be discussed.