A parametric ray tracing study of superluminous auroral kilometric radiation wave modes

We present a ray tracing study on the propagation characteristic of the superluminous wave modes (R-X, L-O) that are generated as auroral kilometric radiation ( AKR) in the Earth's magnetosphere by using a cavity plasma density model and a global core plasma density model. Numerical calculations have been performed for the two modes which originate at three altitudes, r = 1.0, 1.5, and 2 RE in the source cavity along a 70 degrees night field line. We show that the propagation characteristic of the R- X and the L-O modes is found to be essentially controlled by the source cavity position and the plasmapause position, in particular because the reflection occurs at the sharp density edges of the two positions. During high geomagnetic activity (e. g., K-p = 6, where K-p is the geomagnetic activity index), the plasmapause position moves inward, closer to the Earth, the R-X and L-O modes can propagate downward and even through the equatorial plane in the radiation belts under appropriate conditions, since no reflection occurs. However, during low geomagnetic activity (e. g., K-p = 2), both modes basically propagate at higher latitudes because of the reflection in the vicinity of the plasmapause and the source cavity. Furthermore, as the wave frequency or the originated altitude is higher, the reflection becomes weaker, and correspondingly the AKR observational latitude is lower. The wave damping along the ray path is found to be substantially small, primarily because high resonant energies will occur, while specifically Landau damping will not for the electrons in gyroresonance with superluminous waves. The results above provide a new insight into the propagation characteristic of R-X and L-O modes and predict that the two modes may be physically present in the radiation belts of the Earth particularly during strong geomagnetic activities. This needs to be further supplemented by observational analysis.