Hybrid simulations of EMIC waves in a dipolar magnetic field

Abstract

[1] We employ 2.5-D electromagnetic, hybrid simulations that treat ions kinetically via particle-in-cell methods and electrons as a massless fluid to study the generation, propagation and nonlinear evolution of electromagnetic ion cyclotron (EMIC) waves in a dipolar magnetic field. The source of the instability is a small population of hot (keV) protons, representing the ring current ions injected in the equatorial region throughout the run. In this paper we investigate the impacts of the level of temperature anisotropy of these source ions, as well as the location of their injection on the EMIC wave excitation. We simulate wave generation using a ratio of perpendicular to parallel temperature ranging between 4 and 1.6 and show that parallel propagating EMIC waves with circular polarization are generated in the equatorial regions at frequencies below the proton cyclotron frequency. Examination of the Poynting vector shows that waves propagate along the magnetic field away from the equator and reach the ionospheric boundary where they are absorbed due to the simulation boundary conditions. Wave reflection does not occur anywhere along the path of propagation between the equator and the ionosphere. Refraction due to magnetic field gradients results in waves becoming more oblique as they propagate to higher latitudes. As a result, their polarization also changes with one transverse component vanishing and the development of a compressional component. The results also show a reduction in the total density and magnetic field in the region of EMIC wave generation. Although the reduction in density is partially due to the pressure associated with the injection of the hot protons, we show that EMIC waves contribute considerably to the level of density reduction. The nonlinear evolution of the EMIC waves results in the generation of field aligned electrostatic waves in the equatorial and low latitude regions. It is also shown that changing the level of temperature anisotropy while keeping the injection region fixed not only affects the maximum amplitudes reached but also the region of wave growth. Similarly, for a given temperature anisotropy the results vary as the injection region is changed.