Modeling the properties of plasmaspheric hiss: 1. Dependence on chorus wave emission

Abstract

There is increasing evidence that plasmaspheric hiss is formed by the evolution of a portion of chorus waves that are excited in the plasmatrough and propagate into the plasmasphere. Comparison between the statistical spatial distributions of these two emissions in the morning sector during active times from THEMIS over ∼3 years shows that the two emissions have comparable peak intensities but are distinct in their spatial distributions. We present a modeling study of the hiss spectrum, based on ray tracing, by taking the observed chorus source region as an input in the magnetosphere, which contains cold and suprathermal electrons. Our modeling results show that we are able to reproduce the main features of typical hiss, including the frequency spectrum, wave normal angle and spatial distribution. However, the simulated hiss intensity is weaker (∼15 dB less) than the observed intensity, which suggests some modest internal amplification inside the plasmasphere. The responses of hiss to variations in the spatial distribution, wave normal angle distribution and frequency distribution of the source chorus are examined. We find that the majority of hiss formation is due to a small portion of chorus emission originating within ∼3 RE from the plasmapause, with wave normal directions pointing toward the Earth at an angle of 30°-60°, and over a frequency range of 0.1-0.3 fce. If the chorus power is made to increase closer to the plasmapause, the hiss intensity and the peak frequency also increases, which roughly mimics active geomagnetic conditions. Variations of the chorus source distribution do not significantly affect the wave normal angle distribution and frequency distribution of hiss, but does impact the absolute intensity of the resulting hiss. Copyright 2012 by the American Geophysical Union.