The Origin of Plasmaspheric Hiss

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Plasmaspheric hiss is an incoherent, band-limited emission found predominantly in the dense-plasma region surrounding the Earth, known as the plasmasphere. Hiss propagates as an electromagnetic plasma wave in the whistler mode, and is able to resonate with the high-energy electrons in the Van Allen radiation belts. The importance of hiss in controlling the structure and dynamics of the radiation belts has long been established, but the origin of hiss itself has been an open problem for over four decades. This project will develop a newly discovered mechanism involving chorus waves that appears to be able to naturally account for the observed frequency band, the incoherent nature, the day/night asymmetry, and the dependence on geomagnetic activity. An existing numerical ray tracing code will be used to calculate wave propagation, together with Landau damping using fluxes measured by the Combined Release and Radiation Effects Satellite (CRRES). By properly weighting the power distribution of chorus waves and tracking the evolution of chorus into hiss using ray tracing, the distribution of hiss intensity, frequency band, and wave normal angle will be quantified as a function of distance from the Earth, magnetic local time, and latitude. The effects of geomagnetic activity on the distribution of hiss characteristics will then be analyzed by recalculating the ray database under a number of different magnetospheric conditions. In the final year of the study, the project will determine contribution to hiss from (a) global lightning activity, (b) the effects of azimuthal propagation, and (c) propagation in plasmaspheric plumes. The computed hiss distributions will be compared with observations from satellite measurements. The research will be done primarily by a young research scientist and a postdoctoral scholar. Understanding the physics of the radiation belts is important to understanding and being able to forecast space weather.