Multipoint Observations of Compressional Pulsations: Testing Possible Generation Mechanisms

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The ultra-low frequency (3 to 15 minute periods) electromagnetic waves called Pc 5 pulsations have been studied for over 50 years. They are an important magnetospheric phenomenon because they convert energy from one form to another and transmit energy from one location to another. Although there have been a number of physical explanations proposed for these waves their origin remains unknown. This proposal seeks to determine the origin of one class of these waves ? compressional Pc 5 pulsations. The Principal Investigator of this proposal will be assisted by a graduate student in carrying out this research protocol. These are waves in the Pc 5 frequency range for which there are oscillations along the direction of the background magnetic field. Three mechanisms have been proposed for these waves: drift mirror instabilities due to pressure anisotropies, drift bounce instabilities and injection of blobs of plasma sheet plasma into the inner magnetosphere. The combination of THEMIS observations near the equator and Cluster observations in polar orbit now makes it possible to determine which of these mechanisms are operating. The following questions have been identified for study: 1. What mechanism generates compressional pulsations in the dawn and dusk magnetosphere? 2. What are the characteristics of the compressional pulsations, including their spatial extent and latitudinal structure? 3. What mechanism generates the subcategory of compressional waves exhibiting frequency doubling (the frequency of the compression component of the waves is twice that of the transverse components) and how do suprathermal particles respond to the pulsations?