Aspects of ELF/VLF chorus generation mechanism: Source location and motion

Observations by the Wide Band Data (WBD) plasma wave instrument, on the four Cluster spacecraft, of chorus emissions at geomagnetic latitudes -10° < lambdam < 10° and L shells 4 < L < 5 have been used to estimate the location of the compact sources of chorus emissions. The interpretation of frequency differences exhibited by the same chorus emissions observed on different spacecraft in terms of a differential Doppler shift has lead to a simple model involving rapidly moving sources traveling at speeds comparable to the parallel resonant velocity of counter-streaming gyroresonant electrons. Even though this property of the sources of chorus was discussed as a possibility in early models of chorus generation, such source motion was not observed previously and a physical interpretation of the source motion was not previously presented. In this thesis a physical interpretation of the rapid motion of sources of chorus waves is given, which builds upon previous models of chorus emissions and energetic electron phase bunching and which delineates the relationship between rapid temporal growth and saturation of chorus and the rapid movement of the source of these waves. For selected cases in 2003, the source characteristics (location, velocity, and frequency dependence of these parameters) and the linear wave growth rate are estimated using broadband measurements of the plasma wave electric field observed by the WBD instrument and energetic electron fluxes observed by the Plasma Electron And Current Experiment (PEACE) instrument onboard Cluster. For these cases, the particle distribution functions estimated from data are used to calculate the linear amplitude growth due to cyclotron resonance. Results indicate that chorus sources move for ∼ 6000 km along the Earth's magnetic field lines at a velocity of ∼ 0.05 c. The emitted chorus waves at the source are assumed to have a wide range of wave normal angles, but the rays reaching the spacecraft are generally only those with small wave normal angles. In general, the direction of motion of the source, either away or toward the Cluster spacecraft, appears to be determined by the wave amplitude growth rate.