Generation of Whistler Chorus Emissions and Resulting Wave-Particle Interactions

On July 24, 2003, when the Cluster satellites crossed the magnetic equator at about 4.5 RE radial distance on the dusk side (~ 15 MLT) emissions were observed below the local electron gyrofrequency in two bands, one band above one-half the gyrofrequency and the other band below one-half the gyrofrequency, which is typical of chorus emissions propagating in the whistler wave mode. A careful analysis of the emissions for this event has shown that Cluster passed through the wave generation region. Simultaneous electron particle data from the PEACE instrument in the generation region indicated the presence of a mid-energy electron population (~ 100’s of eV) that had a highly anisotropic temperature distribution with the perpendicular temperature about 10 times the parallel temperature. To understand this event in which the satellite passed directly through the whistler wave generation region and in which the free energy source (i.e., temperature anisotropy) was readily identified, a linear theory and particle in cell simulation study has been carried out to understand the physics of the wave generation, saturation and energy redistribution. The theoretical results show that for this event the anisotropic electron distribution can linearly excite obliquely propagating whistler mode waves in the upper band, i.e., greater than one-half the electron gyrofrequency. Non-linear processes excite waves in the lower band, i.e., less than one-half the electron gyrofrequency. The instability saturates by a combination of a decrease in the anisotropy of the mid-energy electrons as well as by heating of the cold electron population.