Abstract
Observations on four Cluster spacecraft of upper band chorus emissions near the magnetic equator (at magnetic latitudes between -10° and 10° and L shells ranging between L = 4 and L = 5) with the WBD instrument, have been used to estimate the dimensions of the compact source regions of ELF/VLF 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 waves was considered in previous models of generation of ELF/VLF chorus, it was never properly explained taking into account a rapidly moving source region. A physical interpretation of rapid motion of sources of chorus waves is presented, using previous models of chorus emissions and charged particle non linear phase bunching, indicating the relationship between the process of chorus generation and the rapidly movement of the source region of these waves. For selected cases in 2003, the source characteristics (size, velocity and frequency dependence of these parameters) and the non linear wave growth rate are estimated using broadband measurements of the electric field observed by the WBD instrument, fluxes measurements and anisotropy as observed by the PEACE particle instrument on Cluster. For these cases, the estimation of the pitch angle distribution is used to calculate the non linear amplitude growth by cyclotron resonance, as was done by Bell et al [2000]. Our results indicate that chorus sources extend over ~6000 km along the field lines and their sources are moving with a velocity of ~0.05*c. We also found that the emitted chorus waves at the source are assumed to have a wide range of wave normal angles, but the rays reaching the spacecraft seem to be the ones with lower angles (with some exceptions). The proper values of wave amplitude growth rate and phase bunching time, determine the direction of motion of the source, either away or toward the Cluster spacecraft.