Multi-spacecraft observations of chorus dispersion and source location

We report the source locations and emission characteristics of 62 chorus events identified with a ray tracing technique. Chorus waves have a characteristic frequency/time slope on a spectrogram that is a combination of frequency separation by propagation dispersion and a time variation of emission frequency. A cross-correlation technique comparing data from multiple Cluster spacecraft quantifies the frequency separation due to propagation dispersion. The comparison of the data cross-correlations with the simulated cross-correlations allows the identification of the source region. Any remaining frequency/time slope in the single spacecraft spectrograms not accounted for by the cross-correlations is then used to determine the time variation of emission frequency. The final source region and time variation of emission frequency for each individual chorus event are consistent with both the cross-correlation and single spacecraft data for that event. The 62 chorus source regions are located near the magnetic equator and are, in general, more extended parallel to the Earth's magnetic field than perpendicular to it. Histograms of wave normal and group velocity angles emitted by the aggregate of simulated sources are presented. It is found that waves in both the upper and lower band of chorus are emitted in a broad spectrum of wave normal angles with some preference for the Gendrin angle in the lower band. All waves, however, propagate along the Earth's magnetic field with little variation. Finally, the time variation of emission frequency for these events ranges from 1--20 kHz/sec. For most events these rates account for at least 2/3 of the chorus frequency/time slope with the rest being due to propagation dispersion.