Statistics of multispacecraft observations of chorus dispersion and source location

We report emission characteristics of 52 chorus events on 23 August 2003 and10 events on three other days, modeled with a ray tracing technique. Chorus waves have acharacteristic frequency/time variation that is a combination of frequency separation bypropagation dispersion and a time-dependent source frequency emission drift. A cross-correlation technique comparing data from multiple Cluster spacecraft quantifies thefrequency variation owing to propagation dispersion. The comparison of the datacross correlations with the simulated cross correlations allows the identification of acorrelation region which has at least one common point with the chorus source region.Any remaining frequency/time variation in the single-spacecraft spectrograms notaccounted for by the cross correlations is then used to determine the time-dependentsource frequency emission drift. The final modeled correlation region and sourcefrequency emission drift for each chorus event is consistent with both the cross-correlationand single-spacecraft data. The modeled correlation regions are located near the magneticequator and are, in general, more extended parallel to the Earth’s magnetic field thanperpendicular to it. It is found that waves with frequencies above and below 1/2 theequatorial electron cyclotron frequency on the magnetic field line of the spacecraft (lowerand upper band, respectively) are emitted in a broad spectrum of wave normal angles.There is also some preference for lower band waves observed at the spacecraft tohave been emitted near the Gendrin angle and at earthward-pointing wave normal anglesof between 20 and 30 . The latter result is close to the range of wave normal anglesshown recently to be connected with chorus that propagates into the plasmasphereand evolves into the incoherent plasmaspheric hiss spectrum, known to be connectedto pitch angle scattering and loss of electrons in the electron slot region. Finally, thetime-dependent source frequency emission drift for these eventsranges from 1to20 kHz/s.For most events these rates account for at least 2/3 of the chorus frequency/time variationwith the rest being due to propagation dispersion.