Statistics of multispacecraft observations of chorus dispersion and source location

Aaron W. Breneman, Craig A. Kletzing, Jolene Pickett, Jaroslav Chum, Ondrej Santolik

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We report emission characteristics of 52 chorus events on 23 August 2003 and 10 events on three other days, modeled with a ray tracing technique. Chorus waves have a characteristic frequency/time variation that is a combination of frequency separation by propagation dispersion and a time-dependent source frequency emission drift. A crosscorrelation technique comparing data from multiple Cluster spacecraft quantifies the frequency variation owing to propagation dispersion. The comparison of the data cross correlations with the simulated cross correlations allows the identification of a correlation region which has at least one common point with the chorus source region. Any remaining frequency/time variation in the single-spacecraft spectrograms not accounted for by the cross correlations is then used to determine the time-dependent source frequency emission drift. The final modeled correlation region and source frequency emission drift for each chorus event is consistent with both the cross-correlation and single-spacecraft data. The modeled correlation regions are located near the magnetic equator and are, in general, more extended parallel to the Earth's magnetic field than perpendicular to it. It is found that waves with frequencies above and below 1/2 the equatorial electron cyclotron frequency on the magnetic field line of the spacecraft (lower and 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 to have been emitted near the Gendrin angle and at earthward-pointing wave normal angles of between -20° and -30°. The latter result is close to the range of wave normal angles shown recently to be connected with chorus that propagates into the plasmasphere and evolves into the incoherent plasmaspheric hiss spectrum, known to be connected to pitch angle scattering and loss of electrons in the electron slot region. Finally, the time-dependent source frequency emission drift for these events ranges from 1 to 20 kHz/s. For most events these rates account for at least 2/3 of the chorus frequency/time variation with the rest being due to propagation dispersion.

Original languageEnglish (US)
Article numberA06202
JournalJournal of Geophysical Research: Space Physics
Issue number6
StatePublished - Jun 1 2009

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