Multi-Spacecraft Observations of Chorus Dispersion and Source Location

We report Cluster Wide Band Data (WBD) receiver observations of correlated chorus elements with different frequency/time characteristics as seen on multiple spatially separated Cluster spacecraft. Because chorus packets disperse as they propagate careful comparison of the signals from multiple spacecraft can provide new information about the origin of these waves. A cross-correlation analysis is used to quantify the dispersive time delay between each frequency of a chorus element as it arrives at Cluster spacecraft pairs. This data cross-correlation is then compared with a ray-tracing technique in order to identify source locations that are consistent with the observed delays. We also consider a time-variable source that emits frequencies that increase as a function of time. This frequency drift rate is adjusted to force the frequency/time variation of the simulated chorus element on a single spacecraft to match that observed. This process yields possible source locations for each spacecraft, whose location and extent are a function of the amount of source frequency emission delay. By requiring the individual spacecraft source regions to intersect with the multi-spacecraft source regions, a common static source region at L » 3.9 (McIlwain parameter) and MLAT » -5.9± (magnetic latitude) is identified for a single event and upper and lower bounds are placed on the amount of source emission frequency drift within the source.