Characteristics of the drift rate of the rising tone chorus waves measured by the Van Allen Probes

Whistler mode chorus waves are common very low frequency coherent electromagnetic emissions in Earths magnetosphere. This type of emission is routinely observed in the frequency range below electron gyrofrequency as repeated narrowband signals of short duration with rising (or falling in less than in 5% of cases) frequency. Understanding the drift rate of the rising frequency is an important step in resolving the long-standing problem of nonlinear chorus generation. In this study, we examine lower-band chorus wave packets measured by the Van Allen Probes. We are focused on the events when elements of the same chorus are simultaneously detected by both spacecraft. We examine the relationship between the drift rate and the characteristic of the wave packets, such as frequency, amplitude, wave normal angle, and the ambient plasma parameters. It has been found that the drift rate depends on the intensity of the chorus emission: wave packets with larger amplitude tend to have a higher drift rate. Further investigation showed that this dependence is not necessarily inherited from the generation processes in a source region but can be caused by the density irregularities in micro ducts encountered by the waves as they propagate away from the equator. This suggestion is in agreement with the presented results obtained by means of the full-wave modeling.