Subpacket structure in strong VLF chorus rising tones: characteristics and consequences for relativistic electron acceleration

Van Allen Probes in situ observations are used to examine detailed subpacket structure observed in strong VLF (very low frequency) rising-tone chorus elements observed at the time of a rapid MeV electron energization in the inner magnetosphere. Analysis of the frequency gap between lower and upper chorus-band waves identifies f(ceEQ), the electron gyrofrequency in the equatorial wave generation region. Initial subpackets in these strong chorus rising-tone elements begin at a frequency near 1/4 f(ceEQ) and exhibit smooth gradual frequency increase across their > 10 ms temporal duration. A second much stronger subpacket is seen at frequencies around the local value of 1/4 f(ce) with small wave normal angle (< 10 degrees) and steeply rising df/dt. Smooth frequency and phase variation across and between the initial subpackets support continuous phase trapping of resonant electrons and increased potential for MeV electron acceleration. The total energy gain for individual seed electrons with energies between 100 keV and 3 MeV ranges between 2 and 15%, in their nonlinear interaction with a single chorus element.

Automated summary

In this study, detailed subpacket structure in strong VLF rising-tone chorus elements observed by Van Allen Probes in the inner magnetosphere is examined. The analysis of the frequency gap between lower and upper chorus-band waves helps identify f(ceEQ) and understand the electron gyrofrequency in the equatorial wave generation region. The observations show smooth frequency increase in initial subpackets near 1/4 f(ceEQ) and strong subpacket with steeply rising frequency at values around 1/4 f(ce).