Remote Sensing of Radiation Belt Energetic Electrons Using Lightning Triggered Upper Band Chorus

Observations of magnetospheric VLF (~3-30 kHz) chorus being triggered by lightning induced whistlers are rare but provide a unique opportunity to remotely diagnose wave-particle interactions in the Earth's radiation belts. The observations presented here are unique in that whistlers originating from lightning in the southern hemisphere are seen to trigger upper band chorus repeatedly over the course of two hours, yielding 12 observation cases with very similar characteristics. Each whistler exhibits a distinct upper frequency cutoff and only the lowest L-shell whistler is observed to trigger upper band chorus. The observed cutoff frequencies along with L-shell and cold plasma density derived from dispersion analysis are used to estimate the anisotropy of the hot plasma distribution in the magnetosphere. Resulting anisotropy estimates are in good agreement with previous in-situ measurements and past work. While the anisotropy of the hot plasma distribution determines wave growth in the linear regime, access to the nonlinear regime requires the in situ wave amplitude to exceed the threshold for phase trapping of energetic electrons. Calculation of the threshold for each whistler shows that the upper band triggering of chorus corresponds to a lower nonlinear threshold. The results suggest that while upper band chorus is less favorable to be spontaneously generated, the conditions in this band are more conducive for triggering of the chorus instability by an external input wave.