Multistation observations of ELF/VLF whistler mode chorus

摘要

An array of seven ELF/VLF receivers in Alaska is utilized for direction finding and determination of ionospheric exit point of whistler mode chorus waves from the Earth's magnetosphere. Each receiver records both orthogonal horizontal magnetic components of the chorus waves. All sites use GPS-synchronized sampling, allowing for the localization of ionospheric exit points utilizing both arrival azimuth and time of arrival lag between sites. Results show two distinct groups of cases, emissions with singular ionospheric exit points and those with multiple exit points. Singular exit point cases exhibit migration and mode conversion as a function of propagation distance from the source point. The multiple exit point case shows chorus waves impinging on the ionosphere over a spread in magnetic latitude, suggesting nonducted propagation. Ray tracing for this case is unable to reproduce the observations unless a cold plasma density different than that predicted by geomagnetic conditions is used. It is proposed that chorus elements cross the transionospheric boundary after experiencing scattering from meter-scale irregularities. Additionally, subionospheric VLF remote sensing is used to detect precipitation onto the ionosphere of energetic radiation belt electrons that have been pitch angle scattered by individual chorus emission packets. VLF perturbation signatures are consistent with precipitation fluxes being dominated by electrons with energies less than 1 MeV.