A Method for Imaging Energetic Particle Precipitation With Subionospheric VLF Signals

Abstract

AbstractEnergetic particle precipitation (EPP) is a key loss mechanism for radiation belt particles. Quantification of the precipitation loss rate feeds into the electron lifetimes used by radiation belt models and is needed to improve understanding of radiation belt dynamics. EPP deposits most of its energy in the D‐region ionosphere, a layer so weakly ionized that it is not observed using standard ionosphere measurement techniques. However, very low frequency (VLF) radio signals propagate great distances because of the naturally occurring waveguide formed by Earth’s surface and the D‐region. If the ground conductivity is known along the propagation path to a receiver, then the amplitude and phase of a VLF transmitter signal can be used to infer the average conductivity of the D‐region ionosphere. This article simulates the propagation of narrowband VLF signals through realistic ionosphere profiles enhanced by EPP. By using a distributed array of VLF receivers, the observations can be simultaneously inverted to estimate the spatial extent of a precipitation patch. These images of the ionosphere are generated using the local ensemble transform Kalman filter. We demonstrate this method with several simulated observation experiments, including four EPP events. Precipitation patches are identified in daytime, but accurate estimation of nighttime ionospheres remains a challenge.