Atmospheric Signatures of Radiation Belt Precipitation and their Relationship to Precipitating Flux and Spectra

Abstract:

Precipitation of radiation belt particles in the upper atmosphere is one of the key loss mechanisms for radiation belt fluxes. Currently, uncertainties in the theoretical precipitation loss rates lead to large discrepancy in electron lifetimes used in radiation belt models; loss rates or lifetimes used in current papers vary by an order of magnitude. Observation of radiation belt precipitation through its atmospheric signatures has the potential to dramatically improve these loss rates, since these atmospheric signatures are a direct result of precipitation. However, quantifying the loss rates from these signatures requires an understanding of the relationship between the precipitating flux and spectra and the resulting atmospheric signatures.

In this paper we present a forward model-based overview of atmospheric signatures of radiation belt precipitation, connecting precipitating fluxes and spectra to atmospheric signatures. We use Monte Carlo modeling of electron precipitation together with models of D-region chemistry to predict electron density enhancements, and couple those to a very-low-frequency (VLF) propagation model to predict signatures observed in VLF subionospheric remote sensing. We further provide model estimates of optical and X-ray signatures from different existing and proposed observing platforms. Most critically, we investigate the variation in observed signatures with different precipitating fluxes and spectra, to determine the invertibility of these signatures: how feasible is it to determine the source precipitation from one or more of these signatures? Results show that X-ray and optical signatures provide excellent insight into the precipitating flux, but some ambiguity remains concerning the precipitating spectrum. The VLF and radar methods, however, are sensitive to narrow ranges of electron energies; when used in conjunction with X-ray and/or optical, these methods can help constrain the precipitating spectrum.