A quantitative comparison of lightning‐induced electron precipitation and VLF signal perturbations

Abstract

VLF signal perturbations recorded on the Holographic Array for Ionospheric/Lightning Research (HAIL) are quantitatively related to a comprehensive model of lightning‐induced electron precipitation (LEP) events. The model consists of three major components: a test‐particle model of gyroresonant whistler‐induced electron precipitation, a Monte Carlo simulation of energy deposition into the ionosphere, and a model of VLF subionospheric signal propagation. For the two representative LEP events studied, the model calculates peak VLF amplitude perturbations within a factor of three of those observed, well within the expected variability of radiation belt flux levels. The phase response of the observed VLF signal to precipitation varied dramatically over the course of the two nights and this variability in phase response is not properly reproduced by the model. The model calculates a peak in the precipitation that is poleward displaced ∼6° from the causative lightning flash, consistent with observations. The modeled precipitated energy flux (E > 45 keV) peaks at ∼1 × 10−2 (ergs s−1 cm−2), resulting in a peak loss of ∼0.001% from a single flux tube at L ∼ 2.2, consistent with previous satellite measurements of LEP events. The precipitation calculated by the model is highly dependent on the near‐loss‐cone trapped radiation belt flux levels assumed, and hence our main objective is not to compare the model calculations and the VLF signal observations on an absolute basis but is rather to develop metrics with which we can characterize the VLF signal perturbations recorded on HAIL in terms of the associated precipitation flux. Metrics quantifying the ionospheric density enhancement (NILDE) and the electron precipitation (Γ) along a VLF signal path are strongly correlated with the VLF signal perturbations calculated by the model. A conversion ratio Ψ, relating VLF signal amplitude perturbations (ΔA) to the time‐integrated precipitation (100–300 keV) along the VLF path (Ψ = Γ/ΔA), of 1.2 ± 0.3 × 1010 (el m−1/dB) is suggested for precipitation events of similar location and characteristics to those examined. The total precipitation (100–300 keV) induced by one of the representative LEP events is estimated at ∼1.8 ± 0.4 × 1016 electrons, calculated directly from the conversion ratio Ψ and observations of VLF signal perturbations.