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

Abstract

[1] 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°f rom the causative lightning flash, consistent with observations. The modeled precipitated energy flux (E > 45 keV) peaks at $1 Â 10 À2 (ergs s À1 cm À2 ), 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 (N ILDE ) and the electron precipitation (G) along a VLF signal path are strongly correlated with the VLF signal perturbations calculated by the model. A conversion ratio Y, relating VLF signal amplitude perturbations (DA) to the time-integrated precipitation (100-300 keV) along the VLF path (Y = G/DA), of 1.2 ± 0.3 Â 10 10 (el m À1 /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 Â 10 16 electrons, calculated directly from the conversion ratio Y and observations of VLF signal perturbations.

Very low frequency subionospheric remote sensing of thunderstorm‐driven acoustic waves in the lower ionosphere

“…We follow the method of Peter and Inan [] to compare the density disturbance to the measured perturbation amplitude. Since the VLF wave is most sensitive to Δ n e and Δ ν e in a narrow altitude range around h R , we define the “integrated line density perturbation” N ILDP as the change in electron density integrated along the transmitter‐receiver path, through an altitude range Δ h = 6 km centered around h R : …”

Terrestrial VLF transmitter injection into the magnetosphere

“…‘Whistlers’ launched from lightning can be subsequently observed in the conjugate region if guided along field‐aligned irregularities known as ducts, but may also propagate in a non‐ducted mode which refract but do not precisely follow magnetic field lines. Whistler mode VLF emissions from lightning are known to interact with energetic electrons (>100 keV) in the radiation belts via gyroresonance, producing both free‐running triggered emissions [ Helliwell , 1965], and inducing electron precipitation onto the ionosphere [ Helliwell et al , 1973] via pitch angle scattering [ Peter and Inan , 2007]. Lightning has been shown to play a role in energetic electron losses from the radiation belts [ Gemelos et al , 2009] and may contribute to the formation of the slot region between the two radiation belts [ Vampola , 1977].…”

VLF observations of ionospheric disturbances in association with TLEs from the EuroSprite‐2007 campaign

“…We consider a particular type of ionospheric disturbance, early VLF events, which are generated by lightning and can perturb the amplitude of a VLF transmitter signal by 0.2–6 dB. The term early refers to the fact that the perturbation occurs nearly in coincidence (within 20 ms) of the lightning stroke [ Inan and Rodriguez , 1993], which distinguishes them from lightning‐induced electron precipitation events (LEP) [ Peter and Inan , 2007]. Within early VLF events, early/fast events are characterized by a rapid onset duration (<20 ms), whereas early/slow events are characterized with an onset duration of 100–1000 ms.…”