Nighttime D region electron density profiles and variabilities inferred from broadband measurements using VLF radio emissions from lightning

Abstract

Lightning discharges radiate most of their electromagnetic energy in the very low frequency (VLF, 3–30 kHz) and extremely low frequency (ELF, 3–3000 Hz) bands and are, consequently, an effective tool for remotely sensing the nighttime ionospheric D region electron density profile. Using broadband lightning‐generated VLF signals, we derived the night‐to‐night variations of the midlatitude ionospheric D region electron density profiles. For 16 nights, between 1 July and 4 August 2004, we examined measured VLF data from lightning occurring near the United States east coast (∼530–860 km away from Duke University) and received at Duke University from 0400 to 0600 UT. From these observed VLF radio atmospherics (sferics), we extracted the nighttime D region electron density profiles covering the range of electron densities from 100 to 103 cm−3, in the altitude range of approximately 70–95 km, using a two‐dimensional, laterally homogenous model of VLF propagation in the Earth‐ionosphere waveguide. The inferred electron density profile variabilities were in good agreement with those from past nighttime rocket experiments at similar latitudes. Using the rocket‐measured profiles in our propagation simulations, we determined that the two‐parameter exponential D region electron density profiles we inferred were the best exponential fit, in the electron density range of ∼3 to ∼500 cm−3, to the rocket‐measured D region electron density profiles. In an initial effort to determine the sources of the observed variabilities, we compared the SAMPEX precipitating electron measurements to the electron density profiles inferred during July 2000. The results indicate that high‐energy electron precipitation might account for at least part of the night‐to‐night variations of the D region electron densities at the midlatitudes.