Terrestrial VLF transmitter injection into the magnetosphere

Abstract

[1] Very Low Frequency (VLF, 3-30 kHz) radio waves emitted from ground sources (transmitters and lightning) strongly impact the radiation belts, driving electron precipitation via whistler-electron gyroresonance, and contributing to the formation of the slot region. However, calculations of the global impacts of VLF waves are based on models of trans-ionospheric propagation to calculate the VLF energy reaching the magnetosphere. Limited comparisons of these models to individual satellite passes have found that the models may significantly (by >20 dB) overestimate amplitudes of ground based VLF transmitters in the magnetosphere. To form a much more complete empirical picture of VLF transmitter energy reaching the magnetosphere, we present observations of the radiation pattern from a number of ground-based VLF transmitters by averaging six years of data from the DEMETER satellite. We divide the slice at $700 km altitude above a transmitter into pixels and calculate the average field for all satellite passes through each pixel. There are enough data to see 25 km features in the radiation pattern, including the modal interference of the subionospheric signal mapped upwards. Using these data, we deduce the first empirical measure of the radiated power into the magnetosphere from these transmitters, for both daytime and nighttime, and at both the overhead and geomagnetically conjugate region. We find no detectable variation of signal intensity with geomagnetic conditions at low and mid latitudes (L < 2.6). We also present evidence of ionospheric heating by one VLF transmitter which modifies the trans-ionospheric absorption of signals from other transmitters passing through the heated region.

Analysis of experimentally validated trans‐ionospheric attenuation estimates of VLF signals

“…Given the findings of Cohen and Inan [] and Cohen et al . [], in this paper we use the FWM to compute trans‐ionospheric attenuation curves for comparison to Helliwell and explain any discrepancies.…”

“…Recent findings by Cohen and Inan [] and Cohen et al . [] provide the first cases of consistent agreement between satellite‐based observations and modeling results for magnetospheric injection from terrestrial VLF transmitters.…”

“…[] provide the first cases of consistent agreement between satellite‐based observations and modeling results for magnetospheric injection from terrestrial VLF transmitters. Cohen and Inan [] analyzed thousands of DEMETER satellite passes over 6.5 years over each of a dozen VLF transmitters to provide radiation maps at 700 km altitude with 25 km resolution, providing significantly more averaging and spatial resolution than previous studies on this topic. Using these maps, the total power injected into the magnetosphere from each transmitter was calculated for both daytime and nighttime.…”

Terrestrial antenna for high power VLF radiation into the magnetosphere

“…VLF signals pass through the ionosphere into the magnetosphere with some attenuation where they propagate as Whistler Mode waves and interact with the energetic electrons trapped in the earth's magnetic field (Van Allen radiation belts) [1,2]. This interaction results in energetic electrons being precipitated out of the radiation belts into the ionosphere [3,4].…”

“…A portion of the energy from these transmitters leaks into the magnetosphere to interact with the energetic particles [8]. Cohen concludes that at night as much as 12% of the energy from the VLF transmitter at the North West Cape of Australia leaks into the magnetosphere [2].…”