Effects of VLF Transmitter Waves on the Inner Belt and Slot Region

Abstract

Signals from very low frequency (VLF) transmitters can leak from the Earth‐ionosphere wave guide into the inner magnetosphere, where they propagate in the whistler mode and contribute to electron dynamics in the inner radiation belt and slot region. Observations show that the waves from each VLF transmitter are highly localized, peaking on the nightside in the vicinity of the transmitter. In this study we use ∼5 years of Van Allen Probes observations to construct global statistical models of the bounce‐averaged pitch angle diffusion coefficients for each individual VLF transmitter, as a function of L*, magnetic local time (MLT), and geographic longitude. We construct a 1‐D pitch angle diffusion model with implicit longitude and MLT dependence to show that VLF transmitter waves weakly scatter electrons into the drift loss cone. We find that global averages of the wave power, determined by averaging the wave power over MLT and longitude, capture the long‐term dynamics of the loss process, despite the highly localized nature of the waves in space. We use our new model to assess the role of VLF transmitter waves, hiss waves, and Coulomb collisions on electron loss in the inner radiation belt and slot region. At moderate relativistic energies, E∼500 keV, waves from VLF transmitters reduce electron lifetimes by an order of magnitude or more, down to the order of 200 days near the outer edge of the inner radiation belt. However, VLF transmitter waves are ineffective at removing multi–megaelectron volt electrons from either the inner radiation belt or slot region.

Empirically Estimated Electron Lifetimes in the Earth's Radiation Belts: Van Allen Probe Observations

“…At energies greater than 1 MeV, the lifetime profiles show a somewhat different character, with less radial dependence and values in the 5-to 10-day range throughout the outer zone. As we detail in the companion paper, the general structure of the lifetime profiles as a function of energy and L is consistent with quasilinear pitch angle diffusion by various scattering mechanisms, such as Coulomb collisions (e.g., Cunningham et al, 2018) and VLF transmitters (e.g., Ma et al, 2017;Ross et al, 2019) for long lifetime effects and hiss/EMIC waves for shorter lifetime effects (e.g., below ∼1 MeV for hiss (e.g., Ripoll et al, 2019) and above ∼2 MeV for EMIC (e.g., Kersten et al, 2014)).…”

Empirically Estimated Electron Lifetimes in the Earth's Radiation Belts: Comparison With Theory

“…It is possible that including this additional wave power could reduce the lifetimes. For example, very recent works Ross et al, 2019) fit a Gaussian function to each VLF transmitter station, and the resulting lifetimes are somewhat lower than those calculated here, but again only by a factor of 2-3.…”

An Investigation of VLF Transmitter Wave Power in the Inner Radiation Belt and Slot Region

“…In this case the estimated power is an upper limit. Using the full cold plasma dispersion relation, we find that the wave magnetic field derived from the wave electric field assuming a wave normal angle of 65 • is a factor of ∼0.65 less than that derived for parallel propagation for transmitters in the region 1.2 < L * < 1.5 (Ross et al, 2019). This would reduce the total intensity from 5 to 3 pT 2 in the region 1.3 ≤ L * ≤ 1.4.…”