Abstract
Very-Low-Frequency (VLF) transmitters operate worldwide mostly at frequencies of 10–30 kilohertz for submarine communications. While it has been of intense scientific interest and practical importance to understand whether VLF transmitters can affect the natural environment of charged energetic particles, for decades there remained little direct observational evidence that revealed the effects of these VLF transmitters in geospace. Here we report a radially bifurcated electron belt formation at energies of tens of kiloelectron volts (keV) at altitudes of ~0.8–1.5 Earth radii on timescales over 10 days. Using Fokker-Planck diffusion simulations, we provide quantitative evidence that VLF transmitter emissions that leak from the Earth-ionosphere waveguide are primarily responsible for bifurcating the energetic electron belt, which typically exhibits a single-peak radial structure in near-Earth space. Since energetic electrons pose a potential danger to satellite operations, our findings demonstrate the feasibility of mitigation of natural particle radiation environment.
Highlights
Very-Low-Frequency (VLF) transmitters operate worldwide mostly at frequencies of 10–30 kilohertz for submarine communications
The spatiotemporal variations of a double-peaked radial profile of energetic electron fluxes were observed at energies of tens of kiloelectron volts (keV) at L < 3.0 during a 15-day period from 20 February to 6 March in 2016, using high-resolution electron flux measurements from Radiation Belt Storm Probes Ion
Characterized by local flux minima at L ~ 2.0–2.2 and resulting from the decay of electron fluxes at energies of tens of keV, the bifurcated electron belt was distinct from the typical structure of energetic electrons peaking at L ~ 2.0–2.524 before 12 UT on 21 February
Summary
Very-Low-Frequency (VLF) transmitters operate worldwide mostly at frequencies of 10–30 kilohertz for submarine communications. While propagating mostly within the Earth-ionosphere waveguide, which is bounded by the terrestrial surface and the lower ionosphere at altitudes about 90 km, VLF transmitter signals can penetrate through the imperfectly reflecting ionosphere, being guided by the gradients of the Earth’s magnetic field, to leak a portion of their power into the Earth’s magnetosphere primarily at L < 35–8 (where L is the geocentric distance in Earth radii of the location where the corresponding magnetic field line crosses the geomagnetic equator) As a result, these transmitter signals, together with naturally occurring plasma waves originating from lightning, plasmaspheric hiss and magnetosonic waves at low L-shells, encounter a population of geomagnetically trapped energetic electrons up to ~1 MeV. Our results provide quantitative direct evidence to link operations of VLF transmitters at ground to changes of the energetic electron environment in geospace
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