Artificial modification of Earth's radiation belts by ground-based very-low-frequency (VLF) transmitters

Wave-particle interactions play a fundamental role in the dynamic variability of Earth's donut-shaped radiation belts that are highly populated by magnetically trapped energetic particles and characteristically separated by the slot devoid of high energetic electrons. Owing to the continuous accumulation of high-quality wave and particle measurements from multiple satellites in geospace, the important contribution of ground-based very-low-frequency (VLF) transmitter waves to the electron dynamics in the near-Earth space has been unprecedently advanced, in addition to those established findings of the significant effects of a variety of naturally occurring magnetospheric waves. This paper focuses on the artificial modification of Earth's inner radiation belt and slot by artificial VLF transmitter emissions. We review the global distributions of VLF transmitter waves in geospace, their scattering effects on radiation belt electrons in terms of both theoretical and observational analyses, and diffusion simulation results of wave-particle interactions along with data-model comparisons. We start with a brief review of the radiation belt electron dynamics and an introduction of anthropogenic VLF transmitter waves. Subsequently, we review the global morphology of in situ VLF transmitter waves corresponding to different transmitter locations, including their day-night asymmetry, geographic distributions, seasonal and geomagnetic activity dependence, and wave propagation features. Existed theoretical and observational analyses of electron scattering effects by VLF transmitter waves are then reviewed to approach the underlying physics that can modulate the spatio-temporal variations of the electron radiation belts. Further Fokker-Planck electron diffusion simulations and their comparisons with realistic satellite observations clearly indicate that VLF transmitter emissions can effectively remove energetic electrons to produce a radially bifurcated electron belt, thereby quantitatively confirming the direct link between operations of VLF transmitters at ground and changes of the energetic electron environment in space. We finally discuss the unsolved problems and possible future research in this area, which has important implications for potential mitigation of the natural particle radiation environment with active means.