We quantify the electron scattering effects of simultaneous plasmaspheric hiss and magnetosonic waves that occurred in two neighboring time intervals but with distinct wave intensity profiles on 21 August 2013. Their combined scattering is found capable of causing electron distribution variations largely distinguishable from the consequences of individual waves. The net effect of electron diffusion relies strongly on the relative dominance of the two wave intensities, which also controls the relative contribution of each wave mode. In combination, MS waves slow down the hiss‐induced loss of ~100 keV electrons, and hiss efficiently inhibits the electron butterfly distribution caused by MS waves to produce a gradual acceleration process. Our results strongly suggest that comprehensive simulations of the radiation belt electron dynamics should carefully incorporate the combined scattering and complex competition resulting from simultaneous occurrences of various magnetospheric emissions, including, but not limited to, plamaspheric hiss and magnetosonic waves.

In the past few decades, numerous efforts have been made to advance our understanding of the Earth's radiation belt electron dynamics (Li & Hudson, 2019; Ripoll et al., 2020, and references therein), where wave-particle interactions play an important role in various electron acceleration and loss processes (Baker, 2021;Thorne, 2010; Thorne et al., 2021, and references therein). The study of Zhang et al. (2021) reported, for the first time, an upper limit of radiation belt electron fluxes over a wide energy range from hundreds of keV to multi-MeV based on 7-year DEMETER and 6-year Van Allen Probes measurements. The observed upper flux limit of the 100 keV-1 MeV electrons is roughly inversely proportional to the kinetic energy in the outer belt, which is consistent with theoretical predictions by Summers and Shi (2014) based on the Kennel-Petschek (KP) theory (Kennel & Petschek, 1966). In the KP theory, the pitch-angle diffusion due to wave-particle interactions leads to particle precipitation into the ionosphere and results in trapped anisotropic electron population, which itself generates whistler-mode waves and leads to further precipitation, and the wave growth rate is limited by the wave damping, leading to self-limited electron fluxes. In addition, based on superposed epoch analysis of 70 geomagnetic storms, the study of Olifer et al. ( 2021) revealed consistency of the upper limit of electron fluxes with energies <∼850 keV between the observations and the theoretical results from KP theory. However, the upper limit of MeV electron fluxes is not well captured by KP theory in either Olifer et al. (2021) or Zhang et al. (2021), since the electron fluxes at such high energies typically do not result in wave growth, and this high energy upper flux limit still remains not fully understood.

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.

Whistler mode hiss acts as an important loss mechanism contributing to the radiation belt electron dynamics inside the plasmasphere and plasmaspheric plumes. Based on Van Allen Probes observations from September 2012 to December 2015, we conduct a detailed analysis of hiss properties in plasmaspheric plumes and illustrate that corresponding to the highest occurrence probability of plumes at L = 5.0–6.0 and MLT = 18–21, hiss emissions occur concurrently with a rate of >~80%. Plume hiss can efficiently scatter ~10‐ to 100‐keV electrons at rates up to ~10−4 s−1 near the loss cone, and the resultant electron loss timescales vary largely with energy, that is, from less than an hour for tens of kiloelectron volt electrons to several days for hundreds of kiloelectron volt electrons and to >100 days for >5‐MeV electrons. These newly obtained statistical properties of plume hiss and associated electron scattering effects are useful to future modeling efforts of radiation belt electron dynamics.