Radiation Belt Electron Acceleration Driven by Very‐Low‐Frequency Transmitter Waves in Near‐Earth Space

• Published in: Geophysical research letters Vol. 49; no. 10
• Main Authors: Hua, Man, Bortnik, Jacob, Ma, Qianli, Bernhardt, Paul A.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.05.2022
• Subjects: Acceleration; Amplification; Belt drives; Diffusion; Electron acceleration; Electron precipitation; Hiss; Lightning; Pitch (inclination); Plasma waves; quasi‐linear diffusion simulation; Radiation; radiation belt electron acceleration; Radiation belts; Rocket exhaust; Scattering; Transmitters; Very Low Frequencies; VLF transmitter waves; VLF waves; Wave amplitude; wave–particle interactions; Whistlers
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2022GL099258

We perform two‐dimensional Fokker–Planck diffusion simulations to quantify the role of Very‐Low‐Frequency (VLF) transmitter waves and other naturally occurring plasma waves in electron acceleration over L = 1.5–3.0. VLF transmitter waves play a dual role in electron acceleration at higher energies from ∼200 to ∼700 keV through energy diffusion, and losses at lower energies below ∼100 keV through pitch angle scattering. Due to the now‐achievable rocket exhaust driven amplification (REDA) of VLF waves suggested by recent studies, control of wave‐induced acceleration can be actively tested with various VLF wave intensities in space. With amplification by a factor of 5, the acceleration by VLF transmitters can overcome the losses by lightning‐generated whistlers and plasmaspheric hiss, leading to net acceleration in the combined scattering. The acceleration occurs within 1 min with amplification factor of 50 dB, which is promising to be observable in the future REDA experiment, representing a feasible test of the theory. Plain Language Summary Very‐Low‐Frequency (VLF) transmitter waves have been well known to drive important electron precipitation in the near‐Earth space. However, the capability of VLF transmitters to accelerate electrons has mostly been overlooked and assumed to be negligible. By performing two‐dimensional Fokker–Planck diffusion simulations, we quantify the role of VLF transmitters and other naturally occurring plasma waves in electron acceleration in the inner belt and slot region. We found that VLF transmitter waves can simultaneously accelerate higher energy electrons and drive electron loss at lower energies. However, the net acceleration is typically suppressed by stronger pitch angle diffusion due to lightning‐generated whistlers and plasmaspheric hiss. Recent studies have reported the achievable rocket exhaust driven amplification (REDA) of VLF transmitter waves by 20–30 dB. We adopt various amplification factors to determine the preferred wave amplitude of VLF transmitters for acceleration. The critical amplification factor is 5, when the acceleration is enough to overcome the combined scattering causing net acceleration. The acceleration occurs within 1 min with amplification factor of 50 dB, which can be achieved in the future REDA experiment. Our study provides the first estimation of the capability and efficiency of VLF transmitter waves to accelerate electrons and provides a feasible test to theoretical predictions. Key Points VLF transmitter waves play a dual role in electron acceleration from ∼200 to ∼700 keV and loss below ∼100 keV at L = 1.5–3.0 The slow electron acceleration by VLF transmitter waves is typically suppressed by the strong pitch angle scattering due to hiss and LGW Amplifying VLF wave intensity by a factor of 5 can drive acceleration by combined scattering, which is enhanced with stronger VLF waves