Superfast precipitation of energetic electrons in the radiation belts of the Earth

Earth's outer radiation belt, a torus-shaped region near the planet, contains energetic electrons. These electron fluxes dramatically increase during geomagnetic storms, posing a threat to satellites. Understanding the mechanisms controlling these fluxes, which involve a balance between acceleration and loss processes, is crucial. Whistler waves play a significant role in both accelerating electrons to relativistic energies and scattering them in pitch angle, leading to precipitation into the atmosphere. Classical quasi-linear theory, which models this interaction as a diffusion process, predicts an upper limit for electron precipitation. However, this paper investigates observations that challenge this theoretical limit.

The quasi-linear theory of electron precipitation assumes moderately fast diffusive interactions with plasma waves, implying that precipitating electron fluxes cannot exceed trapped electron fluxes. Previous research has explored whistler-wave interactions with electrons, primarily in the quasi-linear regime, where the random-phase assumption is made. However, this assumption may not hold for intense wave packets. Coherent nonlinear interactions, such as advection and trapping, can lead to significantly faster electron transport than predicted by quasi-linear theory. While faster nonlinear processes can still lead to precipitation, these processes have not been fully integrated into existing radiation belt models.

This study utilizes data from low-altitude Electron Losses and Fields Investigation (ELFIN) spacecraft and conjugate high-altitude Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft. ELFIN provides high-resolution measurements of energetic electron fluxes at all latitudes, while THEMIS measures near-equatorial plasma waves. The researchers analyzed conjugate measurements from ELFIN and THEMIS, focusing on events where the precipitating electron fluxes exceeded the trapped fluxes, a phenomenon termed "loss-cone overfilling." Numerical simulations were conducted to model electron interactions with intense oblique whistler waves, considering nonlinear Landau resonance. These simulations incorporated observed equatorial electron spectra, wave intensities, and frequencies from THEMIS to predict electron energy and pitch-angle distributions at ELFIN's altitude. The statistical significance of loss-cone overfilling was assessed by analyzing five months of ELFIN data in the dayside inner magnetosphere.

The key finding is the frequent observation of "superfast" precipitation where the loss-cone fluxes significantly exceed the trapped fluxes, contradicting the predictions of quasi-linear theory. This superfast precipitation was observed by ELFIN at low altitudes, and correlated with intense oblique whistler-mode waves observed by THEMIS at higher altitudes. Numerical simulations confirmed that nonlinear electron interactions with these oblique waves via Landau resonance can explain the observed loss-cone overfilling. Specifically, the intense oblique waves trap electrons and rapidly transport them into the loss cone, resulting in a substantial increase in precipitating fluxes, particularly for electrons with energies below 200 keV. Analysis of five months of ELFIN data indicated that loss-cone overfilling (jprec/jtrap > 1) occurred in approximately 10% of all precipitation events at L ~ 10–12, and 5% at L ~ 6–9. ERG satellite observations supported the presence of this phenomenon at lower energies (10–20 keV), further strengthening the proposed mechanism. The energy increase during this process, due to magnetic moment conservation, was shown to reach up to several tens of keV before electrons are released.

The findings demonstrate that nonlinear interactions with intense oblique whistler-mode waves are a significant driver of energetic electron precipitation, and a mechanism not adequately represented in current radiation belt models. The high occurrence rate of loss-cone overfilling highlights the importance of incorporating nonlinear wave-particle interactions into these models to accurately predict radiation belt fluxes and the strength of space-atmosphere coupling. The superfast precipitation can significantly enhance electron losses, suppressing the source electron fluxes and potentially impacting the acceleration process to relativistic energies. The effect extends to lower atmospheric altitudes, potentially influencing atmospheric properties and even the local climate.

This study reveals the prevalence of superfast electron precipitation in Earth's radiation belts due to nonlinear interactions with oblique whistler-mode waves. This process, significantly exceeding the limits of classical quasi-linear theory, is shown to be a significant contributor to electron losses and energy deposition in the upper atmosphere. The high occurrence rate underscores the need to include nonlinear effects in models of radiation belt dynamics and space-atmosphere coupling. Future research should focus on further refining the modelling of these nonlinear interactions, incorporating them into comprehensive radiation belt models and investigating the long-term consequences of this phenomenon on atmospheric chemistry and climate.

The study primarily focuses on dayside observations to minimize the influence of isotropic electron precipitation from the plasma sheet. Further investigation is needed to fully understand the role of superfast precipitation at other local times and under different geomagnetic conditions. While the simulations successfully reproduce the observed loss-cone overfilling, the complex nature of wave-particle interactions and the simplifications in the model warrant further validation and refinement.