Luisa Capannolo

We investigate the association between wave‐driven relativistic (>700 keV) electron precipitation (REP) and the two main geomagnetically effective solar wind (SW) drivers: coronal mass ejections (CMEs) and high‐speed streams (HSSs). We combine SW observations from OMNI and a data set of clear wave‐driven REP events (covering 2012–2023, from low Earth orbit electron measurements from the Polar Operational Environmental Satellites and Meteorological Operational satellites) to determine when and under which conditions CMEs and HSSs lead to REP. Out of 91 CMEs and 309 HSSs, 76%, and 70% are associated with REP, respectively, indicating that both drivers are effective at triggering relativistic electron loss into the atmosphere. However, only ∼25% of all REP events over 2012–2023 are associated with these major SW structures, suggesting that large‐scale drivers are not required in most cases. We also found that the strongest controlling factor for REP occurrence is not the driver type or storm intensity alone, but the preconditioning of the outer belt: drivers associated with REP typically impact a magnetosphere with elevated fluxes of trapped >700 keV electrons. REP occurs primarily during the main phase, although HSS‐driven REP is more often sustained into the recovery phase. In terms of properties, HSS‐driven REP appears broader in radial extent and is associated with higher electron flux, whereas CME‐driven REP shows larger precipitation‐to‐trapped flux ratios, indicating more efficient wave scattering. These results support a scenario in which SW forcing, storm timing, and radiation belt preconditioning jointly control the occurrence and properties of wave‐driven REP.

Hiss waves frequently occur in the plasmasphere or plumes, playing a key role in energetic electron loss in the Earth's inner magnetosphere. While previous studies have linked hiss wave enhancements in the outer plasmasphere (just inside the plasmapause) to electron injections during substorms, their evolution across various substorm phases remains unclear. Using Van Allen Probes observations over 2013–2019, we evaluate hiss wave evolution during various phases of substorm activity. At L > 4, both hiss wave intensity and energetic electron flux increase shortly after substorm onset, first on the morning side, then progress to later magnetic local times (MLTs) at a rate of ∼1–3 hr MLT per hr in universal time (UT), eventually stabilizing near 13 MLT. Stronger substorms result in larger and faster intensification in hiss wave intensity and have more significant impact at lower L‐shells. Our results highlight the global variation of hiss waves during substorms.

We analyze the solar wind (SW) properties associated with relativistic electron precipitation (REP) observed at low‐Earth orbit. A statistical analysis is performed on the SW associated with ∼7,000 REP events likely driven by wave‐particle interactions. We analyze OMNI data to quantify the SW properties prior to REP and reveal temporal patterns that are favorable for electron precipitation. Compared to typical values, SW associated with REP typically exhibits a stronger North‐South interplanetary magnetic field B _z component and higher plasma density, indicating dayside reconnection and compression. REP events observed at low L shell ( L ≲ 4), particularly from noon to post‐midnight, are triggered by enhanced B _z and density. Dayside REP is associated with slightly faster SW, whereas dawnside REP coincides with high plasma density without strong B _z . A typical SW trend leading to REP exhibits enhanced density, magnetic field increase, and a B _z minimum before the REP observation. Through k ‐means clustering, we further identify three distinct SW temporal profiles. Dayside REP at high L shells is associated with ∼500 km/s speeds, duskside REP is associated with enhanced SW density prior to REP, and REP over noon‐to‐dusk at low L shells is triggered by strong dayside reconnection. Furthermore, we found that REP events are more frequent during the declining phase of the solar cycle, with a 6‐month periodicity in occurrence rate. The findings in this study are critical to establishing a relationship between SW and REP, enabling future modeling efforts to predict REP from SW observations.

Pitch‐angle diffusion is a key mechanism driving radiation belt electron loss into the atmosphere. However, traditional bounce‐averaged models, which assume complete atmospheric absorption for loss cone electrons, cannot accurately quantify the loss cone fluxes. This shortfall arises primarily from two issues: bounce‐averaging assumptions generally breakdown within the loss cone, and atmospheric backscatter effects are omitted. In this study, we use a modified bounce‐averaged pitch‐angle diffusion model to show that the inclusion of backscatter effects inside the loss cone can adequately estimate the pitch‐angle distribution of electrons inside the loss cone for the case of Electromagnetic Ion Cyclotron (EMIC) wave‐driven precipitation. In this modified model, atmospheric backscatter effects are incorporated as an additional backscatter‐induced pitch‐angle diffusion (), and an attenuation of atmospheric absorption (). Here, we quantify the values of and using Geant4‐based Monte Carlo atmospheric backscatter simulation and applied to model electron precipitation driven by EMIC waves. Comparisons with ELFIN satellite observations during EMIC events reveal that the inclusion of atmospheric backscatter resolves previously reported discrepancies at electron energies . For electrons, where EMIC wave diffusion is dominant over backscatter‐induced diffusion, backscatter effects become negligible. Notably, we find that accounts for nearly all the observed enhancements in the loss cone flux, while plays a secondary role.

We analyze the solar wind (SW) properties associated with relativistic electron precipitation (REP) observed at low-Earth orbit (LEO). A statistical analysis is performed on the SW associated with ~7,000 REP events likely driven by wave-particle interactions. We analyze OMNI data to quantify the SW properties prior to REP and reveal temporal patterns that are favorable for electron precipitation. Compared to typical values, SW associated with REP typically exhibits a stronger North-South IMF (interplanetary magnetic field) Bzcomponent and higher plasma density, indicating dayside reconnection and compression. REP events observed at low L shell (L≲4), particularly from noon to post-midnight, are triggered by enhanced Bz and density. Dayside REP is associated with slightly faster SW, while dawnside REP coincides with high plasma density without strong Bz. A typical SW trend leading to REP exhibits enhanced density, magnetic field increase, and a Bz minimum before the REP observation. Through k-means clustering, we further identify three distinct SW temporal profiles. Dayside REP at high L shells is associated with ~500 km/s speeds, duskside REP is associated with enhanced SW density prior to REP, and REP over noon-to-dusk at low L shells is triggered by strong dayside reconnection. Furthermore, we found that REP events are more frequent during the declining phase of the solar cycle, with a 6-month periodicity in occurrence rate. The findings in this study are critical to establishing a relationship between SW and REP, enabling future modeling efforts to predict REP from SW observations.

Pitch-angle diffusion is a key mechanism driving radiation belt electron loss into the atmosphere. However, traditional bounce-averaged models, which assume complete atmospheric absorption for loss cone electrons, cannot accurately quantify the loss cone fluxes. This shortfall arises primarily from two issues: bounce-averaging assumptions generally breakdown within the loss cone, and atmospheric backscatter effects are omitted. In this study, we use a modified bounce-averaged pitch-angle diffusion model to show that the inclusion of backscatter effects inside the loss cone can adequately estimate the pitch-angle distribution of electrons inside the loss cone for the case of Electromagnetic Ion Cyclotron (EMIC) wave-driven precipitation. In this modified model, atmospheric backscatter effects are incorporated as an additional backscatter-induced pitch-angle diffusion (Db), and an attenuation of atmospheric absorption (Fb). Here, we quantify the values of Db and Fb using Geant4-based Monte Carlo atmospheric backscatter simulation and applied to model electron precipitation driven by EMIC waves. Comparisons with ELFIN satellite observations during EMIC events reveal that the inclusion of atmospheric backscatter resolves previously reported discrepancies at electron energies 1 MeV electrons, where EMIC wave diffusion is dominant over backscatter-induced diffusion, backscatter effects become negligible. Notably, we find that Db accounts for nearly all the observed enhancements in the loss cone flux, while Fb plays a secondary role.

Whistler‐mode chorus and hiss waves play an important role in Earth's radiation belt electron dynamics. Direct measurements of whistler wave‐driven electron precipitation and the resultant pitch angle distribution were previously limited by the insufficient resolution of low Earth orbit satellites. In this study, we use recent measurements from the Electron Losses and Fields INvestigation CubeSats, which provide energy‐ and pitch angle‐resolved electron distributions to statistically evaluate electron scattering properties driven by whistler waves. Our survey indicates that events with increasing precipitating‐to‐trapped flux ratios (evaluated at 63 keV unless otherwise specified) correlate with increasing trapped flux at energies up to ∼750 keV. Weak precipitation events (precipitation ratio 5 over midnight‐to‐noon local times during disturbed geomagnetic conditions. These results are crucial for characterizing the whistler‐mode wave driven electron scattering properties and evaluating its impact on the ionosphere.

Electromagnetic ion cyclotron (EMIC) waves are known to be efficient for precipitating >1 MeV electrons from the magnetosphere into the upper atmosphere. Despite considerable evidence showing that EMIC‐driven electron precipitation can extend down to sub‐MeV energies, the precise physical mechanism driving sub‐MeV electron precipitation remains an active area of investigation. In this study, we present an electron precipitation event observed by ELFIN CubeSats on 11 January 2022, exclusively at sub‐MeV energy at L ∼ 8–10.5, where trapped MeV electrons were nearly absent. The THEMIS satellites observed conjugate H‐band and He‐band EMIC waves and hiss waves in plasmaspheric plumes near the magnetic equator. Quasi‐linear diffusion results demonstrate that the observed He‐band EMIC waves, with a high ratio of plasma to electron cyclotron frequency, can drive electron precipitation down to ∼400 keV. Our findings suggest that exclusive sub‐MeV precipitation (without concurrent MeV precipitation) can be associated with EMIC waves, especially in the plume region at high L shells.

Whistler mode waves scatter energetic electrons, causing them to precipitate into the Earth's atmosphere. While the interactions between whistler mode waves and electrons are well understood, the global distribution of electron precipitation driven by whistler mode waves needs futher investigations. We present a two‐stage method, integrating neural networks and quasi‐linear theory, to simulate global electron precipitation driven by whistler mode waves. By applying this approach to the 17 March 2013 geomagnetic storm event, we reproduce the rapidly varying precipitation pattern over various phases of the storm. Then we validate our simulation results with POES/MetOp satellite observations. The precipitation pattern is consistent between simulations and observations, suggesting that most of the observed electron precipitation can be attributed to scattering by whistler mode waves. Our results indicate that chorus waves drive electron precipitation over the premidnight‐to‐afternoon sector during the storm main phase, with simulated peak energy fluxes of 20 erg/cm²/s and characteristic energies of 10–50 keV. During the recovery phase, plume hiss in the afternoon sector can have a comparable or stronger effect than chorus, with peak fluxes of ∼1 erg/cm²/s and characteristic energies between 10 and 200 keV. This study highlights the importance of integrating physics‐based and deep learning approaches to model the complex dynamics of electron precipitation driven by whistler mode waves.