Precipitation of Relativistic Electrons Into Atmosphere From Earth's Radiation Belts

The near-Earth space environment is filled with a collisionless gas of charged particles—protons and electrons—some populations of which can reach extreme energies. In particular, relativistic electrons populating the outer radiation belt—a donut-shaped region at ∼ 30 , 000 ~ 30,000 km from Earth—can attain energies of several MeV (multiples of their rest energy, m e c 2 ≈ 0.511 mₑc²≈ 0.511 MeV). Being charged, these electrons are largely controlled by the strong dipolar magnetic field of the Earth, and their motion is, in principle, regular and predictable. However, an ocean of plasma waves filling near-Earth space (Jaynes, 2025) strongly perturbs electron dynamics and can scatter electrons into the Earth's atmosphere. There, their interaction with atmospheric constituents produces a wide range of space weather phenomena that affect technological systems and everyday life. In a recent study, Xiang et al. (2026) demonstrated for the first time how a remarkably broad energy range of relativistic electrons can be involved in atmospheric precipitation and identified the key plasma wave mode responsible for this process. Using combined measurements from the Relativistic Electron and Proton Telescope integrated little experiment-2 onboard the Colorado Inner Radiation Belt Experiment CubeSat (CIRBE/REPTile-2; see Li et al., 2024; Li, 2024) and the Exploration of Energization and Radiation in Geospace satellite (Arase/ERG; see Miyoshi et al., 2018) (see Figure 1), together with detailed theoretical analysis, Xiang et al. (2026) addressed a long-standing question in radiation belt—and more broadly, space weather—physics: what is the precise energy spectrum of relativistic electron precipitation? Schematic view of the event: low-altitude Colorado Inner Radiation Belt Experiment (CIRBE) and near-equatorial Arase (ERG) probe the relativistic electron dynamics in the outer radiation belt. This advance builds on decades of radiation belt modeling efforts (see Thorne et al., 2021, and references therein) and a rich set of spacecraft observations that progressively revealed different elements of the precipitating electron spectrum puzzle (e.g., SAMPEX, FIREBIRD, and ELFIN; see Blum et al., 2015; Johnson et al., 2021; Zhang et al., 2022). The CIRBE/REPTile-2 measurements presented in Xiang et al. (2026), however, provide particularly clear evidence of a mechanism capable of producing electron precipitation across an exceptionally wide energy range. Within the outer radiation belt, different plasma waves preferentially interact with electrons of different energies. Electrons below the first tens of keV—those responsible for beautiful auroral displays and enhanced ionization of the upper ionosphere (above 100 km; see Nishimura et al., 2020)—are primarily scattered by electron cyclotron harmonic waves (e.g., Ni et al., 2016, and references therein), broadband electrostatic waves (e.g., Shen et al., 2024, and references therein), and whistler-mode waves (e.g., Kasahara et al., 2018; Nishimura et al., 2010, and references therein). In contrast, relativistic electrons (above ∼ 0.5 ~ 0.5 MeV), which can penetrate down to ∼ 50 ~ 50 km altitude and produce X-ray emission extending even lower (e.g., Xu et al., 2021), are generally thought to be precipitated by electromagnetic ion cyclotron waves (e.g., Angelopoulos et al., 2023, and references therein). Despite this apparent theoretical separation between low- and high-energy precipitation mechanisms, low-altitude satellites have frequently detected precipitation spanning a very broad energy range—from tens of keV up to several MeV (e.g., Zhang et al., 2022). A leading candidate for driving such broadband precipitation is a specific class of whistler-mode waves (see discussion in Artemyev et al., 2024; Kang et al., 2024; Miyoshi et al., 2020). These waves can be guided by plasma density irregularities, allowing them to propagate far from their equatorial source region. Along their propagation path, they resonate with progressively higher-energy electrons, thereby extending precipitation to increasingly relativistic energies and ultimately producing a very wide energy spectrum. Xiang et al. (2026) confirm this intriguing scenario through direct observations of broadband electron precipitation occurring simultaneously with whistler-mode waves detected far from their equatorial source. By resolving the long-standing question of how a broad energy range of electrons can be precipitated by a single wave mode, this study opens a new and compelling challenge for the radiation belt community: how to incorporate guided whistler-mode waves into state-of-the-art simulations and accurately reproduce relativistic electron losses and their impact on the Earth's atmosphere. A. Artemyev thanks S. Kamaletdinov for assistance with the figure and acknowledges support from NASA Grant 80NSSC25K7749. The authors declare no conflicts of interest relevant to this study. Data were not used, nor created for this research.