Relativistic Electrons — The Key to Coherent Auroral Emissions

The study investigates the role of relativistic electron precipitation (REP) in the generation of coherent auroral radio emissions in the 1–10 MHz band, with a particular focus on the mechanisms governing cyclotron maser instability in Earth's magnetosphere. Using a multi-instrument, multi-scale dataset comprising satellite measurements from Van Allen Probes, Arase, and THEMIS, in conjunction with ground-based observatories such as EISCAT and HAARP, the author identifies a critical energy threshold of (1.82 ± 0.18) MeV for REP events capable of triggering coherent radio emissions Through Bayesian threshold analysis and wavelet-based temporal-spectral correlation techniques, the paper establishes a statistically significant correlation (r ≈ 0.82, p < 0.001) between suprathreshold electron fluxes and narrowband radio signatures characterized by high quality factors (Q > 15) and right-hand circular polarization. Spectral analysis reveals emissions with peak frequencies tied to local gyrofrequencies and narrow bandwidths (Δf ≈ 0.1–0.3 MHz), temporally coincident with electron microbursts on sub-100 ms timescales. The proposed mechanism is grounded in cyclotron resonance conditions and anisotropic electron velocity distributions, satisfying the classical criteria for electron cyclotron maser instability. The results not only provide empirical confirmation of theoretical predictions but also challenge existing paradigms of quasi-linear diffusion by highlighting the importance of nonlinear phase-locking and particle bunching effects. Moreover, the findings carry significant implications for space weather modeling. Specifically, the identified energy threshold and emission characteristics offer a novel diagnostic for real-time forecasting of ionospheric radio noise environments during geomagnetic disturbances. The study further recommends incorporating relativistic electron-driven emissions into next-generation magnetosphere-ionosphere coupling models and global radiation belt simulations.