2024/03/01 by Kyungguk Min · 1 voice
Physics and Astronomy · Earth and Planetary Sciences · #Ionosphere and magnetosphere dynamics #Solar and Space Plasma Dynamics #Tropical and Extratropical Cyclones Research
paper · doi:10.1029/2023ja032387
openalex publication_date 2024/03/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/05/21
Abstract Narrowband (Δ f ≲ 0.1 f cp ), high‐frequency (0.9 f cp ≲ f < f cp ) electromagnetic ion cyclotron (EMIC) waves, or HFEMIC waves for short, are a relatively new type of EMIC waves, where f cp is the proton cyclotron frequency. Here, we investigate the instability threshold conditions and the nonlinear evolution of HFEMIC waves at parallel propagation. First, linear theory analysis is extended to a regime relevant to HFEMIC waves (parallel proton beta β ‖ p < 0.01). The instability threshold follows a similar anisotropy‐ β ‖ p relation and requires a large value of anisotropy ( T ⊥ p / T ‖ p ≳ 10) for wave growth. As a result of decreasing group velocity, the convective growth rate at a fixed threshold exhibits a similar inverse relation with β ‖ p . Heavy ions affect the instability only weakly, primarily through the introduction of stop bands. Second, we carry out one‐dimensional hybrid simulations in a parabolic background magnetic field, with initial parameters constrained by observation. Despite the narrow source region (within about ±3° latitude), HFEMIC waves can grow well above the thermal noise level due in large part to a small group velocity of HFEMIC waves. The saturation level is well within the range of observational amplitudes, and the quasilinear process primarily determines the wave evolution. Finally, we demonstrate that the present results compare favorably to the recent statistical results, thereby supporting anisotropic low‐energy protons as free energy source for HFEMIC waves.