Properties of Intense H-Band Electromagnetic Ion Cyclotron Waves: Implications for Quasi-Linear, Nonlinear, and Nonresonant Wave-Particle Interactions

Resonant interactions between relativistic electrons and electromagnetic ion cyclotron (EMIC) waves provide an effective loss mechanism for this important electron population in the outer radiation belt. The diffusive regime of electron scattering and loss has been well incorporated into radiation belt models within the framework of the quasi-linear diffusion theory, whereas the nonlinear regime has been mostly studied with test particle simulations. There is also a less investigated, nonresonant regime of electron scattering by EMIC waves. All three regimes should be present, depending on the EMIC waves and ambient plasma properties, but the occurrence rates of these regimes have not been previously quantified. This study provides a statistical investigation of the most important EMIC wave-packet characteristics for the diffusive, nonlinear, and nonresonant regimes of electron scattering. We utilize 3 years of observations to derive distributions of wave amplitudes, wave-packet sizes, and rates of frequency variations within individual wave-packets. We demonstrate that EMIC waves typically propagate as wave-packets with similar to 10 wave periods each, and that similar to 3-10% of such wave-packets can reach the regime of nonlinear resonant interaction with 2-6 MeV electrons. We show that EMIC frequency variations within wave-packets reach 50-100% of the center frequency, corresponding to a significant high-frequency tail in their wave power spectrum. We explore the consequences of these wave-packet characteristics for high and low energy electron precipitation by H-band EMIC waves and for the relative importance of quasi-linear and nonlinear regimes of wave-particle interactions. Most intense H-band electromagnetic ion cyclotron (EMIC) wave packets are short but 3%-10% may lead to nonlinear interactionsThe power spectrum of intense H-band EMIC wave packets contains a significant high-frequency tailResonant interactions with intense H-band EMIC wave packets likely play an important role in sub-relativistic electron precipitation