Resonant scattering of outer zone relativistic electrons by multiband EMIC waves and resultant electron loss time scales

TLDR

The study aims to clarify the role of EMIC waves in radiation‑belt dynamics by analyzing resonant scattering of outer‑zone relativistic electrons and their loss time scales across EMIC wave bands, L shells, and wave‑normal‑angle models. The authors conduct a comprehensive analysis of EMIC‑wave–induced resonant scattering of outer‑zone relativistic (>0.5 MeV) electrons, evaluating loss time scales as functions of EMIC wave band, L shell, and wave‑normal‑angle model. The results show that H⁺‑band EMIC waves dominate scattering of 1–4 MeV electrons, while He⁺ and O⁺ bands dominate higher‑energy ultrarelativistic electrons; scattering efficiency rises with L shell, falls with obliquity, and the latitudinally varying wave‑normal‑angle model reduces H⁺‑band rates at low pitch angles, producing rapid loss‑cone filling and a top‑hat pitch‑angle distribution within minutes to hours.

Abstract

Abstract To improve our understanding of the role of electromagnetic ion cyclotron (EMIC) waves in radiation belt electron dynamics, we perform a comprehensive analysis of EMIC wave‐induced resonant scattering of outer zone relativistic (>0.5 MeV) electrons and resultant electron loss time scales with respect to EMIC wave band, L shell, and wave normal angle model. The results demonstrate that while H + ‐band EMIC waves dominate the scattering losses of ~1–4 MeV outer zone relativistic electrons, it is He + ‐band and O + ‐band waves that prevail over the pitch angle diffusion of ultrarelativistic electrons at higher energies. Given the wave amplitude, EMIC waves at higher L shells tend to resonantly interact with a larger population of outer zone relativistic electrons and drive their pitch angle scattering more efficiently. Obliquity of EMIC waves can reduce the efficiency of wave‐induced relativistic electron pitch angle scattering. Compared to the frequently adopted parallel or quasi‐parallel model, use of the latitudinally varying wave normal angle model produces the largest decrease in H + ‐band EMIC wave scattering rates at pitch angles < ~40° for electrons > ~5 MeV. At a representative nominal amplitude of 1 nT, EMIC wave scattering produces the equilibrium state (i.e., the lowest normal mode under which electrons at the same energy but different pitch angles decay exponentially on the same time scale) of outer belt relativistic electrons within several to tens of minutes and the following exponential decay extending to higher pitch angles on time scales from <1 min to ~1 h. The electron loss cone can be either empty as a result of the weak diffusion or heavily/fully filled due to approaching the strong diffusion limit, while the trapped electron population at high pitch angles close to 90° remains intact because of no resonant scattering. In this manner, EMIC wave scattering has the potential to deepen the anisotropic distribution of outer zone relativistic electrons by reshaping their pitch angle profiles to “top‐hat.” Overall, H + ‐band and He + ‐band EMIC waves are most efficient in producing the pitch angle scattering loss of relativistic electrons at ~1–2 MeV. In contrast, the presence of O + ‐band EMIC waves, while at a smaller occurrence rate, can dominate the scattering loss of 5–10 MeV electrons in the entire region of the outer zone, which should be considered in future modeling of the outer zone relativistic electron dynamics.