Resonant scattering and resultant pitch angle evolution of relativistic electrons by plasmaspheric hiss

We perform a comprehensive analysis to evaluate hiss‐induced scattering effect on the pitch angle evolution and associated decay processes of relativistic electrons. The results show that scattering by the equatorial, highly oblique hiss component is negligible. Quasi‐parallel approximation is good for evaluation of hiss‐driven electron scattering rates ≤ 2 MeV. However, realistic wave propagation angles as a function of latitude must be considered to accurately quantify hiss scattering rates above 2 MeV, and ambient plasma density is also a critical parameter. While the first‐order cyclotron and the Landau resonances are dominant for hiss scattering < 2 MeV electrons, higher‐order resonances become important and even dominant at intermediate pitch angles for ultrarelativistic (≥ 3 MeV) electrons. Hiss‐induced electron pitch angle evolution shows an initially rapid transport from high to lower pitch angles, with a gradual approach toward equilibrium, and a final exponential decay as a whole. Although hiss scattering rates near the loss cone control the pitch angle evolution and the ultimate loss of ultrarelativistic electrons, the scattering bottleneck significantly affects the loss rate and leads to characteristic top hat‐shaped pitch angle distributions at energies < 1 MeV. Decay timescales are on the order of a few days, tens of days, and > 100 days for 500 keV, 2 MeV, and 5 MeV electrons, respectively, consistent with recent observations from the Van Allen Probes and indicating that scattering by hiss can realistically account for the long‐term loss process and the pitch angle evolution of relativistic electrons in the plasmasphere following storm time injections.