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Electromagnetic ion cyclotron (EMIC) waves can cause relativistic electron scattering and atmospheric precipitation, primarily via cyclotron resonant interactions in the Earth's radiation belts. However, the conventional quasilinear resonance theory suggests that the cyclotron resonance condition is not satisfied for 90 degrees pitch angle (PA) electrons, which constitute the majority of electrons in the outer radiation belt, such that scattering mainly affects low-PA electrons. In contrast to this theory, using test particle calculations, we demonstrate that even exactly 90 degrees PA electrons can be significantly scattered by large-amplitude EMIC waves. The finite wave force results in the parallel transport of 90 degrees PA electrons away from the equator, corresponding to intrinsically nonresonant scattering. This can lead to parallel velocity that meets cyclotron resonance conditions as local PA deviates from 90 degrees. Different types of resonance are identified depending on the wave normal angle, that is, first-and second-order resonances for parallel and oblique waves, respectively.

Plain Language Summary The Earth's outer radiation belts are filled with very high energy (on the order of 106 electron volts) electrons. The electron population of the belts often experiences dynamic changes, ranging from decreases to the background noise level to increases to a few orders of magnitude. Electromagnetic ion cyclotron (EMIC) waves have long been considered a cause of electron flux decreases through loss into the atmosphere due to the scattering of electron motion. However, a currently popular theory, called "quasilinear resonance," suggests that the scattering of electrons that circulate primarily in the perpendicular plane relative to the background magnetic field (so-called "equatorial particles") is inefficient. Such equatorial electrons typically constitute the majority of the particles in radiation belts. In our new study, we use a different approach (test particle calculations) to demonstrate that such equatorial electrons can actually be scattered by EMIC waves when the wave amplitude is sufficiently large. This finding implies that EMIC waves affect the electron distribution far more significantly and in a different way than expected based on conventional quasilinear theory. The results are applicable to related problems in other planetary magnetospheres and astrophysical situations.