Analyzing EMIC Waves in the Inner Magnetosphere Using Long‐Term Van Allen Probes Observations

Abstract

Abstract With 64‐month magnetic data from Van Allen Probes, we have studied not only the global distribution, wave normal angle ( θ ), and ellipticity ( ε ) of electromagnetic ion cyclotron (EMIC) waves, but also the dependence of their occurrence rates and magnetic amplitudes on the AE * index (the mean value of AE index over previous 1 hr). Our results show that H + band waves are preferentially detected at 5 ≤ L ≤ 6.5 , in the noon sector. They typically have small θ ( <30° ) and weakly left‐hand polarization but become more oblique and linearly polarized at larger magnetic latitudes or L‐shells. With the increase of AE * index, their occurrence rate significantly increases in the noon sector, and their source region extends to dusk sector. He + band waves usually occur in the predawn and morning sectors at 3 ≤ L ≤ 4.5 . They generally have moderate θ ( 30 ° − 40° ) and left‐hand polarization and also become more oblique and linearly polarized at larger latitudes or L‐shells. There is a clear enhancement of occurrence rate and amplitude during active geomagnetic periods, especially in the dusk and evening sectors. O + band waves mainly occur at 3 ≤ L ≤ 4 in the predawn sector. They have either very small θ ( <20° ) or very large θ ( >50° ), and typically linear or weakly right‐hand polarization. During active periods, they mostly occur at the midnight sector and L < 3.5 . As a valuable supplement to previous statistical studies, our result provides not only a more compresentive EMIC wave model for evaluating their effects on the radiation belt, but also detailed observational constraints on generation mechanisms of EMIC waves.