Day-night asymmetry in lower band chorus waves

Chorus waves play a critical role in the acceleration and loss of energetic electrons in the Earth’s inner magnetosphere. Based on the wave normal angle (θ), lower band (LB) chorus waves are commonly classified by the Gendrin angle (θg) into quasi-parallel (θg) and oblique (θ>θg) LB chorus waves. In this study, we have statistically investigated the characteristics of quasi-parallel and oblique LB chorus waves based on observations by Van Allen Probe B. We found that quasi-parallel LB chorus waves exhibit pronounced day-night asymmetry in both spatial distribution and the variation of wave properties as a function of magnetic latitude (MLAT). They appear over a wide range of MLATs at high L shells on the dayside, whereas they are more confined on the nightside. In contrast, oblique LB chorus waves show no significant day-night asymmetry in either spatial distribution or latitudinal variations of wave properties. Moreover, our statistical results reveal that equatorward propagating quasi-parallel waves can extend to higher MLATs (up to MLAT=±20°) at higher L shells on the dayside, but are limited to within ±5° on the nightside. Oblique waves, by comparison, show restricted equatorward propagation within |MLAT|L shells on the dayside.