Study on Source Region and Generation Mechanism of Oblique Whistler-Mode Waves in the Earth's Magnetosphere

Oblique whistler-mode waves are as important as quasi-parallel waves in regulating electron dynamics in radiation belts. However, their generation mechanism and corresponding source region are still under debate. By analyzing nearly 7-year Van Allen Probe-A data, we have thoroughly investigated lower-band (<0.5 (Formula presented.)) and upper-band (>0.5 (Formula presented.)) oblique whistler-mode waves in the Earth's inner magnetosphere. We found the majority of poleward propagating oblique waves preferentially occur at relatively larger magnetic latitudes (MLATs), and their favored MLATs are strongly dependent on magnetic local time. Combined with ray tracing simulation results, it is proposed that parallel waves generated at the equator propagate toward poles and turn oblique to the ambient magnetic field, leading to oblique waves detected at large |MLAT|. This propagation effect is the main cause of oblique whistler-mode waves in the Earth's magnetosphere. There also exist some equatorward propagating oblique waves, which are mainly confined within (Formula presented.) around midnight and considered to be locally excited. Our study provides a comprehensive understanding of the generation and distribution of oblique whistler-mode waves in the Earth's magnetosphere.