Electron Scattering by Highly Oblique Quasi‐Electrostatic Chorus Waves Under Realistic Magnetospheric Conditions

Using Van Allen Probes data (2013–2015), we report that highly oblique chorus waves frequently occur in low‐density regions where . These waves exhibit an electric‐to‐magnetic energy ratio , much higher than the typical for quasi‐parallel chorus, indicating a quasi‐electrostatic nature, termed highly oblique quasi‐electrostatic (HOQE) chorus. Incorporating realistic wave properties and plasma environments, numerical modeling shows that HOQE lower‐band chorus with the central wave normal angle can efficiently scatter relativistic electrons up to 2 MeV near equatorial pitch angles primarily via higher‐order resonance. Additionally, HOQE upper‐band chorus drives strong diffusion of tens of keV electrons at moderate pitch angles. These findings reveal a previously overlooked pathway for MeV electron loss, advancing our understanding of chorus wave‐particle interactions.