Beam-Driven Electron Cyclotron Harmonic Waves in Earth's Magnetotail

摘要

Although electron cyclotron harmonic (ECH) waves are the primary contributor to plasma sheet electron scattering loss, experimental verification of their most widely accepted excitation mechanism, loss-cone instability, has been lacking for decades. Using 10 years of time history of events and macroscale interactions during substorms satellite observations, we investigate ECH wave properties near dipolarization fronts, the predominant source of such waves. To our surprise we find that more than 30% of observed ECH waves have moderately oblique (~70°) wave normal angles (WNA), much less than the ~85° expected from classical loss-cone instability. These moderately oblique WNA ECH waves carry a strong field-aligned electric field that is used to identify them. They are often observed with cold, dense electrons that exhibit enhanced parallel flux at a few hundred eV energy, which suggests that low-energy counterstreaming beams (likely of ionospheric origin) might be their free energy source. By solving the linear dispersion relation for parameters representative of such plasma sheet electron distributions, we confirm that ECH waves at WNA ~ 70° can indeed be driven unstable by such beams. Our work reveals a previously unknown excitation mechanism for ECH waves and exposes the need for quantifying the conditions for and relative importance of beam-driven waves compared to those excited by the loss-cone instability in Earth's plasma sheet.