Extremely intense whistler mode waves near the bow shock: Geotail observations

Abstract

Very intense whistler mode waves with amplitudes up to ∼1 nT were frequently recorded by the Waveform Capture on board the Geotail satellite in the bow shock. The intense whistler waves cover a frequency range from 0.02 to 0.56Ωe (10 ∼ 400 Hz) with durations from tens to hundreds of milliseconds. These intense whistler waves propagate in a direction quasi‐parallel to the ambient magnetic field with an average angle of 22°. Most (∼80%) of the intense whistler waves propagate in a downstream direction, while only 20% of them propagate toward solar wind. Quasi‐perpendicular shocks with shock normal angles from 66° to 86° are associated with the intense whistler waves. The quasi‐parallel propagation indicates that the electron cyclotron resonance is the dominant mechanism for the excitation of the intense whistler waves. It is found that loss cone electrons with temperature anisotropy at the foot of quasi‐perpendicular bow shocks are the major source of the intense whistler waves. The electron beams, formed by an electric field within the bow shock, are very likely the source of those whistlers propagating in an upstream direction. Trapping of electrons by these intense whistler waves may partially explain the flat‐topped electron distribution at the downstream edge of the bow shock.