Whistler mode waves in the magnetotail

Abstract

Whistler mode waves in the magnetotail, especially the plasma sheet and the plasma sheet boundary layer, are frequently detected by a waveform capture on board Geotail satellite. The whistler mode waves with frequencies above 10 Hz are often narrowband and short‐lived. Statistical study shows that these whistler mode waves propagate in a direction quasi‐parallel to the ambient magnetic field with an average wave normal angle of 23°. This suggests that the electron cyclotron resonance is dominant for excitation of the whistler mode waves. Their frequencies range mainly from 0.05 to ∼ 0.5Ωe, where Ωe is the electron cyclotron frequency. Their average frequency is 0.21Ωe. Amplitudes of the whistler mode waves cover a range from a few picoteslas to ∼100 pT. Their mean amplitude is 44 pT or 10−2 B0, where B0 is the magnitude of the ambient magnetic field. One interesting feature is that the whistler mode waves propagate mostly in a direction either parallel or antiparallel to B0. The whistler mode waves are very likely excited by energetic electron beams, which arise from reconnections in the near/deep magnetotail. Another significant feature of the whistler mode waves is that the correlation between their amplitudes and phase speeds. A higher phase speed usually associates with a lower amplitude. Whistler mode waves in the near magnetotail (−100 RE < x <−10 RE) and the deep magnetotail (−210 RE < x < −100 RE) are very much the same in many respects. However, one significant difference is that the average electron resonant energy is higher in the near magnetotail (11 keV) than that in the deep magnetotail (1.6 keV). Observations of whistler mode waves may provide a way to monitor energetic electrons and processes of reconnection in the magnetotail.