Obliquely propagating electromagnetic drift ion cyclotron instability

Identification of wave characteristics during current disruption events in the near-Earth geomagnetic tail region is important for understanding the substorm onset mechanism. The present paper discusses a linear stability analysis in the ion cyclotron frequency range of a plasma possessing the temperature anisotropy and cross-field flow. It is found that the ion cyclotron drift waves propagating in quasi-perpendicular direction with respect to the ambient magnetic field are characterized by low frequencies (omega less than or similar to 0.5 Omega(i)), while quasi-parallel waves have frequencies close to the ion cyclotron frequency (omega similar to Omega(i)). This finding is consistent with the observation by THEMIS spacecraft of a current disruption event in which similar high-and low-frequency band structures are present. The interpretation of the low-frequency mode as the quasi-perpendicular mode is consistent with a recent data analysis. The present paper shows that the high-frequency quasi-parallel mode can simultaneously be excited with the low-frequency quasi-perpendicular mode.