Obliquely propagating electromagnetic drift ion cyclotron instability

Abstract

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 (ω ≲ 0.5Ωi), while quasi‐parallel waves have frequencies close to the ion cyclotron frequency (ω ∼ Ωi). 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.