Theory of the current‐driven ion cyclotron instability in the bottomside ionosphere

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Abstract
A theory of the current‐driven electrostatic ion cyclotron (EIC) instability in the collisional bottomside ionosphere is presented. It is found that electron collisions are destabilizing and are crucial for the excitation of the EIC instability in the collisional bottomside ionosphere. Furthermore, the growth rates of the ion cyclotron instability in the bottomside ionosphere maximize for k⊥ρ i ≥ 1, where 2π/k⊥ is the mode scale size perpendicular to the magnetic field and ρi the ion gyroradius. Realistic plasma density and temperature profiles typical of the high‐latitude ionosphere are used to compute the altitude dependence of the linear growth rate of the maximally growing modes and critical drift velocity of the EIC instability. The maximally growing modes correspond to observed tens of meter size irregularities, and the threshold drift velocity required for the excitation of EIC instability is lower for heavier ions (NO+, O+) than that for the lighter ions (H+). Dupree's resonance‐broadening theory is used to estimate nonlinear saturated amplitudes for the ion cyclotron instability in the high‐latitude ionosphere. Comparison with experimental observations is also made. It is conjectured that the EIC instability in the bottomside ionosphere could be a source of transversely accelerated heavier ions and energetic heavy‐ion conic distributions at higher altitudes.