Toward an Integrated View of Ionospheric Plasma Instabilities: 4. Behavior in the Transitional Valley Region

Abstract

AbstractBehavior of unstable plasma waves generated by the Farley‐Buneman instability (FBI) and the gradient drift instability (GDI) is analyzed in the transitional valley region near 120 km in altitude. The analysis is based on the expression for the FBI/GDI growth rate γ that has been recently generalized to include ion inertia effects for arbitrary altitude and wavelength, within the limits imposed by the fluid and local approaches. It is found that the ion inertia leads to a different instability behavior when the convection component is between the two critical values determined by the ion acoustic speed Cs and the ratio ri between the ion collision and gyrofrequency. The most interesting case occurs near 120 km, just below where ri=1. From analysis of electron density gradients G=∇n/n that result in marginal instability condition γ=0 (i.e., critical gradients G0), there exists a critical scale where G0=0 and below which all waves are unstable to FBI. Above this scale, G0>0 and gradients need to be sufficiently strong G>G0 for the plasma to become unstable through GDI. There also exists a maximum in dependence , which refers to the least unstable scale and gradient. For convection outside of the specified range, no critical or least unstable scale exists, which is a typical situation outside of the transitional valley region. Overall, this analysis shows that the FBI convection thresholds and the GDI critical gradients are modified by the ion inertia and that the effects are most pronounced in the transitional valley region near 120 km.