High‐altitude limit of the gradient drift instability

The gradient drift instability for F region plasma clouds is investigated retaining ion inertia effects. With the assumption that the ion gyrofrequency Ω is greater than ion inertial time changes (∂/∂t+Vi·▽) and the ion‐neutral collision frequency νin, a quadratic dispersion equation results. In the limit νin ≫ 4cE0/BL (E0 and B are the ambient electric and magnetic fields, and L is the plasma cloud density gradient scale length), the growth rate γ reduces to the usual result cE0/BL. In the opposite limit νın ≪ 4cE0/BL, γ = (cE0νin/BL)1/2. This result can explain why high‐altitude (∼450‐km) barium releases, where νin is less than unity and a factor of 10–100 smaller than it is at lower altitudes (∼200 km), striate on time scales similar to the lower altitude releases.