Convective stabilization of ionospheric plasma clouds

We derive a stability criterion for the large‐scale structuring of ionospheric plasma clouds due to the E × B gradient drift instability. For the equilibrium we consider a cylindrical two‐dimensional water bag cloud aligned along a uniform magnetic field that is polarized by a uniform neutral wind. We perform a stability analysis that allows three‐dimensional perturbations (in r, θ, and z), and we consider both local and global modes. We find that when the parallel wave number kz exceeds a threshold value, exponentially growing poloidal eigenmodes form which are localized on the “backside” of the cloud. This is in contrast to the kz = 0 limit in which there are no exponential solutions. As kz is increased further, the unstable modes localize at a finite angle away from the backside, at a point where the diamagnetic propagation velocity Vd balances the convective flow velocity of the background plasma around the cloud, Vb. We find that the E × B gradient drift instability is stable when Vd > Vb so that the cloud is no longer susceptible to large‐scale structuring. We apply these results to ionospheric barium clouds and estimate that they will cease structuring when L⊥ < (cT/eB)(M + 2)/2Vn where L⊥ is the transverse size of the cloud, T = Te + Ti is the total temperature, M = nc/nb (nc is the cloud density, and nb is the background density), and Vn is the neutral wind velocity. For mid‐latitude barium releases at ∼180 km we estimate L⊥ ∼ 160–480 m, which is consistent with observations.