Fluctuations associated with sheared velocity regions near auroral arcs

Abstract

The theory of electrostatic, collisional plasma modes that are driven unstable by a sheared field‐aligned ion flow velocity (i.e., Vio=Vo(x)eˆz, Bo=Boêz) is presented. The threshold condition for the instability is V′o/νin>2 (1+Ti/Te) and the unstable modes are characterized by k∥«k⟂ such that λ∥(=2π/k∥)»λm and 2πρi<λ⟂(=2π/k⟂)≲Lv, where V′o=dVo/dx, νin is the ion‐neutral collision frequency, λm is the electron mean‐free‐path, ρi is the ion gyroradius and Lv is the velocity shear scale length. It is shown that the linear instability provides a direct means of exciting the observed [Basu et al., 1986, 1988] sub‐km scale irregularities in the auroral F‐region. The spectral characteristics of the density and the electric field fluctuations that can be inferred from the theory are found to be consistent with the observations.