Nonlinear evolution of convecting plasma enhancements in the auroral ionosphere: 2. Small scale irregularities

Abstract

Using analytical and numerical simulation techniques, the linear stability and nonlinear evolution of small scale (∼0.1–1 km) density irregularities in local unstable regions of large scale convecting auroral plasma enhancements have been studied. Our results show that these small scale size irregularities are driven unstable primarily by the effects of convection through the E×B gradient drift instability. We find that the density irregularities, in the nonlinear regime, in a plane nearly perpendicular to the magnetic field resemble steepened striation‐like structures (elongated in the north‐south direction for equatorward convection) that can form and cascade from kilometer to tens of meter scale sizes on the order of half an hour. The one‐dimensional spatial power spectra of the density irregularities in the north‐south P(ky) ∝ ky−n and east‐west P(kx) ∝ kx−n can be described by inverse power laws with n ≃ 2–3. Finally, we propose and demonstrate, using a crude model, that a two‐step process, in which small scale irregularities can grow on longer wavelength nonlinear structures, can account for the experimentally observed L shell aligned nature of the irregularities.