Formation of fingerlike structures in fragmentation of small‐scale patchy aurora

Abstract

Recent high‐resolution video imagery reveals a dynamic feature of small‐scale (< 20 km) patchy aurora during a substorm recovery phase, that is, fingerlike structures with a few kilometers' scale are formed through the patch fragmentation. Various kinetic processes such as ion finite Larmor radius effects can be involved because the scale mapped into the plasma sheet is much smaller than the ion inertia length. However, such a scale is likely to be strongly damped in a high‐β plasma regime by the ion gyroviscosity effect. In this study, we make several 2.5‐dimensional simulations at the system of each position s along a field line, perpendicular to the ambient magnetic field, by means of the three‐field reduced magnetohydrodynamic equations. The model includes the effects of ion diamagnetic drift and electron parallel motion. In a low‐β regime of s = 4 RE, we find that an ion diamagnetic drift wave grows isotropically to form a patchy structure with a scale of ≈10 km. As the field‐aligned current increases, these patches are fragmented and fingerlike structures with scales of 2–5 km appear owing to the nonlinear electron inertia effect. The scales of patches and fingers and the east–westward finger development, as well as the time scale, are fairly consistent with those of recent observations. On the other hand, at the plasma sheet (ion beta βi = 1, s = 7.5 RE), fingerlike patterns cannot be formed owing to a predominance of large‐scale vortex patterns.