Small‐scale auroral current sheet structuring

We simulate the 3‐D evolution of a thin current sheet as it impinges on the ionosphere from a magnetospheric source in a manner analogous to that which may occur during the onset of an auroral substorm. We consider two scenarios: one in which electron inertia alone acts to allow motion between the plasma and the geomagnetic field, and a second where a resistive layer at the interface between the ionosphere and magnetosphere is included. These two scenarios in our fluid model are intended to represent what have become known as “Alfvénic” and “Quasi‐static” or “Inverted‐V” aurora, respectively. In the absence of resistivity the evolution is shown to be driven by a combination of Kelvin‐Helmholtz and tearing instabilities leading to vortices similar to folds and the eventual break‐up of the planar arc into distorted fine‐scale sheets and filamentary currents. The later stage of this evolution is driven by an instability on the steep transverse current gradients created by the former instabilities. With a resistive layer present the K‐H instability dominates leading to the formation of auroral curls. We show how these evolutionary processes can be ordered based on the ratio of the transverse electric and magnetic fields (ΔE X /ΔB Y ) across the current sheet relative to the Alfvén speed, and demonstrate how the evolution is dependent on wave reflection from the topside ionosphere.