Ionospheric feedback instability and active discrete auroral forms

Abstract We present results from time‐dependent, three‐dimensional numerical simulations of ULF Alfvén waves generated by the ionospheric feedback instability at high latitudes. The goal of this study is to understand physical mechanisms responsible for the formation of active discrete auroral forms (curls and folds) typically observed during the substorm onset. Our simulations demonstrate that active feedback of the auroral ionosphere on magnetic field‐aligned currents carried by ULF Alfvén waves can explain wide variety of auroral structures. The main reason for this variety is a susceptibility of the electrodynamics of the coupled magnetosphere‐ionosphere system to parameters of the electric field and plasma in the ionosphere and magnetosphere of the Earth. Due to the highly nonlinear character of the magnetosphere‐ionosphere interactions, different combinations of these parameters lead to different spatial and temporal behavior of the magnetic field‐aligned currents producing aurora. One of the main conclusions from our study is that the ionosphere is responsible for the formation of small‐scale curls and folds in the discrete aurora and for the intensification of magnetic field‐aligned currents, observed during substorms. The results from our 3‐D simulations are applied for the explanation of structure of discrete auroral arcs observed during the 29 October 2013 substorm at Fort Yukon, Alaska.