Narrowing of the discrete auroral arc by the ionosphere

We investigate the role of the ionosphere in the development of intense, narrow discrete auroral arcs. Our study shows that interactions between a pair of downward and upward magnetic field‐aligned currents (FACs) and the ionosphere can lead to the narrowing of the upward current channel and broadening of the downward current channel such that the total width of the initial current pair remains the same. In this case the intensity of the upward current increases, and the intensity of the large‐scale downward current decreases. Conditions promoting this asymmetry between the upward and downward FACs include low ionospheric conductivity (≤1 mho) and a moderate magnitude of the current density (≤5 μ A/m 2 ). Simulations show that the ionosphere causes significant asymmetry not only in the structure and amplitude of the currents but also in the structure and amplitude of the corresponding parallel electric fields. The dynamics of the parallel electric field in the upward current channel is similar to the dynamics of the current itself, but the dynamics of the field in the downward current channel differs from the dynamics of the current. The major difference is that the width of the downward current channel becomes broader with time but the scale size of the parallel electric field becomes smaller inside the channel. This effect is quite important for understanding the parallel electron acceleration in the auroral zone. In particular, it may explain why the so‐called “black” auroral arcs, which are produced by the electrons flowing along the magnetic field lines from the ionosphere, always appear in the form of very narrow, discrete dark lines.