A new mechanism for interpreting the motion of auroral arcs in the nightside ionosphere

A new mechanism is proposed for predicting and interpreting the motion of auroral arcs observed in the nightside ionosphere during the expansion phase of a substorm. This mechanism is centred on the idea that such arcs act as visible manifestations of the arrival of earthward‐propagating shock waves in the near‐Earth magnetosphere. These shock waves are generated at a near‐Earth X‐line, and propagate at the local Alfvén speed. Because of the non‐uniform nature of the magnetised plasma in the magnetotail, dispersion results in a change in the shape of the wave fronts as the shocks propagate towards the ionosphere. Theoretical analysis shows that a variety of arc motions can occur as a result of this dispersion, depending on factors such as the reconnection rate, the location of the reconnection site, and gradients in the magnetic field strength and plasma density.