A possible mechanism for the formationof an eastward moving auroral spiral

Auroral spirals have different morphologies and origins. In this study, we propose a possible mechanism for the formation of an eastward-moving auroral spiral, which was observed in Tromsø, Norway, during the expansion phase of a substorm on 18 September 2013. During this time, the Cluster and THEMIS-A spacecraft were located ∼7 RE duskward of the spiral generator region in the magnetotail. Prior to the spiral observation, concurrent magnetic field dipolarizations, bursty bulk flows and electron injections were measured by the Cluster satellites. At the same time, a local Kelvin-Helmholtz-like vortex street in the magnetic field was detected, which was likely caused by the bulk flows. The vortex street presumably propagated towards the source region of the spiral due to a high dawnward velocity of the flow bursts. The observations suggest that the spiral may have been generated by an associated vortex mapped along the magnetic field lines to the ionosphere. Future research of the generation of auroral spirals requires higher resolution monitoring of the ionosphere.