On the Formation of Auroral Spirals.

The paper contains a detailed analysis of the formation of an auroral spiral based on hitherto not published observations by the all‐sky camera in Kilpisjärvi, Northern Finland. We conclude that spirals appearing during a substorm form by a modification of the interface between tail and magnetosphere, the location of the generator current of the westward electrojet. Driven by the arriving flow bursts, this current is subject to ruptures by the appearance of a sequence of hook‐like structures. These structures can move eastward with speeds up to 3 km/s. The propagation is attributed to a constructive magnetic fracture process driven from behind by the power of the arriving flow bursts. Poleward bending and extension of a hook‐like structure, followed by a turning to the west and then equatorward, is the first step in spiral formation. It becomes the primary spiral arm, if a poleward arm grows out of weaker auroral structures, poleward and eastward of it. We suggest that the upward field‐aligned currents related to the bright spiral arms are largely balanced by adjacent downward currents. The electric fields associated with the connecting Pedersen currents are consistent with the counter‐clockwise motion. An important additional ingredient in the observed configuration is an eastward directed flow field, which is the generator of an additional upward current and possibly crucial for the spiral formation. Electric field data from literature throw confusing light on the propagation of a spiral, whether like a vessel in the ocean or by incorporating the magnetic flux ahead of it. Plain Language Summary: Auroral spirals are a fascinating phenomenon in the night sector during substorms. An event, observed by the all sky camera in Kilipisjärvi, is interpreted in the light of the physical processes at substorm onset. The forces exerted by an arriving flow burst on the magnetic field of the near‐dipolar magnetosphere generate an eastward directed current, which is the generator of the auroral electrojet (AEJ). This current can be broken into sections by an instability suggested to occur at strong flow gradients. Out of such sections, a spiral may form in two steps. First, a hook‐like structure, arising from the eastern end of the section, will extend poleward, then westward and finally equatorward thus forming the primary spiral arm. The poleward arm grows out of residues of the rupture process by draping themselves around the upper end of the primary arm. The luminous spiral arms, generated by upward currents, must be closely accompanied by downward currents located in the dark regions of the spiral. The electric fields, connected with the current closure in the ionosphere, are consistent with the counter‐clockwise sense of spiral formation. The creation of convolved auroral forms such as spirals serves an enhanced dumping of the entering energy. Key Points: Ruptures of the generator current lead to the formation of hook‐like structures out of which a spiral can evolveA primary spiral arm extending poleward in a counter‐clockwise sense is completed into a spiral by reformation of residues from the ruptureThe upward field‐aligned currents, which are associated with the luminous structures, are largely balanced by adjacent downward currents [ABSTRACT FROM AUTHOR]