During increased geomagnetic activity, the quiet auroral arcs become more active, producing forms like spirals, curls and folds (e.g., Hallinan and Davis, 1970; Davis and Hallinan, 1976). Localized upward field-aligned currents (Rostoker, 1987; Bythrow and Potemra, 1987) are associated with these structures. Two large scale deformation not discussed here are the auroral bulge and the omega band.
In the work by Steen et. al. (1988) formation of fold structures was attributed to the strong large scale electric field intensifications, which produced also measurable enhancements in Ti through frictional heating. The arc brightness over the whole all-sky camera field of view responded uniformly to the electric field intensification, and only the hole of the fold was not characterized by any strong ionospheric electric field (or Ti) signature.
- Bythrow, P. F., and T. A. Potemra, Birkeland currents and energetic particles associated with optical auroral signatures of a westward travelling surge, J. Geophys. Res., 92, 8691-8699, 1987.
- Davis, T. N., and T. J. Hallinan, Auroral spirals, 1. Observations, J. Geophys. Res., 81, 3953, 1976.
- Hallinan, T. J., and T. N. Davis, Small-scale auroral arc distortions, Planet. Space Sci., 18, 1735, 1970.
- Rostoker, G., The Kelvin-Helmhotz instability and its role in the generation of the electric currents associated with Ps6 and westward travelling surges, in Magnetotail Physics, edited by A. T. Y. Lui, The Johns Hopkins University Press, Baltimore, Md., 1987.
- Steen, Å., P. N. Collis, and I. Häggström, On the development of folds in auroral arcs, J. Atmos. Terr. Phys., 50, 301-313, 1988.