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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
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.