Coupling of Kelvin‐Helmholtz and current sheet instabilities to the ionosphere: A dynamic theory of auroral spirals

The Kelvin-Helmholtz instability has often been invoked to describe transport at the magnetopause. However, recent observations of vortices in the postnoon sector of the auroral zone [Elphinstone et al., 1993] have indicated that structure of these vortices is inconsistent with the evolution of the standard Kelvin-Helmholtz instability, and is instead consistent with the theory of auroral spirals proposed by Hallinan [1976]. Models of the Kelvin-Helmholtz instability coupled to the ionosphere have generally neglected the effect of the field-aligned current which is generated by the velocity shear. This current must close in the ionosphere, and leads to the presence of a magnetic shear in addition to the velocity shear. This magnetic shear is susceptible to a dynamic current sheet instability, which has many of the same properties as the Kelvin-Helmholtz instability but produces vortices which are wound in a sense consistent with spiral observations. This instability, which may grow much faster than Kelvin-Helmholtz, may provide a basis for a dynamic theory of auroral spirals.

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