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 tr ansport 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 t he evolution of the standard Kelvin-Helmholtz instability, and is inst ead consistent with the theory of auroral spirals proposed by Hallinan [1976]. Models of the Kelvin Helmholtz instability coupled to the ion osphere have generally neglected the effect of the field-aligned curre nt which is generated by the velocity shear. This current must close i n the ionosphere, and leads to the presence of a magnetic shear in add ition to the velocity shear. This magnetic shear is susceptible to a d ynamic current sheet instability, which has many of the same propertie s 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 basi s for a dynamic theory of auroral spirals.