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.