Velocity Shear Driven Aurora

Abstract:

Auroral bright spots and streamers and their associated current systems are are a signature of active magnetospheric dynamics at Earth. Dayside spots move along the poleward boundary of the oval toward the nightside consistent with a picture of KH structures moving tailward along the flank. On the nightside, the structures move equatorward and are thought to be associated with earthward traveling fast flows. These dayside and nightside structures are linked by their close association with gradients in the flow. We utilize a quasi-static magnetosphere-ionosphere coupling model driven by a velocity vortex to determine how the field-aligned current structure associated with the vortices depends on ionospheric and boundary layer parameters. The model predicts a maximum current that is well approximated by

$$

j_{\parallel,max} = {2 \Sigma_p V_0 B_0 b \over \Delta_m + 2 b L^2/\Delta_m}

$$

where $V_0$ is the velocity of the vortex, $\Delta_m$ is the width of the vortex, $b = B_i/B_m$ is the magnetic field scaling factor between the magnetosphere and ionosphere, $L = \Sigma_P/\kappa$ is the auroral scale length, where $\Sigma_P$ is the Pedersen conductivity and $\kappa$ is the Knight parameter. This model suggests maximum current/electron energy flux when the vortex size mapped to the ionosphere is approximately 1.5 $L$ and drops off sharply for larger or smaller scales. We examine an event in detail where THEMIS detects Kelvin-Helmholtz structures at the magnetopause and DMSP detects auroral spots, monoenergetic electrons, and field-aligned currents on the dusk flank showing consistency with the model. We also discuss applications of the model to Jupiter and Saturn where auroral spots are also thought to result from KH vortices.