A magnetohydrodynamic simulation study of Kronian field-aligned currents and auroras

Magnetohydrodynamic simulations of the interaction of Saturn's magnetosphere with the solar wind indicate that Kelvin-Helmholtz (K-H) waves can form on the dayside magnetopause when the interplanetary magnetic field (IMF) is northward. Dayside magnetic reconnection occurs at Saturn for northward IMF, and the combination of K-H waves and reconnection caused enhanced vorticity in Saturn's magnetosphere. We have used a very high resolution version (Delta x = 0.1 R-S) of our simulation code to study the consequences of the vortices and reconnection for the generation of field-aligned currents (FACs) and auroras in Saturn's ionosphere. We found three bands of alternating FACs toward and away from the dawn side of the ionosphere and two sets on the dusk side. The K-H waves generated a series of toward and away currents along the dayside magnetopause. In the ionosphere they appear as a series of spots of up and down currents. The K-H field-aligned currents are adjacent to nearly continuous currents located from 16: 00 LT, to around past midnight, and to about 07:00 LT. The largest currents densities (j(parallel to) > 5 x 10(-8) A m(-2)) are found at the highest latitudes. They map to the magnetopause and to the near-Saturn tail region. We used the away current density and the Knight relationship to estimate the energy flux related to discrete auroras and obtained similar to 1 mW m(-2) in the region with the strongest currents. This gives approximately 7 GW for the auroral UV emitted power. We found a region of enhanced thermal energy flux in the region where cusp auroras are observed.