Jupiter's Main Auroral Emission for Different Solar Wind Conditions

Abstract:

We study the temporal change of Jupiter's magnetosphere and aurora due to changing solar wind conditions. In particular, we examine how the the main auroral emission is affected by the solar wind density. Using three dimensional global MHD simulations, we perform three different runs, with: 1) quiet solar wind conditions (ram pressure of 0.05 nPa), 2) disturbed solar wind conditions (ram pressure of 0.17 nPa), and 3) very disturbed solar wind conditions (ram pressure of 0.34 nPa). We show that the response of the main auroral emission depends on local time: at noon, the main oval is only weakly affected by the variations in the solar wind; whereas on the night side, the main emission becomes brighter when the solar wind ram pressure increases. For instance, 10 hours after the high density solar wind reached the magnetosphere, the peak in parallel electrical current on the night side is 20% and 40% stronger for the disturbed and very disturbed solar wind conditions, respectively. The main auroral emission begins to change three hours after the solar wind density enhancement strikes the bow-shock and it takes approximately three days for the magnetosphere to adjust to the new solar wind conditions. The total electrical current flowing out of the ionosphere is then 30% (50%) higher for the (very) disturbed solar wind conditions than for the quiet solar wind conditions. In addition, for the three simulations, a localized enhancement of the main oval emission is periodically observed around noon local time (inside the main oval discontinuity). A very similar enhancement has already been observed with the Hubble Space Telescope in Far-UV images by Palmaerts et al. (JGR, under review). In our simulations, the localized peak is not caused by fluctuations in the solar wind, but is always associated with a region of negative radial velocity in the equatorial plane at the position where the corotation breaks down. The shearing motions associated with this negative radial velocity region produce strong gradients for Bz in the azimuthal direction, which causes an enhancement of the electrical current.