Abstract
Abstract. We propose a simple model of the flow and currents in Saturn's polar ionosphere. This model is motivated by theoretical reasoning, and guided quantitatively by in situ field and flow data from space missions, ground-based IR Doppler measurements, and Hubble Space Telescope images. The flow pattern consists of components which represent (1) plasma sub-corotation in the middle magnetosphere region resulting from plasma pick-up and radial transport from internal sources; (2) the Vasyliunas-cycle of internal plasma mass-loss down the magnetospheric tail at higher latitudes; and (3) the polar Dungey-cycle flow driven by the solar wind interaction. Upstream measurements of the interplanetary magnetic field (IMF) indicate the occurrence of both extended low-field rarefaction intervals with essentially negligible Dungey-cycle flow, and few-day high-field compression regions in which the Dungey-cycle voltage peaks at a few hundred kV. Here we model the latter conditions when the Dungey-cycle is active, advancing on previous axi-symmetric models which may be more directly applicable to quiet conditions. For theoretical convenience the overall flow pattern is constructed by adding together two components - a purely rotational flow similar to previous axi-symmetric models, and a sun-aligned twin vortex representing the dawn-dusk asymmetry effects associated with the Vasyliunas-and Dungey-cycle flows. We calculate the horizontal ionospheric current associated with the flow and the field-aligned current from its divergence. These calculations show that a sheet of upward-directed field-aligned current flows at the boundary of open field lines which is strongly modulated in local-time by the Dungey-cycle flows. We then consider implications of the field-aligned current for magnetospheric electron acceleration and aurorae using two plasma source populations (hot outer magnetospheric electrons and cool dense magnetosheath electrons). Both sources display a strong dawn-dusk asymmetry in the accelerating voltages required and the energy fluxes produced, resulting from the corresponding asymmetry in the current. The auroral intensities for the outer magnetosphere source are typically ~50 kR at dawn and ~5 kR at dusk, in conformity with recent auroral observations under appropriate conditions. However, those for the magnetosheath source are much smaller. When the calculated precipitating electron energy flux values are integrated across the current layer and around the open closed field line boundary, this yields total UV output powers of ~10 GW for the hot outer magnetosphere source, which also agrees with observations.
Highlights
Recent theoretical interest in the dynamics of Saturn’s magnetosphere has been stimulated by new data from the Cassini Saturn orbiter space mission (e.g. Dougherty et al, 2005; Krimigis et al, 2005; Young et al, 2005), by ground-based IR Doppler measurements of ionospheric flows (Stallard et al, 2004), and by studies of the planet’s aurorae using the Hubble Space Telescope (HST) (e.g. Gerard et al, 1995, 2004; Cowley et al, 2004a; Prangeet al., 2004)
The model parameters have been guided by Voyager plasma velocity measurements on closed field lines (e.g. Richardson, 1986, 1995; Richardson and Sittler, 1990), by groundbased IR Doppler measurements of ionospheric flows in the polar cap (Stallard et al, 2004), and by remote sensing studies of the planet’s aurorae using the HST (Gerard et al, 1995, 2004; Cowley et al, 2004a; Prangeet al., 2004)
The physical nature of the flow pattern in the model consists of elements which are intended to represent subcorotational plasma flow in the middle magnetosphere resulting from plasma pick-up and radial transport from internal sources, and the Vasyliunas- and Dungey-cycles of convection at higher latitudes
Summary
Recent theoretical interest in the dynamics of Saturn’s magnetosphere has been stimulated by new data from the Cassini Saturn orbiter space mission (e.g. Dougherty et al, 2005; Krimigis et al, 2005; Young et al, 2005), by ground-based IR Doppler measurements of ionospheric flows (Stallard et al, 2004), and by studies of the planet’s aurorae using the Hubble Space Telescope (HST) (e.g. Gerard et al, 1995, 2004; Cowley et al, 2004a; Prangeet al., 2004). Weaker aurorae of a similar nature can be observed during interplanetary rarefaction conditions of intermediate field strength These studies indicate that the solar wind interaction is important at Saturn, as it is at Earth (see, e.g. the review by Cowley et al, 2003). A qualitative picture of the overall plasma flow and electric current system in Saturn’s coupled solar wind-magnetosphere-ionosphere system has been discussed by Cowley et al (2004a), which contains co-existing elements related both to internally-driven processes and the solar wind interaction. This picture forms the basis of the theoretical model of the flow and current in Saturn’s polar ionosphere developed here. We begin by outlining the overall physical picture presented by Cowley et al (2004a), which forms the background of the detailed theoretical model which follows in Sects. 3 and 4
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