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Ann. Geophys., 24, 1029-1055, 2006
www.ann-geophys.net/24/1029/2006/
© European Geosciences Union 2006
A model of the plasma flow and current in Saturn's polar ionosphere under conditions of strong Dungey cycle driving
C. M. Jackman and S. W. H. Cowley
Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
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.
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