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Ann. Geophys., 25, 1433-1463, 2007
www.ann-geophys.net/25/1433/2007/
© European Geosciences Union 2007
Modulation of Jupiter's plasma flow, polar currents, and auroral precipitation by solar wind-induced compressions and expansions of the magnetosphere: a simple theoretical model
S. W. H. Cowley1, J. D. Nichols2, and D. J. Andrews1
1Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK
2Center for Space Physics, Boston University, Boston, MA 02215, USA
Abstract. We construct a simple model of the plasma flow,
magnetosphere-ionosphere coupling currents, and auroral precipitation in
Jupiter's magnetosphere, and examine how they respond to compressions and
expansions of the system induced by changes in solar wind dynamic pressure.
The main simplifying assumption is axi-symmetry, the system being modelled
principally to reflect dayside conditions. The model thus describes three
magnetospheric regions, namely the middle and outer magnetosphere on closed
magnetic field lines bounded by the magnetopause, together with a region of
open field lines mapping to the tail. The calculations assume that the
system is initially in a state of steady diffusive outflow of iogenic plasma
with a particular equatorial magnetopause radius, and that the magnetopause
then moves rapidly in or out due to a change in the solar wind dynamic
pressure. If the change is sufficiently rapid (~2–3 h or less) the
plasma angular momentum is conserved during the excursion, allowing the
modified plasma angular velocity to be calculated from the radial
displacement of the field lines, together with the modified
magnetosphere-ionosphere coupling currents and auroral precipitation. The
properties of these transient states are compared with those of the steady
states to which they revert over intervals of ~1–2 days. Results
are shown for rapid compressions of the system from an initially expanded
state typical of a solar wind rarefaction region, illustrating the reduction
in total precipitating electron power that occurs for modest compressions,
followed by partial recovery in the emergent steady state. For major
compressions, however, typical of the onset of a solar wind compression
region, a brightened transient state occurs in which super-rotation is
induced on closed field lines, resulting in a reversal in sense of the usual
magnetosphere-ionosphere coupling current system. Current system reversal
results in accelerated auroral electron precipitation occurring in the outer
magnetosphere region rather than in the middle magnetosphere as is usual,
with peak energy fluxes occurring just poleward of the boundary between the
outer and middle magnetosphere. Plasma sub-corotation is then re-established
as steady-state conditions re-emerge, together with the usual sense of flow
of the closed field current system and renewed but weakened accelerated
electron precipitation in the middle magnetosphere. Results for rapid
expansions of the system from an initially compressed state typical of a
solar wind compression region are also shown, illustrating the enhancement
in precipitating electron power that occurs in the transient state, followed
by partial reduction as steady conditions re-emerge.
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