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Ann. Geophys., 26, 913-928, 2008
www.ann-geophys.net/26/913/2008/
© European Geosciences Union 2008
Ionospheric feedback effects on the quasi-stationary coupling between LLBL and postnoon/evening discrete auroral arcs
M. M. Echim1,*, M. Roth1, and J. De Keyser1
1BIRA-IASB, Avenue Circulaire 3, 1180 Bruxelles, Belgium
*also at: Institute for Space Sciences, Atomiştilor 409, 077125 Măgurele, Romania
Abstract. We discuss a model for the
quasi-stationary coupling between magnetospheric
sheared flows in the dusk sector and discrete auroral arcs, previously
analyzed for the case of a uniform height-integrated Pedersen
conductivity (ΣP). Here we introduce an ionospheric feedback
as the variation of ΣP with the
energy flux of precipitating magnetospheric electrons
(εem).
One key-component of the model
is the kinetic description of the interface between the duskward LLBL and the
plasma sheet that gives the profile of
Φm, the magnetospheric electrostatic potential.
The velocity shear in the dusk LLBL plays the role
of a generator for the auroral circuit closing through
Pedersen currents in the auroral ionosphere.
The field-aligned
current density, j||,
and the energy flux of precipitating electrons are given by
analytic functions of the field-aligned potential drop, ΔΦ,
derived from standard kinetic models of the adiabatic motion of
particles.
The ionospheric electrostatic potential, Φi (and
implicitely ΔΦ) is determined from the
current continuity equation in the ionosphere.
We obtain values of ΔΦ of the order
of kilovolt and of j|| of the order
of tens of μA/m2 in thin regions of the order of
several kilometers at 200 km altitude. The spatial scale
is significantly smaller and the peak values
of ΔΦ, j|| and
εem are higher than in the case of a uniform
ΣP.
Effects on the postnoon/evening auroral arc
electrodynamics due to variations of dusk LLBL and solar wind
dynamic and kinetic pressure are discussed.
In thin regions (of the order of kilometer) embedding
the maximum of ΔΦ we evidence a non-linear regime of
the current-voltage relationship.
The model predicts also
that visible arcs form
when the velocity shear in LLBL is above
a threshold value depending on the generator and ionospheric plasma
properties. Brighter arcs are obtained for increased velocity shear in
the LLBL; their spatial scale remains virtually
unmodified. The field-aligned potential drop tends to decrease with increasing LLBL density.
For higher values of the LLBL electron temperature
the model gives negative field-aligned potential drops
in regions adjacent to upward field-aligned currents.
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