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Ann. Geophys., 24, 1685-1693, 2006
www.ann-geophys.net/24/1685/2006/
© European Geosciences Union 2006
Simulation of the interchange instability in a magnetospheric substorm site
O. V. Mingalev, I. V. Golovchanskaya, and Y. P. Maltsev
Polar Geophysical Institute, 14a, Fersman street, Apatity, 184209 Russia
Abstract. We perform modeling of the interchange instability driven
by longitudinal pressure asymmetry in the region of the pressure buildup
that forms in the inner magnetosphere at the substorm growth phase. The
simulation refers to the dawnward side of the Harang discontinuity and times
after Bz IMF turning northward. The solution for the equilibrium state
indicates tailward flows associated with vortices, which is in agreement
with a previous finding of Ashour-Abdalla et al. (1999, 2002). We show that
in the regions of equilibrium field-aligned currents (FACs), small initial
perturbations in pVγ (p is the isotropic plasma pressure, V is the
unit magnetic flux tube volume, γ=5/3 the adiabatic exponent), set up as
ripples inclined to azimuth, grow in time. For the background FAC of
~10-6 A/m2, the linear growth rate of the instability is
~6 min. Starting from the 12th min of evolution, the perturbations
exhibit nonlinear deformations, develop undulations and front
steepening. An interesting peculiarity in the distribution of the associated
small-scale FACs is that they become asymmetric with time.
Specifically, the downward currents are more localised, reaching densities
up to 15×10-6 A/m2 at the nonlinear stage. The upward
FACs are more dispersed. When large enough, these currents are
likely to produce the aurora. We also run our simulation for the initial
perturbations of large transverse scales in order to demonstrate that the
interchange instability can be responsible for pressure and cross-tail
current spatial variations of great extent.
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