Topic/Type: 1.1 Space & astrophysical plasmas, Oral
National Institute for Fusion Science / Graduate University for Advanced Studies Toki, Gifu, Japan
Spontaneous excitation of quiet auroral arcs has been discussed in terms of feedback interactions of the magnetosphere and the ionosphere. The magnetosphere-ionosphere (M-I) coupling system is destabilized, when the ExB convection flow exceeds a threshold . The feedback instability grows with the ionospheric density and field-aligned current perturbations that would be associated with visible auroras. In previous studies of the feedback instability, the linear response of shear Alfven waves or its two-fluid extensions were often employed for describing the cross-field dynamics of the magnetospheric plasma [2, 3]. Three-dimensional global simulations of the feedback instability involved the nonlinear magnetohydrodynamic (MHD) effects [4, 5], while the coarse resolution employed made detailed studies of the nonlinear dynamics of auroral arcs quite difficult.
In our recent study, the reduced MHD equations are applied to the magnetosphere for construction of the M-I coupling system. The ionospheric behaviors are modeled by two-fluid equations. The new M-I coupling model enables us to explore nonlinear evolutions of the feedback instability. A preliminary simulation for a slab configuration demonstrates nonlinear saturation of the feedback instability growth. It is also found that the Kelvin-Helmholtz-like mode grows in the saturation phase, where role-up of thin vortex sheets is observed. Simultaneously, one finds splitting of arc-like structures of the ionospheric density perturbation. Extension of the computational model to the dipole configuration is currently in progress, and the result will also be reported at the conference.
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