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Author(s):
T. Watanabe, H. Oya, K. Watanabe and T. Sato
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Title:
Comprehensive Simulation Study on Local and Global Development of Auroral Arcs and Field-Aligned Potentials
Date of publication:
Oct. 1992
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Key words:
magnetosphere-ionosphere coupling, auroral arc, field-aligned potential, feedback instability, computer simulation, magnetohydrodynamics
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Abstract:
Extensive three-dimensional computer simulations of the magnetosphere-ionosphere (M-I) coupling are performed to study self-excitation of auroral arcs with special emphasis on 1) nonlinear evolution of the feedback instability in the M-I coupling system, 2) controlling mechanisms of the auroral arc structure, 3) formation of a field-aligned electric potential structure in association with the development of the feedback instability, and 4) effects of the parallel potential generation on auroral arc development. As was already shown by Sato [1978] and Watanabe and Sato [1988], it is reconfirmed that the feedback instability produces a longitudinally elongated, latitudinally striated structure where the upward field-aligned current and the ionospheric density are locally enhanced. On top of this the present extended study reveals the following important new features : 1) The global distribution of the striation structure is primarily governed by the magnetospheric convection pattern and the ionospheric density distribution. 2) There appears a significant dawn-dusk asymmetry in the auroral arc formation, even though the apparent geometrical relationship is symmetric. This dawn-dusk asymmetry reflects the geometrical fact that the ionospheric Pedersen current closing the magnetospheric current is anti-symmetric with respect to the noon-midnight plane, while the self-closed Hall current is symmetric. 3) The recombination effect plays a significant role in the global, as well as local, development of the auroral arc structure. The nonlinearlty of recombination, in conjunction with the closure of an arc-associated local field-aligned current system, acts to destroy an old arc and creates a new arc in a different but adjacent position. This results in a peculiar behavior of auroral arc formation and deformation. 4) A V-shaped field aligned potential structure is created in association with an auroral arc. Rapid increase in the electron density and the local upward field-aligned current of an arc arises as a result of enhanced ionization by precipitating electrons accelerated by the parallel potential. 5) A drastic oscillatory behavior of appearance and disappearance of auroral arcs is obtained when the ionization effect is strong. The period is primarily given by the Alfven bounce time.
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