A magnetospheric energy principle for hydromagnetic stability problems
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概要
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A magnetospheric energy principle is formulated to study hydromagnetic stability of a magnetospheric plasma. The magnetospheric plasma is either in a two-dimensional or three-dimensional static equilibrium. It is surrounded by lateral perfectly conducting walls and ideal ionospheres in both cases and also by dawn-dusk periodic boundaries in the two-dimensional case. The two-dimensional case has a translational symmetry and has no unperturbed magnetic field component in the dawn-dusk direction. Unlike the conventional energy principle for a plasma surrounded by a perfectly conducting wall, field lines are assumed to vertically thread the ionospheric boundary, which is not a perfectly conducting rigid wall. Ideal ionospheric boundary conditions are obtained, so that the force operator becomes self-adjoint and the magnetospheric energy principle is valid. There are four ideal ionospheric boundary conditions to satisfy these requirements: horizontally free, conducting, free, and rigid. A change in the potential energy becomes equal to the sum of the change in the fluid energy and an ionospheric surface contribution, which is negative and thus destabilizing for horizontally free and free ionospheric boundary conditions. A minimization condition for the change in the potential energy is obtained. When an unperturbed field-aligned current vanishes, the horizontally free, conducting, free, and rigid boundary conditions allow interchange, incompressible ballooning, incompressible ballooning, and compressible ballooning modes, respectively. Different characteristics of those three pressure-driven modes are clarified. Existing interchange stability criteria are compared and results of several different numerical stability analyses of ballooning instabilities for different magnetospheric equilibria are discussed systematically in light of the present magnetospheric energy principle.
- American Geophysical Union,Department of Earth and Planetary Physics, University of Tokyoの論文
American Geophysical Union,Department of Earth and Planetary Physics, University of Tokyo | 論文
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