Modeling of the Magnetohydrodynamic Instability

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1. Dependence of the K-H instability on the flow speed and the magnetic field of the magnetosheath was investigated by the 2D MHD simulation. The supersonic shear flow was found to be unstable to the K-H instability. This result suggests that the tail flank boundary of the magnetosphere is also unstable to the K-H instability. By assuming that the unperturbed magnetic field rotates continuously from the magnetosheath direction to the northward direction in the magnetosphere across the magnetopause, it was found that the northward magnetosheath magnetic field is more favorable to the instability than the southward magnetosheath magnetic field. This is because when the magnetosheath magnetic field is southward there appears a magnetic field component parallel to the flow in the rotation of the magnetic field across the magnetopause, which contributes to stabilization of the K-H instability. The velocity boundary layr generated by the K-H instability was found to be thicker when the magnetosheath magnetic field is northward. This result obtained by the simulation is consistent with observations showing that the low latitude boundary layr is thicker when the magnetosheath magnetic field is northward.2. The 3D MHD linear analysis of the K-H instability in the magnetosphere shows that the K-H instability is stabilized by the 3D coupling to the ionosphere (line-tying effect) and the growth rate of the instability decreases with increasing ionospheric Pedersen conductivity. The K-H instability was also found to be stabilized by the unperturbed perpendicular magnetic field in the magnetosphere, which is generated by the ionospheric Pedersen current and is parallel to the flow. When the ionospheric Pedersen conductivity exceeds a critical value, the K-H instability was found to be completely suppressed by the magnetic tension force associated with the unperturbed perpendicular magnetic field in the magnetosphere.