Soliton approach to magnetic holes

Magnetic holes (MHs), depressions in the magnetic field magnitude associated with enhancements in density and kinetic pressure, have been observed in the solar wind, the magnetosheaths of terrestrial planets and in the environments of comets, suggesting that this phenomenon may be a common occurrence in space plasmas. MHs are usually believed to result from the mirror instability which can develop in high-beta plasmas with a temperature anisotropy, T-perpendicular to/T-parallel to > 1. Motivated by the fact that solar wind MHs are often observed in a mirror mode stable environment [Winterhalter et al., 1994], this paper proposes a mechanism for the maintainance of MHs in an equilibrium plasma. We suggest an explanation in terms of magnetically rarefactive ("dark") MHD solitons with anticorrelation of magnetic field and density, which propagate with small velocities at large angles to the ambient magnetic field. This intrinsically nonlinear approach is based on a magnetohydrodynamic plasma description including Hall inertia effects and utilizes the well-developed soliton theory of the Derivative Nonlinear Schrodinger Equation (DNLS) which appears as a partly adequate approximation to the parent Hall-MHD system. The approach introduces an alternative mechanism into the discussion over the physical nature of MHs that is not related to an instability and provides an explanation for various aspects of the observations including amplitude, thickness, and spatial structure of MHs.