Thresholds for magnetic percolation through the magnetopause current layer in asymmetrical magnetic fiels

Physics

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Electric Current, Magnetic Field Reconnection, Magnetic Fields, Magnetic Permeability, Magnetopause, Plasma Turbulence, Distribution Functions, Kinetics, Velocity Distribution, Vlasov Equations

Scientific paper

The Vlasov kinetic approach is used to study the stability of magnetic surfaces with respect to spontaneous excitation of collisionless tearing perturbations within magnetopause current layers (MCLs) with asymmetrical magnetic field profiles. For the unperturbed configuration a 'tractable' (that is, with a minimum number of free parameters) Vlasov equilibrium model describing a tangential discontinuity is developed. In this model, asymmetric MCLs are not electrostatically equipotential configurations and their structure is only determined by the angle of the magnetic field rotation theta(sub O) and the magnetic field asymmetry factor Kappa(sub B) = (B(sub 2) - B(sub 1))/B(sub 2), where B(sub 1) and B(sub 2) are the magnetic field intensities in the adjacent magnetosheath and magnetospheric regions, respectively. The stochastic percolation model by Galeev et al. (1986), based on the symmetrical charge-neutral Harris equilibrium, is generalized for asymmetrical MCLs. Asymmetry in the B field profile strongly modifies the dependence of the marginal MCL thickness (below which the MCL is subjected to percolation) on the polatity of the interplanetary magnetic filed (IMF). For a northward IMF (theta(sub O) less than 90 deg, the percolation is impossible when Kappa(sub B) equal to or greater than 0.4, while for moderate values of Kappa(sub B) (0.15 less than or equal to Kappa(sub B) less than 0.4) only thin MCLs can be percolated. When Kappa(sub B) equal to or greater than 0.3, the maximum thickness of MCLs subjected to percolation is achieved for theta(sub O) greater than 90 deg, that is, for a southward IMF. Assuming that the magnetopause should, on the average, be close to its stability threshold, realistic asymmetrical MCLs (with Kappa sub B greater than 0.3) should be thinner for a northward IMF than for a southbound IMF.

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