| dc.description.abstract | Abstract
The resistive tearing-mode instabilities of Harris sheet magnetic field configuration are studied based on two-dimensional, resistive, compressible MHD models with isotropic and anisotropic pressures, respectively. Different energy closures along with various parameter regimes of magnetic Reynolds number Rm, plasma beta, pressure anisotropy , magnetic By and the ratio of wavelength to the layer thickness are explored to give a systematic analysis. The linear growth rate and the eigenmode structures are calculated from the linear numerical models. The linear solutions are then used as initial perturbations of the nonlinear numerical models to allow the full evolution of resistive tearing-mode instability.
The linear calculations of isotropic resistive tearing-mode instability show that for the range of Rm=10-105 the fastest growth rate increases with decreasing Rm and only slightly increases with increasing plasma beta and By but is not sensitive to the equation of state. While the nonlinear calculations of isotropic tearing-mode instability show that only for large Rm, where the diffusion time is much larger than the linear growth time, the magnetic island may possibly grow substantially and become saturated. For small Rm the plasmoids either diminish in the late stage or do not have apparent growing, that is, the linear analysis is not meaningful for large resistivity cases. The calculations are compared to the magnetic island structure at earth’s magnetopause reconstructed from the single-spacecraft data by Hau and Sonnerup [1999].
The anisotropic resistive tearing-mode instability is studied within the framework of gyrotropic MHD theory for which the standard CGL or double adiabatic laws are replaced with the more generalized double-polytropic equations to incorporate various thermodynamic states of collisionless plasmas. The linear calculations show that the dependence of linear growth rate on the energy equations, plasmas beta as well as the magnetic By component is much more pronounced than that in isotropic plasmas; in particular, the growth rate is larger for smaller and as well as for larger and , a tendency in accordance with the mirror instability criterion based on the double-polytropic MHD model. For certain parameter regime, the growth rate may even increase with increasing magnetic Reynolds number, a result in contrast to the isotropic tearing instability. For sufficiently large pressure anisotropy of , the eigenmode is not purely exponential but contains oscillation with the overall growth rate much larger than the isotropic case. Cases of large growth rate on the order of Alfvén time scale are associated with oscillatory slow-mode structures that may exhibit negative as well as positive density-magnetic field correlation as predicted by Hau and Sonnerup [1993] being one of the anomalous behaviors associated with slow-mode waves in anisotropic plasmas. The nonlinear evolution shows for the first time that the oscillatory solutions may develop in the nonlinear anisotropic MHD model; in particular, the X-line and O-line take place alternatively with magnetic islands growing and saturated with oscillation period of about ten Alfvén transit time. The growth rate is on the order of Alfvén time scale and the plasma flow velocity may reach the Alfvén speed. The presence of mirror-mode wave may result in magnetic bottle structure on top of the X and O lines that is responsible for the enhanced growth rate and oscillation of magnetic reconnection. | en_US |