Tearing-mode instability in anisotropic plasmas: Effects of energy closure

The resistive tearing-mode instability of tangential-discontinuity current sheets is studied within the framework of linear double-polytropic MHD theory. Four sets of polytropic exponents, γ∥ and γ⊥, are chosen to describe various thermodynamic states. It is shown that the dependence of tearing growth rate on the energy equations, plasma beta as well as the magnetic B_y component is much more pronounced than that in isotropic plasmas; in particular, the growth rate is larger for smaller γ∥ and γ⊥ as well as for larger p⊥/p∥ and β⊥. For certain parameter regime, the growth rate may even increase with increasing magnetic Reynolds number, a result in contrast to the pure tearing instability. Cases of large growth rate on the order of Alfvén time scale are associated with oscillatory slow-mode structures that may sometimes exhibit positive density-magnetic field correlation.