  Recent MHD simulations find that the reversal of the local resistive
interchange parameter $D_R$ from negative to positive due to impurity radiation
cooling is able to trigger the resistive tearing mode growth in a tokamak above
a threshold in impurity level. A layer of perturbed Pfirsch-Schl\"{u}ter
current density and resistivity are also induced by the impurity radiation,
which further govern the tearing mode growth and saturation in the nonlinear
stage. The impurity threshold and the tearing mode growth strongly depend on
the parallel thermal conductivity, and such a dependence derives from the
impact on $D_R$ of the fast parallel thermal equilibration along the helical
magnetic field lines.
