  In this study, the evolution of a highly unstable m = 1 resistive tearing
mode, leading to plasmoid formation in a Harris sheet is studied in the
framework of full MHD model using the NIMROD simulation. Following the initial
nonlinear growth of the primary m = 1 island, the X-point develops into a
secondary elongated current sheet that eventually breaks into plasmoids. Two
distinctive viscous regimes are found for the plasmoid formation and
saturation. In the low viscosity regime (i.e. P r . 1), the plasmoid width
increases sharply with viscosity, whereas in the viscosity dominant regime
(i.e. P r & 1 ), the plasmoid size gradually decreases with viscosity. Such a
finding quantifies the role of viscosity in modulating the plasmoid formation
process through its effects on the plasma flow and the reconnection itself.
