Effect of in-plane shear flow on the magnetic island coalescence instability

Using a 2D Viscoresistive Reduced MagnetoHydroDynamic model, the magnetic island coalescence problem is studied in the presence of inplane, parallel shear flows. Extending the analytical work of Waelbroeck et al. [Phys. Plasmas 14, 022302 (2007)] and Throumoulopoulos et al., [J. Phys. A 42, 335501 (2009)] in the sub-Alfvenic flow shear regime for Fadeev equilibrium, the super-Alfvenic regime is studied for the first time numerically. A wide range of values of shear flow amplitudes and shear scale lengths have been considered to understand the effect of sub-Alfvenic and super-Alfvenic flows on the coalescence instability and its nonlinear fate. We find that for flow shear length scales greater than the magnetic island size, the maximum reconnection rate decreases monotonically from sub-Alfvenic to super-Alfvenic flow speeds. For scale lengths smaller than the island size, the reconnection rate decreases up to a critical value v(0c), beyond which the shear flow is found to destabilize the islands. The value of v(0c) decreases with a decrease in the value of shear flow length scale. Interestingly, for our range of parameters, we find suppression of the Kelvin-Helmholtz instability in super-Alfvenic flows even when the shear scale length is smaller than the island width. Observation of velocity streamlines shows that the plasma circulation inside the islands has a stabilizing influence in strong shear flow cases. Plasma circulation is also found to be responsible for the decrease in upstream velocity, causing less pileup of magnetic flux on both sides of the reconnection sheet. Published under an exclusive license by AIP Publishing.

Automated summary

The study investigates the magnetic island coalescence problem in the presence of inplane, parallel shear flows using a 2D Viscoresistive Reduced MagnetoHydroDynamic model. It is found that for shear flow length scales greater than the magnetic island size, the maximum reconnection rate decreases monotonically, while for scales smaller than the island size, a critical value is reached where the shear flow destabilizes the islands. Additionally, the study shows suppression of the Kelvin-Helmholtz instability in super-Alfvenic flows and the stabilizing influence of plasma circulation inside the islands in strong shear flow cases.