The Effects of Shear Flow on Collisionless Magnetic Reconnection in the Heliosphere*

Magnetic reconnection is an effective collisionless dissipation mechanism at current sheets, one which has been argued to be important for accelerating particles and heating plasma. However, recent observations of the current sheets made by Parker Solar Probe close to the sun do not appear to be undergoing reconnection. This suppression is correlated with the presence of a shear flow, and persists, unexpectedly for sub-Alfvénic speeds. Using kinetic particle-in-cell simulations of magnetic reconnection, we investigate this regime with a range of initial shear flows. We show that the reconnection outflow velocity is reduced by shear-dependent heating which occurs when the inflowing ions are pitch angle scattered in the exhaust. We show that while the outflow velocity and the reconnection rate are reduced by an increased shear flow, the total amount of heating generated is increased, as the reconnection process dissipates the shear flow energy. For even modest shear flows (V_shear ~ V_A/2), more shear flow heating is dissipated than magnetic energy. These results have important implications for magnetic reconnection’s efficiency in energy dissipation and non-thermal particle acceleration in systems where shear flows would likely be present, such as the turbulent, weakly collisional solar wind.