Role of photospheric footpoint shear in the impulsive dynamics of the solar corona

A frequently observed feature of a solar flare is its impulsive growth from a relatively quiescent background. This imposes a significant constraint on magnetic reconnection models which propose to account for flares: not only should the time scale be fast, but it must also exhibit an impulsive character. It is shown that the spatial width of the shearing motion of the photospheric footpoints has a significant qualitative effect on the time-evolution of the current sheet and the reconnection electric field in a two-dimensional coronal loop with a neutral line. The reconnection dynamics for localized footpoint shear profiles does not show an impulsive phase and occurs on a Sweet-Parker time scale (∼ η^-1/2), whereas the reconnection dynamics with broad shear profiles occurs on a faster time scale with a weaker dependence on the resistivity (∼ η^-1/5), and furthermore, exhibits an impulsive phase due to flux pile-up near the reconnection layer.