  We investigate the onset of 2D time-dependent magnetic reconnection that is
triggered using an external velocity driver located away from, and
perpendicular to, an equilibrium Harris current sheet. Previous studies have
typically utilised an internal trigger to initiate reconnection, e.g. initial
conditions centred on the current sheet. Numerical simulations solving the
compressible, resistive magnetohydrodynamics equations were performed to
investigate the reconnection onset within different atmospheric layers of the
Sun, namely the corona, chromosphere and photosphere. A reconnecting state is
reached for all atmospheric heights considered, with the dominant physics being
highly dependent on atmospheric conditions. The coronal case achieves a sharp
rise in electric field for a range of velocity drivers. For the chromosphere,
we find a larger velocity amplitude is required to trigger reconnection. For
the photospheric environment, the electric field is highly dependent on the
inflow speed; a sharp increase in electric field is obtained only as the
velocity entering the reconnection region approaches the Alfven speed.
Additionally, the role of ambipolar diffusion is investigated for the
chromospheric case and we find that the ambipolar diffusion alters the
structure of the current density in the inflow region. The rate at which flux
enters the reconnection region is controlled by the inflow velocity. This
determines all aspects of the reconnection start-up process, i.e. the early
onset of reconnection is dominated by the advection term in Ohms law in all
atmospheric layers. A lower plasma-$\beta$ enhances reconnection and creates a
large change in the electric field. A high plasma-$\beta$ hinders the
reconnection, yielding a sharp rise in the electric field only when the
velocity flowing into the reconnection region approaches the local Alfven
speed.
