We investigate the onset of 2D time-dependent magnetic reconnection that istriggered using an external velocity driver located away from, andperpendicular to, an equilibrium Harris current sheet. Previous studies havetypically utilised an internal trigger to initiate reconnection, e.g. initialconditions centred on the current sheet. Numerical simulations solving thecompressible, resistive magnetohydrodynamics equations were performed toinvestigate the reconnection onset within different atmospheric layers of theSun, namely the corona, chromosphere and photosphere. A reconnecting state isreached for all atmospheric heights considered, with the dominant physics beinghighly dependent on atmospheric conditions. The coronal case achieves a sharprise in electric field for a range of velocity drivers. For the chromosphere,we find a larger velocity amplitude is required to trigger reconnection. Forthe photospheric environment, the electric field is highly dependent on theinflow speed; a sharp increase in electric field is obtained only as thevelocity entering the reconnection region approaches the Alfven speed.Additionally, the role of ambipolar diffusion is investigated for thechromospheric case and we find that the ambipolar diffusion alters thestructure of the current density in the inflow region. The rate at which fluxenters the reconnection region is controlled by the inflow velocity. Thisdetermines all aspects of the reconnection start-up process, i.e. the earlyonset of reconnection is dominated by the advection term in Ohms law in allatmospheric layers. A lower plasma-$\beta$ enhances reconnection and creates alarge change in the electric field. A high plasma-$\beta$ hinders thereconnection, yielding a sharp rise in the electric field only when thevelocity flowing into the reconnection region approaches the local Alfvenspeed.
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