  In response to a solar wind dynamic pressure enhancement, the compression of
the magnetosphere generates strong ionospheric signatures and a sharp variation
in the ground magnetic field, termed sudden commencement (SC). Whilst such
compressions have also been associated with a contraction of the ionospheric
polar cap due to the triggering of reconnection in the magnetotail, the effect
of any changes in dayside reconnection is less clear and is a key component in
fully understanding the system response. In this study we explore the
time-dependent nature of dayside coupling during SC by performing global
simulations using the Gorgon MHD code, and impact the magnetosphere with a
series of interplanetary shocks with different parameters. We identify the
location and evolution of the reconnection region in each case as the shock
propagates through the magnetosphere, finding strong enhancement in the dayside
reconnection rate and prompt expansion of the dayside polar cap prior to the
eventual triggering of tail reconnection. This effect pervades for a variety of
IMF orientations, and the reconnection rate is most enhanced for events with
higher dynamic pressure. We explain this by repeating the simulations with a
large explicit resistivity, showing that compression of the magnetosheath
plasma near the propagating shock front allows for reconnection of much greater
intensity and at different locations on the dayside magnetopause than during
typical solar wind conditions. The results indicate that the dynamic behaviour
of dayside coupling may render steady models of reconnection inaccurate during
the onset of a severe space weather event.

Space Physics;Earth and Planetary Astrophysics;Computational Physics;Geophysics