  Light bridges, the bright structure dividing umbrae in sunspot regions, show
various activity events. In Paper I, we reported on analysis of
multi-wavelength observations of a light bridge in a developing active region
(AR) and concluded that the activity events are caused by magnetic reconnection
driven by magnetconvective evolution. The aim of this second paper is to
investigate the detailed magnetic and velocity structures and the formation
mechanism of light bridges. For this purpose, we analyze numerical simulation
data from a radiative magnetohydrodynamics model of an emerging AR. We find
that a weakly-magnetized plasma upflow in the near-surface layers of the
convection zone is entrained between the emerging magnetic bundles that appear
as pores at the solar surface. This convective upflow continuously transports
horizontal fields to the surface layer and creates a light bridge structure.
Due to the magnetic shear between the horizontal fields of the bridge and the
vertical fields of the ambient pores, an elongated cusp-shaped current layer is
formed above the bridge, which may be favorable for magnetic reconnection. The
striking correspondence between the observational results of Paper I and the
numerical results of this paper provides a consistent physical picture of light
bridges. The dynamic activity phenomena occur as a natural result of the bridge
formation and its convective nature, which has much in common with those of
umbral dots and penumbral filaments.
