  Densely packed coronal loops are rooted in plages in the vicinity of active
regions on the Sun. The photospheric magnetic features underlying these plages
are patches of mostly unidirectional magnetic field extending several arcsec on
the solar surface. We aim to explore the transient nature of the magnetic
field, its mixed-polarity characteristics, and the associated energetics in
plages using high resolution observations and simulations. We used photospheric
Fe I 6173 {\AA} spectropolarimetric observations of a decaying active region
obtained from the SST. These data were inverted to retrieve the photospheric
magnetic field underlying the plage as identified in the EUV emission maps
obtained from the SDO/AIA. To obtain better insight into the evolution of
plages, we performed 3D radiation magnetohydrodynamic simulations of
magnetoconvection using the MURaM code. The observations show transient
magnetic flux emergence and cancellation events within the plage on timescales
of a few 100 s and on spatial scales comparable to granules. These transient
events occur at the footpoints of active region plage loops. In one case the
coronal response at the footpoints of these loops is clearly associated with
the underlying transient. The simulations also reveal similar magnetic flux
emergence and cancellation events that extend to even smaller spatial and
temporal scales. Individual simulated transient events transfer an energy flux
in excess of 1 MW m$^{-2}$ through the photosphere. We suggest that the
magnetic transients could play an important role in the energetics of active
region plage. Both in observations and simulations, the opposite-polarity
magnetic field brought up by transient flux emergence cancels with the
surrounding plage field. Magnetic reconnection associated with such transient
events likely conduits magnetic energy to power the overlying chromosphere and
coronal loops.
