  Recurrent, arc-shaped intensity disturbances were detected by EUV channels in
an active region. The fronts were observed to propagate along a coronal loop
bundle rooted in a small area within a sunspot umbra. Previous works have
linked these intensity disturbances to slow magnetoacoustic waves that
propagate from the lower atmosphere to the corona along the magnetic field. The
slow magnetoacoustic waves propagate at the local cusp speed. However, the
measured propagation speeds from the intensity images are usually smaller as
they are subject to projection effects due to the inclination of the magnetic
field with respect to the line-of-sight. Here, we aim to understand the effect
of projection by comparing observed speeds with those from a numerical model.
Using multi-wavelength data we determine the periods present in the
observations at different heights of the solar atmosphere through Fourier
analysis. We calculate the plane-of-sky speeds along one of the loops from the
cross-correlation time lags obtained as a function of distance along the loop.
We perform a 2D ideal MHD simulation of an active region embedded in a
stratified atmosphere. We drive slow waves from the photosphere with a 3
minutes periodicity. Synthetic time-distance maps are generated from the
forward-modelled intensities in coronal wavelengths and the projected
propagation speeds are calculated. The intensity disturbances show a dominant
period between [2-3] minutes at different heights of the atmosphere. The
apparent propagation speeds calculated for coronal channels exhibit an
accelerated pattern with values increasing from 40 to 120 km/s as the distance
along the loop rises. The propagation speeds obtained from the synthetic
time-distance maps also exhibit accelerated profiles within a similar range of
speeds. We conclude that the accelerated propagation in our observations is due
to the projection effect.
