  We report on the first direct observation of a fast-mode wave propagating
along and perpendicular to cool (171 {\AA}) arcade loops observed by the
SDO/AIA. The wave was associated with an impulsive/compact flare, near the edge
of a sunspot. The EUV wavefront expanded radially outward from the flare center
and decelerated in the corona from 1060-760 km/s within ~3-4 minute. Part of
the EUV wave propagated along a large-scale arcade of cool loops and was
partially reflected back to the flare site. The phase speed of the wave was
about 1450 km/s, which is interpreted as a fast-mode wave. A second overlying
loop arcade, orientated perpendicular to the cool arcade, is heated and becomes
visible in the AIA hot channels. These hot loops sway in time with the EUV
wave, as it propagated to and fro along the lower loop arcade. We suggest that
an impulsive energy release at one of the footpoints of the arcade loops causes
the onset of an EUV shock wave that propagates along and perpendicular to the
magnetic field.
