Abstract:
We observe intensity oscillations along coronal fan loops associated with the active region AR 11428. The
intensity oscillations were triggered by blast waves that were generated due to X-class flares in the distant active
region AR 11429. To characterize the nature of oscillations, we created time–distance maps along the fan loops and
noted that the intensity oscillations at two ends of the loops were out of phase. As we move along the fan loop, the
amplitude of the oscillations first decreased and then increased. The out-of-phase nature together with the
amplitude variation along the loop implies that these oscillations are very likely to be standing waves. The period
of the oscillations is estimated to be ∼27 minutes, damping time to be ∼45 minutes, and phase velocity projected
in the plane of sky to be ∼65–83 km s−1
. The projected phase speeds were in the range of the acoustic speed of
coronal plasma at about 0.6 MK, which further indicates that these are slow waves. To the best of our knowledge,
this is the first report on the existence of the standing slow waves in non-flaring fan loops.
