Abstract:
The active region, AR0554, was observed with NIS/CDS on board SoHO to examine the extent and range of oscillations from a range of
features. Among all the NIS spectral lines analysed, significant oscillations were found in Si xii 520 Å, Mg x 625 Å, O v 629 Å, and
He i 522 Å. The periods of the strongest oscillations in these lines were ≈10−20 min. After the dominant 10−20 min oscillations were filtered
out from these lines, only O v 629 Å showed significant (i.e. above the 95% significance level) shorter-period oscillations. Specifically, we
found that weak but significant 3-min oscillations are not confined to the umbra/plume but can be seen in many bright locations. The duration
of these non-umbral 3-min oscillations is mostly ≈20 min. In contrast, the 3-min oscillations within the umbra are strong and stable for longer
than 50 min. The duration could be related to the size of the oscillating source region, rather than the lifetime of the oscillation. To find the
possible sources of these 3-min oscillations outside of the umbra, we compared the oscillations of single pixels in different regions. The results
of our comparison indicate a possible connection between the magnetic fields and the oscillations. Therefore, we suggest that 3-min oscillations
may exist in many magnetic structures, but are often too weak to be seen in an unfiltered signal.
