The extent of 3-min oscillations in regions other than sunspot plumes

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 A, Mg x 625 A, O v 629 A, and He i 522 A. 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 A 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. The detection of waves in the solar atmosphere has been of great interest to both solar and plasma physicists. Observations of oscillatory phenomena in the solar atmosphere have increased dramatically in the past few years. Detailed observational studies, especially with the Solar and Heliospheric Observatory (SoHO) and the Transition Region And Coronal Explorer (TRACE) have provided a strong stimulus to theoretical developments. A new field of study, namely, coronal seismology has emerged, which allows the determination of coronal parameters by using information about the properties of the waves that the medium supports. This field is rapidly developing and providing insights into the processes at work in the corona. Mode frequencies and damping rates are two main observable properties of the waves, and, hence, are important in linking theoretical models with observations. Most of the observations to date have suggested that 3-min oscillations are associated with sunspots. De Moortel et al. (2002) reported the detection of 3-min oscillations in the coronal loops above sunspots and 5-min oscillations in other coronal loops. The observational results in Banerjee et al. (2002) indicate that 3-min oscillations are localized within the umbra. Maltby et al. (1999) found 3-min oscillations in coronal loops that are rooted in the umbra. By examining TRACE and CDS data, Brynildsen et al. (2002) found that the 3-min oscillations of transition region lines are present in the whole umbral region but the 3-min oscillations of coronal lines tend to localize in small regions that coincide with the endpoints of coronal loops. In their study, the 3-min oscillations were not observed with CDS at coronal temperatures within the spots, but were seen in the TRACE 171 A band. In a different joint observing campaign with CDS and TRACE, O’Shea et al. (2002) reported the detection of 3-min oscillations over a larger sunspot as seen in Fe xvi 335 A, a high temperature coronal line in CDS. This could indicate that the detection of oscillations crucially depends on the extent of the oscillating region.