Omnipresent long-period intensity oscillations in open coronal structures

Abstract

Context. Quasi-periodic propagating disturbances in coronal structures have been interpreted as slow magneto-acoustic waves and/or periodic upflows. It is important to understand the nature of these disturbances before proceeding to apply them. Aims. Here we aim to understand the nature of these disturbances from the observed properties using a three-hour imaging sequence from AIA/SDO in two different temperature channels. We also compare the characteristics with a simple wave model. Methods. We searched for propagating disturbances in open-loop structures at three different locations; a fan loop- structure off-limb, an on-disk plume-like structure and the plume/interplume regions in the north pole of the sun. In each of the subfield regions chosen to cover these structures, the time series at each pixel location was subjected to wavelet analysis to find the different periodicities. We then constructed powermaps in three different period ranges, short (2 min – 5 min), intermediate (5 min – 12 min), and long (12 min – 25 min), by adding the power in the individual range above the 99% significance level. We also constructed space-time maps for the on-disk plume structure to estimate the propagation speeds in different channels. Results. We find propagating disturbances in all three structures. Powermaps indicate that the power in the long-period range is significant up to comparatively longer distances along the loop than that in the shorter periods. This nature is observed in all three structures. A detailed analysis on the on-disk plume structure gives consistently higher propagation speeds in the 193 A channel and also reveals spatial damping along the loop. The amplitude and the damping length values are lower in hotter channels, indicating their acoustic dependence. Conclusions. These properties can be explained very well with a propagating slow-wave model. We suggest that these disturbances are more likely to be caused by propagating slow magneto-acoustic waves than by high-speed quasi-periodic upflows. We find that intensity oscillations in longer periods are omnipresent at larger heights even in active regions.