Propagating Oscillations in the Lower Atmosphere under Coronal Holes

TL;DR

This study investigates oscillations in the lower solar atmosphere within coronal holes, revealing upward propagating waves with specific periods and examining their possible relation to Alfvén waves and other MHD modes.

Contribution

It provides new spectroscopic evidence of propagating oscillations in coronal-hole regions and explores their potential connection to Alfvén waves and MHD wave transformations.

Findings

Significant five-minute oscillations in coronal-hole regions

Upward propagation indicated by phase shifts between layers

Line-width oscillations likely related to other MHD modes

Abstract

The subject of this study is oscillations in the lower atmosphere in coronal-hole regions, where the conditions are favorable for propagation between the atmospheric layers. Based on spectroscopic observations in photospheric and chromospheric lines, we analysed the features of the oscillations that show signs of propagation between layers of the solar atmosphere. Using the cross-spectrum wavelet algorithm, we found that both chromospheric and photospheric signals under coronal holes share a range of significant oscillations of periods around five minutes, while the signals outside of coronal holes show no mutual oscillations in the photosphere and chromosphere. The phase shift between the layers indicates a predominantly upward propagation with partial presence of standing waves. We also tested the assumption that torsional Alfv\'en waves propagating in the corona originate in the lower atmosphere. However, the observed line-width oscillations, although similar in period to the Alfv\'en waves observed earlier in the corona of open-field regions, seem to be associated with other MHD modes. If we assume that the oscillations that we observed are related to Alfv\'en waves, then perhaps this is only through the mechanisms of the slow MHD wave transformation.