Intensity oscillations observed with Hinode near the south pole of the Sun: leakage of low frequency magneto-acoustic waves into the solar corona

TL;DR

This study observes ~7 and ~13 minute intensity oscillations in the solar chromosphere and corona near the south pole, suggesting magneto-acoustic wave leakage from magnetic network regions into higher solar layers.

Contribution

It provides evidence of low-frequency magneto-acoustic wave leakage into the solar corona and proposes a new model involving field-free cavity resonators under magnetic canopies.

Findings

~7 min intensity oscillations observed in all spectral lines

~13 min oscillations detected in He II with high probability

Magneto-acoustic waves likely propagate from cavity resonators to the corona

Abstract

Aims. To study intensity oscillations in the solar chromosphere/corona above a quiet-Sun magnetic network. Methods. We analyse the time series of He II 256.32, Fe XI 188.23 and Fe XII 195.12 spectral lines observed by EUV Imaging Spectrometer (EIS) on board Hinode near the south pole. Then we use a standard wavelet tool to produce power spectra of intensity oscillations above the magnetic network. Results. We get ~7 min intensity oscillations in all spectral lines and ~13 min intensity oscillations only in He II with the probability of ~96-98 %, which probably reflects the process of magneto-acoustic wave propagation above the network. Conclusions. We suggest that field-free cavity areas under bipolar magnetic canopies in the vicinity of magnetic network may serve as resonators for the magneto-acoustic waves. The cavities with photospheric sound speed and granular dimensions may produce the waves with the observed periods. The waves may propagate upwards in the transition region/corona and cause observed intensity oscillations.