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

Abstract

Aims: We study intensity oscillations in the solar chromosphere and corona, above a quiet-Sun magnetic network.Methods. We analyse the time series of He II 256.32 Å, Fe XI 188.23 Åand Fe XII 195.12 Åspectral lines, observed close to the south pole, by the EUV Imaging Spectrometer (EIS), onboard Hinode. We use a standard wavelet tool, to produce power spectra of intensity oscillations above the magnetic network.Results. For all spectral lines, we detect intensity oscillations of period of approximately seven minutes; and for the He II 256.32 Åline only, we detect an intensity oscillation of period of thirteen minutes, with a probability of approximately 96-98%, which provides the most likely signature of magneto-acoustic wave propagation above the network. Conclusions: We propose that field-free cavity areas under bipolar magnetic canopies, in the vicinity of a magnetic network, are likely to serve as resonators for the magneto-acoustic waves. The cavities with photospheric sound-speed, and granular dimensions, can produce waves with observed periods. These waves may propagate upwards in the transition region/corona and cause observed intensity oscillations. © 2008 ESO.