Letter to the Editor

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

¹ Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland e-mail: aks@arm.ac.uk
² Abastumani Astrophysical Observatory at I. Chavchavadze State University, Al Kazbegi ave. 2a, 0160 Tbilisi, Georgia e-mail: [dato;temury]@genao.org
³ Department of Applied Physics, Institute of Technology, Banaras Hindu University, Varanasi-221005, India e-mail: bholadwivedi@gmail.com

Received: 25 December 2007
Accepted: 21 February 2008

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 Å and Fe XII 195.12 Å spectral 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 Å line 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.