Network oscillations at the boundary of an equatorial coronal hole

¹ Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany e-mail: tian@mps.mpg.de
² Department of Geophysics, Peking University, Beijing, PR China
³ School of Space Science and Physics, Shandong Univ. at Weihai, Weihai, Shandong, PR China e-mail: xld@sdu.edu.cn

Received: 5 May 2008
Accepted: 20 June 2008

Abstract

Aims. We investigate intensity oscillations observed simultaneously in the quiet chromosphere and in the corona, above an enhanced network area at the boundary of an equatorial coronal hole.

Methods. A Fourier analysis is applied to a sequence of images observed in the 171 Å and 1600 Å passbands of TRACE. Four interesting features above the magnetic network are further investigated by using a wavelet analysis.

Results. Our results reveal that, in both the 171 Å and 1600 Å passbands, oscillations above the magnetic network show a lack of power at high frequencies (5.0–8.3 mHz), and a significant power at low (1.3–2.0 mHz) and intermediate frequencies (2.6–4.0 mHz). The global 5-min oscillation is clearly present in the 4 analyzed features when seen in the 1600 Å passband, and is also found with enhanced power in feature 1 (leg of a large coronal loop) and feature 2 (legs of a coronal bright point loop) when seen in the 171 Å passband. Two features above an enhanced network element (feature 3 and feature 4) show repeated propagating behaviors with a dominant period of 10 min and 5 min, respectively.

Conclusions. We suggest these oscillations are likely to be slow magneto-acoustic waves propagating along inclined magnetic field lines, from the lower solar atmosphere into the corona. The energy flux carried by these waves is estimated of the order of for the 171 Å passband and is far lower than the energy required to heat the quiet corona. For the 1600 Å passband, the energy flux is about , which is about one third of the required energy budget for the chromosphere.