Letter to the Editor

Properties of high-degree oscillation modes of the Sun observed with Hinode/SOT

¹ University of Sheffield, Department of Applied Mathematics, Hicks Building, Sheffield S3 7RH, UK e-mail: U.MitraKraev@sheffield.ac.uk
² W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305, USA
³ National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan

Received: 9 November 2007
Accepted: 6 January 2008

Abstract

Aims.With the Solar Optical Telescope on Hinode, we investigate the basic properties of high-degree solar oscillations observed at two levels in the solar atmosphere, in the G-band (formed in the photosphere) and in the  H line (chromospheric emission).

Methods.We analyzed the data by calculating the individual power spectra as well as the cross-spectral properties, i.e., coherence and phase shift. The observational properties are compared with a simple theoretical model, which includes the effects of correlated noise.

Results.The results reveal significant frequency shifts between the  H and G-band spectra, in particular above the acoustic cut-off frequency for pseudo-modes. The cross-spectrum phase shows peaks associated with the acoustic oscillation (p-mode) lines, and begins to increase with frequency around the acoustic cut-off. However, we find no phase shift for the (surface gravity wave) f-mode. The observed properties for the p-modes are qualitatively reproduced in a simple model with a correlated background if the correlated noise level in the  H data is higher than in the G-band data. These results suggest that multi-wavelength observations of solar oscillations, in combination with the traditional intensity-velocity observations, may help to determine the level of the correlated background noise and to determine the type of wave excitation sources on the Sun.