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Astron. Astrophys. 346, 633-640 (1999)

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1. Introduction

At the solar surface p-mode oscillations are observed mainly as a vertical wave motion. This is a consequence of the refraction of acoustic waves towards the vertical direction; the diffraction occurs because the sound velocity decreases from the solar interior towards the surface. However, a horizontal velocity component generally must also exist, except for the spherically symmetric modes with degree [FORMULA]. In particular the f mode, which essentially is a surface wave, should have horizontal and vertical velocity components of approximately the same magnitude.

Information about the horizontal velocity components can be obtained from the center-to-limb variation of the line-of-sight velocity measured via the Doppler effect. Stix & Wöhl (1974) used sequences of one-dimensional scans made at diverse positions on the solar disk. For each of these positions they computed the distribution of the power in the diagnostic diagram of wave number and frequency, and so could analyze the center-to-limb variation of the wave spectrum. The results confirmed the essentially vertical character of the solar oscillations.

The purpose of the present contribution is to improve the analysis of the horizontal and vertical oscillation components on the basis of the superior data now available from the SOHO spacecraft. The new data are two-dimensional on the solar surface and have much less noise than the one-dimensional ground-based data used in the earlier study. In addition, the field size and the duration are larger so that the resolution in the diagnostic diagram is much better (although we shall again consider limited fields at various disk positions so that the wave-number resolution is not given by the full disk but rather by the field size). With these data we are of course able to pay attention to the particular behavior of the specific modes of global oscillation, the f and p modes. This was not possible in the study of Stix & Wöhl (1974); at that time these modes had not yet been identified observationally, although a theoretical prediction (Ulrich 1970) had been made.

In Sects. 2-4 of this paper we describe the data treatment and the computation of the power spectra. The results are presented in Sect. 5; Sects. 6 and 7 contain a discussion of the geometric and atmospheric effects that influence the observed Doppler shift. This discussion is guided by a model of adiabatic oscillations of small amplitude in a plane-parallel isothermally stratified atmosphere.

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© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999