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Astron. Astrophys. 321, 921-926 (1997)

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1. Introduction

The physical conditions of cool solar prominences embedded in hot coronal surrounding is not yet understood. Numerical calculations (e.g. by Joarder & Roberts 1993) show that prominence oscillations may be driven by magnetic, pressure, and gravity forces. They manifest as internal, external, and hybrid modes (Oliver et al. 1993). This division disappears if the prominence-corona transition region, PCTR, is smooth (Oliver & Ballester 1995). In case of a sharp PCTR, internal prominence oscillations can be detected only if the PCTR is optically thin (Oliver & Ballester 1996).

Hence, the determination of oscillatory modes in prominences gives important physical information. Existing observations (an extended list of references is given by Oliver & Ballester 1995) point to a large variety of periods. Even two simultaneously used telescopes give no coherent results (Balthasar et al. 1993). A possible reason is imperfect guiding of the prominence on the spectrograph slit (cf. Balthasar & Wiehr 1994), since the observer may tend to apply corrections in constant time intervals given by the telescope's drift.

This can be avoided by an automatic guider which stabilizes the image perpendicular to the spectrograph slit, whereas drifts parallel to the slit can be removed in the spectra. Such a 'one-dimensional limb guider' has been realized at the engineer's college, FH Wiesbaden, and was first applied for the present observations. The aim of this paper is to check the existence of 'typical' periods from observations of several prominences guided on the slit with exceptional high spatial accuracy.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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