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

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7. Repeated disturbances

Such a candidate was the prominence at W40N, observed on July, 31 (A) and August, 3 (B and C). However, even this apparently stable prominence repeatedly exhibited disturbances of the kind described by Stellmacher & Wiehr (1973): a region of low (or absent?) emission moved almost radially through the prominence body showing occasionally Doppler shifts at its boundaries (e.g. locations 37" and 42" in Fig. 3). Such events occured in that W40N-prominence on July, 31 (A), August, 3 (B and C), and also on August 1, where no time series but only a few spectra could be taken between clouds. Prominence-G exhibited similar events. Interestingly, both prominences were of the same 'hedgerow type' (cf. Fig. 6) as that observed by Stellmacher & Wiehr (1973).

[FIGURE] Fig. 6. [FORMULA] slitjaw images of a prominence with an enlarged space between the vertical thread structures (above the slit) being a typical candidate for a vertically upward moving disturbance with high Doppler shifts at its borders

The repetitive character of such events as well as the temporal variation of the Doppler velocities at their boundaries may simulate quasi oscillatory motion. Hence, also for such apparently 'quiescent' prominences, evolutionary variations are hardly separable from pure internal oscillations. These, instead, require a statistical analysis from a large number of observations. A corresponding observing routine has been started at the Locarno observatory.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998