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

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5. Results

For selected spatial regions with sufficiently high emission, the power spectra averaged over several rows are given in Fig. 5. Significant power maxima from this figure, summarized in Table 2, show the known period near one hour for prominences A and D, both being observed over a sufficiently long time interval of 2 hours. Since the other time sequences are not long enough for a detection of these periods, our data agree with a general existence of long periods in prominences (cf. Wiehr et al. 1984; Bashkirtsev & Mashnich 1983, 1984).

[FIGURE] Fig. 5. Power of the Doppler shifts as a function of frequency at the locations of prominent Ca [FORMULA]  8542 emission maxima in prominence A (22-30; 48-54), B (73-76), C (5-8; 23-28), D (33-39), E (30-35; 51-57) and F (35-41; 51-60). The dashed lines mark the 90 percent confidence level


Table 2. Doppler periods in the spatial ranges from Fig. 5

Table 2 shows that for all prominences, except B, periods tend to group near 20 min, a value which Wiehr et al. (1989) found in nearly all of their observations. Periods between 3 and 10 min are slightly indicated in all observations with small cycle times, i.e. not in prominences A and D. Allthough the significance of those power maxima hardly exceeds the 90% level, their 'grouping' near periods of 3.5, 4.1, 5.3, 8.2, and 9.4 min may be a hint for their reliability. Prominence-A shows periods near half an hour, which still existed three days later (prominence-B), indicating a long lifetime. Indication for harmonic frequencies, as reported by Balthasar et al. (1988) and by Wiehr et al. (1988), is found in prominence-C at 23-28" (f0 =1.01 mHz) and in prominence-F at 35-41" (f0 =0.834 mHz).

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998