3.2 Spectral indicators

The vast majority of spectroscopic reports of prominence oscillations are based on the analysis of the Doppler velocity. Some other spectral indicators (line intensity and line width) have also been used in the search for periodic variations in prominences and, sometimes, a periodic signal has been recognized in more than one of these indicators. Landman et al. (1977Jump To The Next Citation Point) observed periodic fluctuations in the integrated line intensity and line width with period around 22 min, but not in the Doppler shift. In addition, Yi et al. (1991Jump To The Next Citation Point) detected periods of 5 and 12 min in the power spectra of the line-of-sight velocity and the line intensity. Also, Suematsu et al. (1990Jump To The Next Citation Point) found signs of a ∼ 60 min periodic variation in the Doppler velocity, line intensity, and line width. Nevertheless, the Doppler signal also displayed shorter period variations (with periods around 4 and 14 min) which were not present in the other two data sets. We here encounter a common feature of other investigations, namely that the temporal behaviour of various indicators corresponding to the same time series of spectra do not agree, either because they show different periods in their power spectra (as in Tsubaki et al., 1987Jump To The Next Citation Point) or because one indicator presents a clear periodicity while the others do not (Wiehr et al., 1984Jump To The Next Citation Point; Tsubaki and Takeuchi, 1986Jump To The Next Citation Point; Balthasar et al., 1986Jump To The Next Citation Point; Tsubaki et al., 1988Jump To The Next Citation Point; Suetterlin et al., 1997Jump To The Next Citation Point). Only rarely have the oscillations been detected in several of these spectral indicators at the same time and with the same period, which constitutes a puzzling feature of prominence oscillations. This can be caused by insufficient instrumental sensitivity or by the effect different waves have on the plasma parameters (pressure, magnetic field, …), which in turn may give rise to perturbations of one spectral indicator alone. In addition, Harvey (1969Jump To The Next Citation Point) failed to detect periodic perturbations in the line-of-sight component of the magnetic field in a set of prominences that displayed oscillations of the Doppler velocity. He attributed this to the fact that variations in this magnetic field component were below the observational limit.

Special mention must be made of the study performed by Balthasar and Wiehr (1994Jump To The Next Citation Point), who simultaneously observed the spectral lines He at 3888 Å, H8 at 3889 Å and Ca+IR3 at 8498 Å. From this information they analyzed the temporal variations of the thermal and non-thermal line broadenings, the total H8 line intensity, the He 3888 Å to H8 emission ratio and the Doppler shift of the three spectral lines. The power spectra of all these parameters yield a large number of power maxima, but only two of them (with periods of 29 and 78 min) are present in more than one indicator.

The interpretation of the results just summarized appears difficult. First, the theoretical models (see Section 4) can give the temporal behaviour of the plasma velocity, the density, and other physical parameters, in a prominence. The observations, however, yield information on quantities such as the line intensity or the line width. Hence, a clear identification of the spectral parameters with physical variables (density, pressure, temperature, magnetic field strength) is required before further progress can be achieved. Then, the presence of a certain period in more than one signal could be used to infer the properties of the MHD wave involved. Another useful source of information could be the detection of a given period in one signal but not in the others, such as reported in some works discussed above.

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