Regarding spectroscopic observations, different setups have been used to gather the temporal variation of the spectral indicators and more complexity and refinement has been gained over the years. For example, in some initial studies an entrance hole was placed on a selected area of a prominence (Wiehr et al., 1984). Another technique that provides information about a small region of a prominence is the so-called differential method (Kobanov, 1983; Bashkirtsev and Mashnich, 1984). A very widely used method in the investigation of small amplitude prominence oscillations is to place a spectrograph slit on a prominence (a few examples from a very long list are: Tsubaki and Takeuchi, 1986; Suematsu et al., 1990; Balthasar et al., 1993; Balthasar and Wiehr, 1994; Suetterlin et al., 1997). Then, this yields a time series of spectra on each slit position (see, for example, Figure 4 of Tsubaki and Takeuchi 1986), from which the temporal variation of the spectral indicators (Doppler shift, line intensity, integrated line intensity, line width) can be derived. These time series can be later analyzed to obtain the period, wavelength, etc. of the oscillations (an example is shown in Figure 5 of Tsubaki and Takeuchi 1986). Slit observations have also been conducted from space, using SUMER on SoHO (Blanco et al., 1999; Régnier et al., 2001) and CDS on SoHO (Pouget et al., 2006).
A spectroscopic observation using a slit yields restricted information on the spatial distribution of oscillations and, what is even worse, does not ensure that the same plasma elements are placed on the slit during the observing time. The first of these concerns also applies to the analysis of a two-dimensional data set in which only variations in one direction are considered. Observations using a two-dimensional field of view and with high spatial resolution have diminished these worries, while allowing to study how prominence threads participate of the oscillatory motions. These observations have been conducted both with ground-based telescopes (Yi et al., 1991; Yi and Engvold, 1991; Lin et al., 2003; Lin, 2005; Lin et al., 2005, 2007, 2009) and with space-based telescopes (Okamoto et al., 2007). In addition, two-dimensional Dopplergrams have also been employed (Molowny-Horas et al., 1999; Terradas et al., 2002), although the spatial resolution of this particular observation is not good enough to appreciate the prominence thread structure.
Although most data used in the analysis of small amplitude prominence oscillations come from typical prominence lines, in some cases spectral lines or images formed at hotter temperatures have also been considered. Examples are the He i line at 584.33 Å, formed at 20,000 K (Régnier et al., 2001; Pouget et al., 2006); the Si iv and O iv lines at 1393.76 Å and around 1401 – 1405 Å, formed at transition region temperatures (Blanco et al., 1999); and 195 Å images, with a formation temperature of 1.5 MK (Foullon et al., 2004). Cool prominences or filaments can be identified in coronal lines since the line intensity is reduced by means of two different mechanisms: absorption and volume blocking (Anzer and Heinzel, 2005). In the first case, coronal radiation coming from behind the cool structure is partially absorbed, while in the second case the volume filled with cool plasma does not contribute to coronal emission and in this region the radiative output is reduced as compared with the surrounding corona. These two mechanisms give place to a brightness reduction of coronal lines and allows us to identify the volume occupied by cool and dark structures like prominences or filaments. Arguably, oscillations in the dense prominence affect their rarer neighbourhood, so a joint investigation of the dynamics of the two media has a very promising seismological potential.
Living Rev. Solar Phys. 9, (2012), 2
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