3.1 Detection methods

The investigation of small amplitude prominence oscillations has most often been done by spectroscopical means, but also using images (e.g., Foullon et al., 2004Jump To The Next Citation Point) or filtergrams, i.e., images taken in a given spectral line (e.g., Yi et al., 1991Jump To The Next Citation Point; Yi and Engvold, 1991Jump To The Next Citation Point; Lin, 2005Jump To The Next Citation Point; Berger et al., 2008Jump To The Next Citation Point; Ning et al., 2009bJump To The Next Citation Point,aJump To The Next Citation Point). Regarding these studies using a two-dimensional field of view, in some of them the variations along selected straight paths have been analyzed (Berger et al., 2008Jump To The Next Citation Point; Ning et al., 2009bJump To The Next Citation Point,aJump To The Next Citation Point). This simplifies the study but also reduces the amount of oscillatory information that can be derived (see Figure 2View Image).
View Image

Figure 2: Time slices taken at three heights in a quiescent prominence. The bright sinusoidal patterns are caused by horizontal oscillations of the plasma with periods between 20 and 40 min. The orange lines denote oscillations with phases that approximately match. The slope of these lines implies an upward propagation speed of about 10 km s–1 (projected on the plane of the sky) (from Berger et al., 2008Jump To The Next Citation Point).

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., 1984Jump To The Next Citation Point). Another technique that provides information about a small region of a prominence is the so-called differential method (Kobanov, 1983; Bashkirtsev and Mashnich, 1984Jump To The Next Citation Point). 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, 1986Jump To The Next Citation Point; Suematsu et al., 1990Jump To The Next Citation Point; Balthasar et al., 1993Jump To The Next Citation Point; Balthasar and Wiehr, 1994Jump To The Next Citation Point; Suetterlin et al., 1997Jump To The Next Citation Point). Then, this yields a time series of spectra on each slit position (see, for example, Figure 4 of Tsubaki and Takeuchi 1986Jump To The Next Citation Point), 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 1986Jump To The Next Citation Point). Slit observations have also been conducted from space, using SUMER on SoHO (Blanco et al., 1999Jump To The Next Citation Point; Régnier et al., 2001Jump To The Next Citation Point) and CDS on SoHO (Pouget et al., 2006Jump To The Next Citation Point).

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., 1991Jump To The Next Citation Point; Yi and Engvold, 1991Jump To The Next Citation Point; Lin et al., 2003Jump To The Next Citation Point; Lin, 2005Jump To The Next Citation Point; Lin et al., 2005Jump To The Next Citation Point, 2007Jump To The Next Citation Point, 2009Jump To The Next Citation Point) and with space-based telescopes (Okamoto et al., 2007Jump To The Next Citation Point). In addition, two-dimensional Dopplergrams have also been employed (Molowny-Horas et al., 1999Jump To The Next Citation Point; Terradas et al., 2002Jump To The Next Citation Point), 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., 2001Jump To The Next Citation Point; Pouget et al., 2006Jump To The Next Citation Point); the Si iv and O iv lines at 1393.76 Å and around 1401 – 1405 Å, formed at transition region temperatures (Blanco et al., 1999Jump To The Next Citation Point); and 195 Å images, with a formation temperature of 1.5 MK (Foullon et al., 2004Jump To The Next Citation Point). 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.


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