Quiescent solar filaments are clouds of cool and dense plasma suspended against gravity by forces thought to be of magnetic origin. They form along the inversion polarity line in or between the weak remnants of active regions. Early observations already suggested that their fine structure is apparently composed by many horizontal and thin dark threads (de Jager, 1959; Kuperus and Tandberg-Hanssen, 1967). More recent high-resolution H observations obtained with the Swedish Solar Telescope (SST) in La Palma (Lin et al., 2005) and the Dutch Open Telescope (DOT) in La Palma (Heinzel and Anzer, 2006) have allowed to observe this fine structure with much greater detail (see Lin, 2010, for a review). The measured average width of resolved thin threads is about 0.3 arcsec ( 210 km), while their length is between 5 and 40 arcsec ( 3500 – 28,000 km). The fine threads of solar filaments seem to be partially filled with cold plasma (Lin et al., 2005), typically two orders of magnitude denser and cooler than the surrounding corona, and it is generally assumed that they outline the magnetic flux tubes in which they reside (Engvold, 1998; Lin, 2005; Lin et al., 2005; Okamoto et al., 2007; Engvold, 2008; Martin et al., 2008; Lin et al., 2008). This idea is strongly supported by observations which suggest that threads are skewed with respect to the filament long axis in a similar way to what has been found for the magnetic field (Leroy, 1980; Bommier et al., 1994; Bommier and Leroy, 1998). On the opposite, Heinzel and Anzer (2006) suggest that these dark horizontal filament structures are a projection effect. According to this view, many magnetic field dips of rather small vertical extension, but filled with cool plasma, are aligned in the vertical direction and the projection against the disk produces the impression of a horizontal thread.
Prominences are highly dynamic structures that display flows. These flows have been observed in H, UV, and EUV lines, and their study and characterization are of great interest for the understanding of prominence formation and stability, the mass supply, and the prominence magnetic field structure. In the H line, and in quiescent limb prominences, a complex dynamics with vertical downflows and upflows (Berger et al., 2008) as well as horizontal flows is often observed. The velocities are in the range between 2 and 35 km s–1, while in EUV lines flow velocities seem to be slightly higher. When comparing these values one should be aware that these lines correspond to different temperatures, so probably the reported flow speeds correspond to different parts of the prominence. In active region prominences, flow velocities seem to be higher than in quiescent prominences, even reaching 200 km s–1, and some of these high-speed flows are probably related with the prominence formation itself. In the case of filaments observed on the disk in the H line, horizontal flows in the filament spine are often observed, while in barbs flows are vertical. The range of observed velocities of filament flows is between 5 and 20 km s–1. A particular feature in these observations is the presence of counter-streaming flows, i.e., oppositely directed flows (Zirker et al., 1998; Lin et al., 2003). Because of the physical conditions of the filament plasma, all these flows seem to be field-aligned. For a thorough information about flows in prominences see Labrosse et al. (2010) and Mackay et al. (2010).
Solar prominences are subject to various types of oscillatory motions. Some of the first works on this subject were concerned with oscillations of large amplitude induced by disturbances coming from a nearby flare. Later, observations performed with ground-based telescopes pointed out that many quiescent prominences and filaments display small amplitude oscillations (Harvey, 1969). These oscillations have been commonly interpreted in terms of standing or propagating magnetohydrodynamic (MHD) waves. Using this interpretation, a number of theoretical models have been set up in order to try to understand the prominence oscillatory behaviour. Such as we will point out in the following, the study of prominence oscillations can provide an alternative approach for probing their internal structure. The magnetic field structure and physical plasma properties are often hard to infer directly and wave properties directly depend on these physical conditions. Therefore, prominence seismology seeks to obtain information about prominence physical conditions from a comparison between observations and theoretical models of oscillations.
This review is mainly devoted to small amplitude oscillations, although a brief section deals with large amplitude oscillations. The layout of the review is the following: large amplitude oscillations are succinctly summarized in Section 2; in Section 3, the observational background about small amplitude oscillations is reviewed; in Section 4, theoretical models of small amplitude oscillations based on linear ideal MHD waves in different configurations are described; next, in Section 5, the damping of prominence oscillations produced by different mechanisms is studied from a theoretical point of view; finally, in Section 6, prominence seismology using large and small amplitude oscillations is introduced.
Living Rev. Solar Phys. 9, (2012), 2
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