Recent observations of prominence cores show thin dark threads, thanks to the high spatial resolution that long wavelength instruments can reach (e.g., a few tenths of an arcsec in Hα). However, the highest spatial resolution ground-based instruments, such as the Dutch Open Telescope (DOT)13 or the Swedish Solar Telescope (SST),14 have limitations for high cadence dynamical studies due to the atmospheric seeing. Space observatories such as the Hinode/SOT instrument15 do not have this problem. The situation is different for studies aimed at observing the hotter PCTR and the coronal environment through UV and soft X-ray instruments, for which only 1'' – 2'' resolution could be reached until now (e.g., SOHO/SUMER, Hinode/EIS-XRT, STEREO/EUVI and now SDO/AIA. The lack of suitable resolution for resolving the filament threads at shorter wavelengths makes it difficult to better picture the PCTR and the cool filament as a whole, even when multi-wavelength data are available. However, it is likely that the IRIS mission will obtain new results at higher resolution.
Recent statistical work on threads of quiescent and active region filaments made using Hα filtergrams of the SST instrument by Lin et al. (2005*, 2008*) confirms previous results with better precision. The width of filament threads has a narrow distribution centered on 0.3'' (for comparison, this is the chromospheric fibrils’ average width), even though these authors did not exclude the existence of sub-resolution structures. Their findings imply thread widths from about 150 to 450 km, where the lower limit is given by the instrumental resolution.
The length of the threads is on the order of a few thousand kilometers, which is generally less than the full filament length, particularly for intermediate and polar-crown filaments (e.g., Engvold, 1989), even though exceptions are observed.
We have mentioned that prominences observed at the limb in the Hα or Ca ii bands show fine-scale threads dominantly horizontal or vertical, even though mixed orientations are also present. In general, vertical fine scales dominate quiescent prominences, horizontal threads dominate active region prominences, and intermediate prominences may show both substructures.
Filaments can be stable for a long time or can show different types of global dynamics such as oscillations, lifting off, and eruptions (these will be discussed in Section 5). However, even if a filament is stable its threads are dynamic, showing mass flows along their axes and/or transverse displacement on timescales of a few minutes. Internal flows are inferred from the time variability of the darkening of threads, which allow their identification against the disk. Such variations may last from a few minutes up to 20 min (Engvold, 1976, 1981; Lin et al., 2005*). This could have several origins, one of which could be density variations due to the mass motions. Observing and characterizing the fine scale dynamics can yield important information on the magnetic field topology, estimate its amplitude, and measure velocity variations in time. In addition, the mass motions are an ingredient of the prominence plasma energy balance and mark the loading or draining of mass into and from the structure.
In images of the cool filament core, the plane of the sky velocities can be deduced from the tracking of proper motions. Transverse motion is instead deduced from Doppler shifts. If the full line profile is not available, this may be deduced by using a minimum of three intensity measurements: at the line center and at both sides in the line wings. However, sampling multiple points in both wings is a better solution for studying possible distortions of the line profile (multi-velocity components). This method is often used for the Hα line through tunable filters. The combination of the two measurements can give better estimates of the vector velocity, unless the full geometry of the structure is available.
However, these measurements are affected by the possible presence of sub-resolution dynamics and/or the presence of multiple threads along the line of sight, as well as the instrument spatial resolution and the optical thickness of the medium. In addition, the data inversion method and hypothesis adopted may affect the results. All these aspects are thoroughly discussed in Chae (2007*).
Lin et al. (2005*) using Hα data from the SST revealed the presence of a relative single-thread motion of 2 – 3 km s–1. The typical size of coherent motions (1500 × 15 000 km) of grouped threads has been measured, for example, by Schmieder et al. (1991*).
The most striking aspect of filament bulk motion is counter-streaming: a mass flow along the full filament length and barbs, which is nearly horizontal along the spine and of the same amplitude in both directions in adjacent threads (Zirker et al., 1998; Schmieder et al., 1991*; Lin et al., 2003*). The magnitude of this motion has been estimated to vary from about a few to 30 km s–1. These values were obtained by using different spatial resolutions and diagnostic techniques on data from different types of prominences. In the 1990s literature (e.g., Tandberg-Hanssen, 1995*) it is reported that quiescent prominences do not show this plasma flow or that it is of only a few km s–1, where the highest values are found in active prominences. However, the most recent studies made with the highest–spatial-resolution data seem to indicate higher values also in quiescent prominences. This is the case in the studies by Lin et al. (2003*) (8 km s–1) and Lin et al. (2005*) ( 15 km s–1), for example, where the same time-slice technique and Doppler-shift measurements were applied to different spatial resolution Hα data. Lin et al. (2008*) found flows up to 25 km s–1 in active region filaments.
Chae et al. (2005) and Chae (2007*) made use of the tunable Hα filter at BBSO16 to scan the line. They interpreted the observations using the profiles of single and multi-velocity component cloud models,17 to derive information on the plasma flow along the line of sight in the filament barbs and spine. In both regions they found that some threads are almost stationary while others exhibit a high-speed component of about 15 km s–1 along the line of sight, from which they deduced a total amplitude of the vector velocity of 30 km s–1 (under the assumption of horizontal motion). Using their models, these authors inferred that the stationary component could well be the result of the superposition of opposite velocity components of threads along the line of sight (they estimated five – six threads), as it would occur in the presence of counter-streaming motions. This result could be consistent with, for example, the Lin et al. (2003, 2005*) results when the effects of different spatial resolutions are taken into account.
Doppler shifts both red and blue (but predominantly blue), from a few to 15 km s–1, are observed in threads within barbs and at the filament ends. This line-of-sight velocity decreases approaching the center of the filament (Schmieder and Mein, 1989*; Schmieder et al., 1991*). Under the hypothesis that barbs root down in the chromosphere or photosphere, these motions have been interpreted as a signature of an inclined magnetic field with respect to the vertical to the solar surface. These flows could be the source of the material for the prominence, even though the imbalance between blue and red-shift should be confirmed and better explained. Similar results on plasma flows are obtained through the study of filament destabilization and eruption, as will be discussed in Section 5. Line-of-sight velocity measurements also reveal oscillatory motions of threads that will be discussed in Section 3.1.3.
The bulk motions inside threads have velocities on average lower than the sound speed for prominence plasma (20 – 40 km s–1). This suggests that waves may not be the origin of these motions. The mechanisms at the origin of such motions are still under debate, and are part of the problem of mass supply in prominences. This is discussed further in Section 4.
Investigations of the motions around the filament and inside the channel reveal that fibrils have similar flow patterns as threads, and that both are of lower amplitude than for chromospheric fibrils outside the filament channel (Schmieder et al., 1991*; Lin et al., 2008). This may be a further indication that a common global magnetic structure of the whole filament-filament channel system exists.
Berger et al. (2008*) in the Ca ii 3968 Å H line (Figure 22*) had mainly vertical substructures. They showed downflows in bright streams ( 10 km s–1 with a lifetime of about 10 min, 200 – 700 km wide, labeled D in the figure) sometimes transforming into vortices, turbulence, and oscillatory motions with finer details than previously reported. These downflows were already detected in the past in the prominence footpoints, with less detail and lower resolution (e.g., see Schmieder and Mein, 1989; Schmieder et al., 1991). Despite the difficulty of matching velocity features between data taken at different spatial resolutions, all these studies agree that the velocities are lower than the value expected for a free fall (around 20 km s–1 at the surface for a 15 Mm height prominence), implying the presence of an external unknown upward force.
Chae et al. (2008*) came to a similar conclusion by following the descent of a blob into a prominence observed in Hα, using an optical flow technique to follow the flows. They could identify a vertical upflow path from the prominence base, which transformed first in an almost horizontal path and then in a downflow through the formation of blobs. This path persisted longer than the lifetime of the single emission patch used to trace it.
Other characteristics were found in the work of Chae (2010*), who studied in detail these vertical bright threads and knots in a Ca ii hedgerow prominence observed by SOT. He reported a very small variation of the thread intensity with height and the quasi-absence of flows. He estimated a pressure-scale height greater than the hydrostatic scale height, therefore supporting the idea of the presence of non-hydrostatic support for the prominence plasma. In this work, the downward vertical motion identified by Berger et al. (2008*) was more associated with the bright knots than with the thread itself. It also confirmed that the bright vertical threads are not straight and homogeneous, and suggested either a spatial resolution problem or a different nature of the fine structure than simple straight flux tubes. The implication of these results will be discussed at the end of this chapter.
In Figure 22* it is also possible to recognize dark inclusions (labeled U) called plumes which are the location of turbulent upflows ( 20 km s–1, 180 – 740 km in width). These dark plumes, originating from dark cavities at the base of the prominences, are seen intermittently. The discovery of the plumes opens the question of whether they correspond to the Doppler upflows measured in earlier lower resolution observations of filaments. In more recent works Berger et al. (2010*, 2011) investigated these outflows in more detail in different quiescent prominences. The characteristics of these turbulent flows (listed in Table 3 of Berger et al., 2010) not only confirms the Berger et al. (2008*) values, but also extends the earlier results to a larger set of studied prominences. Studying the evolution of the dark cavity regions at the base of prominences that may be the origin of plumes, they found a growth of the cavity at a velocity consistent with buoyancy. The observed stability of certain cavities with respect to others is seen as the result of the relative strengths of the buoyancy force and magnetic tension of the overlying prominence. Certain cavities can evolve in turbulent plumes through classical Rayleigh–Taylor instability in a magnetized environment. An important aspect highlighted by the authors is the possibility that this mechanism could supply mass to the prominence and the overlying coronal cavity. This would balance the continuous draining of mass in prominences and allow the structure to be seen as quasi stable (over larger scales) for weeks. The authors also supported the idea that the dark appearance is produced by a higher temperature of the plasma outside the sensitivity range of the Ca ii filter (between 2.5 and 12 × 105 K), which feeds the coronal cavity, as has been discussed in Section 2.3. This new interpretation of the small-scale turbulent motions has become a hot topic in the past few years. The effectiveness of the Rayleigh–Taylor instability in the prominence environment has been simulated by Hillier et al. (e.g., 2012a, and references therein) and Hillier et al. (2012b*) assuming a 3D Kippenhahn–Schlüter model. These first efforts could reproduce several properties resembling those derived from observations (such as up and downflows), but quantitative comparisons of results with observations should be further demonstrated, particularly considering the properties of the magnetic field whose strength and direction are difficult to measure.
Oscillating motions of filaments or part of filaments are regularly observed. These periodic motions are generally associated with the presence of magnetohydrodynamic (MHD) waves and their study is called prominence seismology. Oscillations are detected via local flux variations in high spatial and temporal resolution images, and through varying Doppler shifts along the line of sight. The study of prominence oscillations allows the derivation of several important parameters such as plasma density or Alfvén speed, from which the magnetic field strength can be deduced (e.g., Régnier et al., 2001; Lin et al., 2009*).
For an overview of the different modes of oscillations, see Vršnak (1993) and Nakariakov and Verwichte (2005). Oliver and Ballester (2002) and Oliver (2009*) reviewed small-scale–prominence-oscillation observations and modeling, and recommended several important aspects that should be taken into account for a correct and complete analysis of oscillations. Tripathi et al. (2009b) reviewed large-amplitude oscillations, while a general review of oscillations is given in Banerjee et al. (2007), Mackay et al. (2010*) and Arregui et al. (2012), where open questions are discussed. Here are a few general properties and examples.
Oscillating motions are observed both in prominence at the limb and on the disk, indicating that the plane of oscillation may have different directions with respect to the structure. In certain cases the oscillations are observed only in the vertical (e.g., Okamoto et al., 2007) or transverse directions (e.g., Berger et al., 2008). On-disk oscillations were observed in both directions (e.g., Lin et al., 2009).
The parameters of oscillations, like amplitude of the velocity, period and spatial distribution, may vary, implying the influence of several physical mechanisms. In terms of amplitude of the velocity of threads, oscillations are classified as large (a few tens of km s–1) and small (a few km s–1 or less) amplitudes. The measured periods may be divided in three groups: short (< 5 min), intermediate (6 – 20 min) and long (< 20 min).
The triggers of oscillating motions can be internal or external. External agents can be, for example, p mode excitation or EIT or Moreton waves (Ramsey and Smith, 1966) or other CME/flare-initiated disturbances (Jing et al., 2003, 2006; Vršnak et al., 2007; Luna and Karpen, 2012*). Internal agents can be flux emergence or disappearance (Feynman and Martin, 1995*). These perturbations, if large enough, can also lead to the destabilization of the structure, as discussed in Section 5.
For small amplitude oscillations, a range of periods from a few minutes to more than one hour is observed. A large variety in the spatial distribution is also reported. The disturbance rarely involves the whole structure, but most frequently only a few threads, or part of the threads. Different threads of the same structure may be subject to different stimulations: for example, Ning et al. (2009) detected the oscillatory motion of a group of threads associated with their drifting.
The oscillations generally last for a few periods, then decay and disappear from the data, indicating a damping process. Theoretical studies have then to address the different mechanisms that either stimulate or suppress the waves in prominences (see, e.g., Oliver, 2009*). A recent example is given by the self-consistent model by Luna et al. (2012a) and Luna and Karpen (2012*), where their results highlight interesting properties of large-amplitude oscillations. Earlier studies of prominence plasma formation through thermal nonequilibrium showed that thin flux tubes heated asymmetrically at the footpoints develop longitudinal oscillations, while condensation of material fills the threads’ magnetic dips (Antiochos and Klimchuk, 1991; Luna et al., 2012b). Luna and Karpen (2012) found that gravity is the restoring force for these oscillations, and continuous mass accretion onto the prominence threads is responsible for the damping. This investigation also demonstrated that the same forces govern observed prominence oscillations initiated by external disturbances, and that the very existence of these large-amplitude oscillations provide strong evidence for the presence of dips in filament-channel magnetic fields. A new diagnostic tool was revealed: by measuring the frequency of oscillations, the radius of curvature of dips and magnetic field amplitude can be derived from observations.
At the same time, restrictions in the prominence oscillation observations due to technical constraints of the instruments, and the uncertainty regarding the real thread structure, limit the possibility of testing the various proposed scenarios for wave formation and damping. As discussed by Oliver (2009), much work still needs to be done to reconcile the observational aspects with the theoretical predictions, even though recent efforts on both sides are improving the situation.
Due to the lower spatial and temporal resolutions available for the UV-EUV data with respect to the optical domain, studies addressing the dynamic behavior of prominences access mainly bulk and (spatially and temporally) averaged velocities, including sub-feature (blob) motions. Table 6 of Labrosse et al. (2010b*) summarizes the latest studies, in which motions from a few to several tens km s–1 were measured. The different values listed in this Table are found for different features, using different lines of sight and spectral lines with different formation temperatures. These results all indicate the dynamic nature of prominences throughout their layers, highlighting the necessity for increased efforts to access the finest temporal and spatial scales in order to obtain a clear unified picture of these motions.
Line widths of the PCTR emission exhibit signatures of non-thermal motions, even for quiescent structures. Figure 23* reports the results for non-thermal velocities as a function of temperature by Parenti and Vial (2007*) for a quiescent prominence (bottom panel) to be compared to the QS values (top panel). As shown in this figure, this velocity component increases generally with temperature, reaching about 30 km s–1 at log T(K) = 5.5 (e.g., Parenti and Vial, 2007*, and references therein), then decreases for higher temperatures. Some measurements reported a less continuous distribution (Stellmacher et al., 2003). The existence of unresolved fine structure along the line of sight may cause the different results listed in the literature. Non-thermal motions derived from line widths have been interpreted as a signature of waves and used to show that the energy they transport is not enough to balance the radiative losses of the prominence. Another interpretation is microturbulence, although not much attention has been given yet to this aspect. There is also the possibility that this extra line width is the result of the superimposition of the internal motions of the unresolved fine structure. At present, we are not able to disentangle these various possibilities; only when EUV data at higher spatial resolution are available, will we be able to infer further fine scale properties.
In the presence of unresolved structures, it is generally useful to introduce the filling factor, which provides an estimate of the amount of space occupied by the unresolved fine structure in the observed data. This can be defined in various ways, and may depend on the temperature of the emitted radiation, the emitting volume along the line of sight, and the instrument resolution (Labrosse et al., 2010b*). The different estimates found in the literature point at a volumetric filling factor in the range 0.001 – 0.2, which indeed suggests that the PCTR emission comes from a small fraction of the observed volume.
Starting from this information, the next step is to establish how the PCTR is structured. In the various pictures proposed, cool threads can be surrounded by their own PCTR, a bundle of threads can have a common PCTR.
The investigation of the bright H Lyα line is a powerful tool to link the properties of the cooler filament core, formed by threads of sub-arcsec diameter, with its PCTR. This line is indeed formed between the chromosphere and the bottom of the PCTR, so that it images warmer plasma than Hα. Using H Lyα data from the rocket-borne VAULT instrument, Vial et al. (2012) have found that the typical size of threads on the disk is between 1'' and 2'', with a minimum of 0.4''. This is comparable with the picture of cool threads sharing a common PCTR.
Another interesting recent observation brings unique information on the small scale structure of the PCTR and the corona. This was made by the High-resolution Coronal Imager (Hi-C) on board a sounding rocket (Cirtain et al., 2013) which observed, on 11 July 2012, an active region with a filament in the 195 Å waveband. For the first time a spatial resolution of 0.3'' in this waveband was reached. Alexander et al. (2013*) concentrated their analysis of these data on bright threads of the filament and measured their width using the FWHM of the intensity profile across a coherent fine structure. The result gives substructures of about 0.8'' ± 0.1'', which is larger than the spatial resolution of the instrument and the 0.3'' typical width of Hα threads. Even if this instrument is imaging hotter plasma than VAULT, the values found from the data of the two sounding rockets are compatible. The Alexander et al. (2013) thread width could suggest a single thread surrounded by its own PCTR, but this needs to be confirmed by co-spatial and co-temporal high-resolution analysis using both cool threads and PCTR data.
Various methods are used to estimate the number of threads along the line of sight when observing at higher temperatures (which means lower resolution) than H Lyα: inferred from the Doppler and non-thermal velocities, or from the density and differential emission measures (with some assumptions). For example, when the structure of a thread with its own PCTR is assumed, results from EUV observations suggest that the number of threads along the line of sight is from a few to 30 (see Labrosse et al., 2010b for details, Gunár et al., 2011), and their diameters are between a few km and more than 200 km (Wiik et al., 1999; Patsourakos and Vial, 2002*). These values have to be compared with values of about 200 km inferred from observing the cooler plasma in Hα presented in Section 3.1.1. For example, we have seen that Chae (2007) inferred five to six threads along the line of sight using the Hα line profile at less than 0.5'' spatial resolution. These values are consistent with the EUV estimates, even though the latter vary over a larger range. One reason could be the large uncertainties in the EUV data inversion, which has too many free parameters and hence may have multiple solutions.
Cool threads have been estimated to have widths of a fraction of 1'' (see Section 3). This is also consistent with a few tens of threads estimated within the 1'' pixel of the EUV common data, assuming that the threads are all the same and each is enveloped by its own PCTR. However, these comparisons are certainly not exhaustive and different diagnostics have to be investigated. Recent work by Soler et al. (2011*), for example, shows that the damping of kink waves in prominence threads depends on the transverse inhomogeneity scale length. This property offers a promising way to test the different models for the PCTR, but actual application to the data has still to come.
Various interpretations have been raised to explain the different plasma motions mentioned above, with the attempt, where possible, of reconciling limb and on-disk observations of vertical and horizontal fine structures. These motions need to be interpreted in the context of the different global magnetic-field configurations proposed by the different models mentioned in Section 2.4.
Lin et al. (2005*), as other authors, found their observations compatible with the model of filaments made of thin, horizontal, sheared magnetic flux tubes partially filled with condensed coronal plasma: the threads, suggested by Karpen and Antiochos (2008) (and references therein, see also the middle panel of Figure 3*). However, this model cannot easily explain the vertical appearance of threads at the limb.
A different interpretation of threads is given by a group of models (e.g., Low, 1982; Poland and Mariska, 1986; Heinzel and Anzer, 2001; Heinzel et al., 2005; Heinzel and Anzer, 2006) which aim at explaining both the vertical aspect of Hα threads at the limb and the horizontal aspect on the disk. They introduce vertical magnetic dips in a horizontal magnetic field as the location where the thread plasma is concentrated (see also Section 2.4.3). The vertical superposition of these dips gives the observed appearance of vertical threads at the limb and as dark horizontal fine threads when observed from above. This picture can explain, for example, the observations by Chae et al. (2008), Schmieder et al. (2010) and Chae (2010*). The threads form magnetic dips filled with cool material (the bright knots of Chae, 2010*) that deforms by gravity the magnetic structure, to give the impression of vertical threads. In support of this idea Ahn et al. (2010) also identified returning flows that could be a signature of the counter-streaming flows observed in filaments from above. However, in this proposed picture the full filament length flow is not real but the result of a superposition effect. Chae (2010) extended this idea of magnetic dips by proposing the presence of reconnection and interchange of material between knots and their surrounding to justify the long descending paths (up to 15 000 km) and the local impulsive accelerations detected in knots (up to 0.1 km s–2 in absolute value). These new ideas on the presence of multiple dips offer significant promise to solve the paradox of the orientation of the cool threads. Higher–spatial-resolution magnetic-field measurements and PCTR velocity studies will certainly help by better testing these ideas. In addition, setting up more realistic modeling will improve the comparison with the observations. Encouraging first attempts in such a direction include the Hillier et al. (2012b) simulation of plume dynamics. Certainly, the counter-streaming motion along the full filament is an established observational property (within the accessible spatial resolutions) and needs to be accounted for in all models, including those proposing magnetic dips.
The models for the fine magnetic structure should also be tested in terms of compatibility with other observational properties. One example is given by the small-scale turbulent dynamics inside prominences recently observed with Hinode/SOT, and which have been interpreted in terms of plasma buoyancy. Indeed a preliminary numerical MHD simulation on a Kippenhahn–Schlüter prominence model (Hillier et al., 2011) found that such a mechanism may be compatible with the presence of the horizontal magnetic field without being completely inhibited. Further investigations should reveal if and how this process affects the stability of the whole structure.
On the issue of the cool-PCTR fine structure of filaments, we understand that the observational results are generally obtained under certain assumptions, which may vary from one study to the next, and that these results span a large range and may not be in agreement. In particular, in determining the number of threads along the line of sight, values derived from velocity measurements disagree with those derived from the density (Cirigliano et al., 2004). Even if the high resolution Hα data are bringing us closer to resolving a single thread element, much work should be done to improve the data inversion by limiting the free parameters of the used models. A better constraint will arise combining IRIS UV and optical data, allowing systematic studies with similar resolutions in order to establish the coherence in the fine scale structure. This could confirm, for example, the results obtained using the few minutes observations by VAULT H-Lyα and Hi-C 193 Å. To better analyze the plasma conditions inside threads, it is important to compare the fast changes (a few minutes) in a typical thread’s darkness observed by Lin et al. (2005*) with the characteristic time scale for the stationary radiative state. We have mentioned that such variations may be the result of a density change due to mass motion, which affects the amount of absorbed underlying chromospheric emission, or to local temperature changes due to some cooling/heating events. Engvold (1980) has tested this and found that the relaxation time of a hydrogen plasma is on the order of 1 – 2.5 mins, which is of the same order as the observed intensity change. This lack of thermal equilibrium should be better tested and taken into account in the diagnostics and modeling of prominence fine structure.