1.3 Structures and dynamics in the solar atmosphere
1.3.2 Prominences and filaments
Prominences are volumes of plasma that is denser and cooler than the surrounding environment. They are anchored in the photosphere and extend through the corona. They are supported against gravity by the effect of the magnetic field. An example of this kind of structures is shown in the left panel of Figure 1.7, where the plasma can be clearly seen protruding from the limb of the Sun. They form along the polarity inversion lines (PIL) of the magnetic field, i.e., the lines that divide two regions of opposite magnetic field polarity in the photosphere, but the precise mechanism of its formation is still under active research. Their temperatures are typically one hundred times lower than those found in the corona, of the order of 104 K,
reason why the prominence plasma is partially ionized instead of fully ionized. In contrast, they are between one hundred and one thousand times denser than the corona, with densities of the order of 10−12 to 10−10 kg m−3. Their heights are of the order of 104 km, their widths
vary between 4 and 30 thousand kilometers and their lengths are of the order of 105 km.
Filaments are the same type of feature than prominences. The only differences between both are their position in the Sun and their brightness when compared to the surrounding background: prominences extend from the limb of the Sun and appear bright in comparison with the space in the background. On the other hand, filaments are located over the solar disk and show up as dark lines. An example of the appearance of a filament is shown on the right
panel of Figure 1.7. Prominences and filaments can be classified in two categories: quiescent and active. The quiescent class is more stable and can have lifetimes of up to several months. A typical value of the magnetic field strength in these structures is 10 G. Active prominences form faster, are associated with sunspot groups and have much shorter lifetimes, of minutes to hours. They also have stronger magnetic fields. Some prominences may suffer an eruption, detaching from the Sun and ejecting their material into space. More detailed portrayals of the properties of these structures can be found in, e.g., Labrosse et al. , Mackay et al. orParenti .
Figure 1.7: Left: image of a prominence extending from the limb of the Sun (Credit: Gary Palmer / Royal Museums Greenwich). Right: Hα image of a filament (Credit: Y. Lin).
The first historical record of the description of a solar prominence appears in the Laurentian Codex or Chronicle of Novgorod, written in the 14th century by Russian monk Laurentius (see,
Sviatsky ). In that text, the solar eclipse of 1 May 1185 is mentioned as follows: “In the evening there was a sign on the Sun. The night fell on the Earth and stars could be seen [...]. The Sun became like the Moon and from the horns of the crescent came out somewhat like live embers”. However, as it occurs with many aspects of the Sun, most of what is known about prominences has been discovered in the last twenty or thirty years. Earlier observations like those analyzed inde Jager  orKuperus and Tandberg-Hanssen  suggested that prominences and filaments have a fine structure. Nevertheless, this fact was not confirmed until more recent observations with higher resolution were performed (Lin et al. , Heinzel and Anzer , Lin et al. , Okamoto et al. , Berger et al. ) and showed that prominences are composed of long and thin threads or fibrils. The width of the threads is about 200 km and its length vary from ∼ 3500 to ∼ 28, 000 km. The fine structure of prominences can be noticed in Figure 1.8, an image obtained in Hα, which is caused by the radiation emitted
by neutral hydrogen when its electron falls from the third to its second lowest energy level. This line, whose wavelength is 656.3 nm, appears in the red part of the visible range of the electromagnetic spectrum.
Observations in Hα, ultraviolet (UV) and extreme-ultraviolet (EUV) lines also reveal a rich dynamics in prominences and filaments. For instance, mass flows along the threads axes and transverse to them have been frequently reported (Engvold [1976, 1981], Zirker et al.
Figure 1.8: Threads of a quiescent filament observed with the Swedish 1-m Solar Telescope. From Lin.
of the flows in quiescent filaments vary from 5 to 30 km s−1, while higher speeds have been
detected in active region prominences: Chae et al.  reported motions with speeds of up to 40 km s−1 and, later,Chae  detected jet-like and eruptive behaviors with speeds from
80 to 250 km s−1. The presence of flows may lead to the appearance of instabilities like the
Kelvin-Helmholtz instability (see, e.g.,Soler et al. [2012b]). More details about this instability and how is affected by partial ionization are given in Chapter 6 of this Thesis.
In addition to the mass flows mentioned in the previous paragraph, oscillatory motions have also been detected in prominences and filaments. The first observations of this kind of motion correspond to what is now known as large-amplitude oscillations, which are typically caused by disturbances coming from flares. The relation between large-amplitude oscillations and nearby flares was demonstrated by Moreton and Ramsey : waves propagating at speeds between 500 and 1500 km s−1 impact on prominences and cause them to vibrate during
a few periods with amplitudes of the order of 20 km s−1. These oscillations, which are global,
i.e., affect the whole prominence, are quite rare events, although in the last years a growing number of observations has been reported (Eto et al., Jing et al. , Okamoto et al.
,Gilbert et al. ,Luna and Karpen ). However, the nature of large-amplitude oscillations is still poorly understood. More details about the research on this subject can be found inTripathi et al.. The topic of large-amplitude waves in partially ionized plasmas, with an application to the particular case of a quiescent prominence, is addressed in Chapter 5 of the present Thesis.
Prominences are also subject to oscillatory motions of small amplitude, which have been commonly interpreted in terms of standing or propagating magnetohydrodynamic (MHD) waves. This kind of oscillations were first detected in quiescent structures by Harvey , who measured amplitudes of the order of 2 km s−1. With the improvement of the observing
techniques and of the resolution of the instruments, a wider range of oscillations were detected (see, e.g., Landman et al. , Bashkirtsev et al. , Bashkirtsev and Mashnich ,
Wiehr et al. , Balthasar et al. ), which lead to classify them in three categories according to their periods: short (3 to 10 min), intermediate (10 to 40 min) and long (40 to 80 min). Nonetheless, oscillations with periods of less than 1 min (e.g.,Balthasar et al.) and longer than several hours (e.g.,Foullon et al., Pouget et al., Foullon et al.) have also been reported. Small-amplitude oscillations are typically of local nature, i.e., they
do not normally affect the whole prominence at the same time and different parts of a given structure may show dissimilar oscillatory motions. Usually, they are not related to flare activity but the mechanism that triggers them remains unknown. They may be produced by a continu- ous driver like the 5-min photospheric and the 3-min chromospheric oscillations, or by external impulsive agents, like magnetic reconnection events (Vial and Engvold ). Authors like
Harvey  or Yi and Engvold  suggested that Alfv´en waves propagate upwards from the photosphere and the chromosphere and induce a periodic motion in the material of the prominence.
Observations by, e.g., Landman et al.  and Tsubaki and Takeuchi  hinted that small-amplitude oscillations are damped in time and they disappear after a few periods. This behavior was confirmed by later works like Wiehr et al. , Molowny-Horas et al. ,
Terradas et al.  or Lin . To address that behavior, several damping mechanisms, such as thermal processes (see, e.g., Carbonell et al. ,Terradas et al. ), ion-neutral collisions (Forteza et al. [2007,2008]), resonant damping (see, e.g., Ionson,Arregui et al.
, Soler et al. [2009b]) or wave leakage (van den Oord and Kuperus , Schutgens
[1997a,b]), have been proposed. The state of the research about this issue has been reviewed by, e.g.,Oliver,Mackay et al.,Arregui and Ballester orArregui et al.. Among all the mentioned mechanisms, this Thesis focuses on the collisional damping. The effect of the interaction between ions and neutrals was previously investigated by, e.g.,Carbonell et al. , Zaqarashvili et al. [2011b] and Soler et al. [2013a]. Those authors found that friction due to ion-neutral collisions can efficiently dissipate Alfv´en and fast MHD waves, while the damping of slow modes is much smaller. However, they did not explore the range of high-frequency waves, which is one of the motivations of this Thesis. Hence, Chapter 4, where small-amplitude waves in partially ionized plasmas are studied by means of a multi-fluid model, includes applications to the case of quiescent prominences.