The right panel of Figures 2 and 4 show examples of filaments lying in different parts of the Sun. These locations have a common property: a filament is invariably found to overlay the neutral line PIL separating concentrations of photospheric magnetic fields having opposite vertical polarities. At the same location, surrounding the PIL, is the filament channel , which extends up to the corona. The filament channel is thought to be the magnetic structure within which lies the filament plasma.
It can be recognized through the properties of the chromospheric fibrils, which are almost aligned with the underlying neutral line as shown in Figure 7. This does not happen elsewhere outside the channel, where it is possible to find fibrils crossing the PIL. Depending upon the direction of the line of sight, it is possible to see barbs , or lateral plasma extensions see Figures 5 and 6. Some barbs seem to reach down into the chromosphere and possibly into the photosphere, as in the case of, e.
For their role in the formation and stability of filaments, barbs and filament ends are important components that need to be characterized in great detail.
The green circles marks the barbs. Barbs are clearly visible in absorption at the limb and on disk. Both chromospheric fibrils and filament threads are considered to trace the magnetic field, so that they suggest a highly-sheared magnetic field Martin, b almost aligned with the neutral line in the photosphere.
This is true both for the main body and barbs in filaments. The highly-sheared magnetic configuration appears to have a role in prominence eruptions see Section 5. Not all filament channels are filled with filament plasma, even though the observation of empty filament channels is quite rare. In certain cases only segments of a filament are seen along the same PIL.
To understand where and why filaments form are among the key goals of the study of prominences. What we can tell is that an empty filament channel is part of the filament formation process see details in Section 4. An example of an apparently-empty filament channel and its aligned fibrils is given in Figure 7.
Image reproduced by permission from Martin b , copyright by Springer. Filaments seen at the limb show different properties. They may look like structures made of fine vertical threads, like that shown in Figure 22 , or of horizontal threads as in Figure 8.
They may also appear to have more complex morphologies, including arcades, like that shown in Figure 9. Some of these differences may be attributed to different viewing angles at the times the images were recorded. For example, in some cases it is possible that we see just the final end of the structure the barb , which is compatible with seeing mainly vertical substructures.
In contrast an observation of a prominence perpendicular to its spine often reveals almost horizontal substructuring. However, it is not always easy to make this distinction and, if this fine structure traces the fine-scale magnetic morphology, the vertical structuring is not apparently compatible with the horizontal magnetic field deduced from on-disk observations.
Possible solutions to this paradox come from measurements of the magnetic field, as discussed in Section 2. However, clear differences in the fine-scale morphology exist for which we still do not have a complete explanation.
Image reproduced by permission from Okamoto et al. If the morphology of a filament marks its magnetic skeleton, interesting information may be deduced during the liftoff of an erupting prominence. In this case, prominences are stretched and isolated from their environment, giving a better view of their structure.
Some observations of erupting prominences reveal the presence of a helical structure see Figures 10 and 33 , even though it is not clear yet whether this is a consequence of the eruption or whether it was already present during the quiescent period.
In fact, there is also evidence for on-disk filaments with a similar structure, which supports the flux rope models. Further details on prominence eruptions are given in Section 5. For video see appendix. One of the main steps in the comprehension of a prominence is determining its plasma parameters, such as electron density, ionization degree, and temperature.
Hydrogen and helium spectral lines and continua emissions are the main tools used for the diagnostics of the cooler prominence core. It is important to point out the difficulties in the interpretation of the prominence spectrum, which is produced by both optically-thin and thick plasma.
Conversely, for optically-thin lines, we deduce information from the integrated emission along the full depth of the emitting plasma even though the intensity scales as the square of the electron density, for collisionally-excited transitions. It is extremely important to study both core and PCTR environments, in quiet and dynamic conditions, in order to understand the occurrence of prominences, their stability, and destabilization.
However, there is a difficulty when trying to combine the results from these two regions, which are obtained using data with different spatial and temporal resolutions. We recall that the EUV instruments used to observe the PCTR have a spatial resolution about a factor 10 lower than those used to observe the cool core at longer wavelengths.
Therefore, the PCTR is observationally much less detailed, and so are the plasma parameters derived in this region. A recent review Labrosse et al. Here we present a summary of observational results with some updates. The prominence core is dense, cool and mostly made of optically-thick plasma. The radiation process is very complex here because the plasma completely or partially absorbs the incoming and auto-produced radiation, and should be diagnosed through solving a non-LTE non-Local Thermodynamic Equilibrium radiative transfer problem.
Forward methods compare the observed spectrum to a synthetic one obtained from a parameterized prominence model, and solve the associated set of equations for the transfer of radiation through the medium Labrosse et al.
Indirect measurements of plasma parameters can also be obtained using, for example, prominence seismology. The simplest model used to solve the radiative transfer problem is the 1D slab, whose capabilities in deriving important general properties of prominences have been quite-well analyzed.
A clear example of the use of this model is found in Gouttebroze et al. These authors analyzed models characterized by different prominence conditions to derive the behavior of the hydrogen emission, and provide its spectral signatures together with the tools for their interpretation.
Starting from the 1D models, 2D slab and multi-thread prominence models have been developed. Unless a multi-thread model is applied, the prominence is considered as a monolithic body, often static, in contrast to what observations show. Besides this approximation, the results revealed the models to be quite good in reproducing the general properties of the structure.
Interested readers may refer to Labrosse et al. The prominence core plasma can also be optically thin to some radiations, such as those produced by less abundant elements. In this case the plasma parameters can be derived without solving the radiative transfer analysis. Next we review the physical parameters of the prominence core inferred via different techniques. Electron Density. Various methods are used to derive the electron density, for example: broadening of lines due to Stark effect, depolarization by the Hanle effect, or comparing lines with continuum emission.
From recent observation, Lites et al. If the hypotheses used in this work are correct, this is one of the highest values reported in the literature. This will have important bearings on the study of the equilibrium conditions of these structures, implying that the stability conditions must be valid over a large range of density values. This should be an element for constraining models. The temperature in the prominence core can be derived from H-Ly continuum observations Parenti et al.
By fitting these data it was possible to derive, under given approximations, an electron temperature of about K. The values reported in the literature, as derived applying different methods, give a range of values between and K. Gas pressure. The pressure in prominences is in the range 0. This was already reported in the Hvar reference atmosphere Engvold et al. Determining the prominence mass is also required to properly model the prominence equilibrium.
For this purpose, it is necessary to know the geometrical thickness and the whole volume of the structure as well as the ionization degree. There are not many studies aimed at determining the ionization degree. The range of values listed by the Hvar reference atmosphere for hydrogen is still valid 0.
Radiance of the H I Ly continuum as a function of wavelength in a prominence observed in The crosses indicate data corrected for temporal variations. The solid curve represents the fit to the corrected data. Image reproduced by permission from Parenti et al. This extra width is called the EUV filament extension. Line intensity profiles along a cut perpendicular to a filament axis observed on 14 June on the disk and shown in Figure 4 right.
Image reproduced by permission from Vial et al. As a consequence, in order to estimate the whole mass of the filament we need to investigate both wavelength regions. This is also confirmed by non-LTE radiative transfer modeling of filaments by Anzer and Heinzel A recent multi-wavelength study by Heinzel et al. Some of the techniques mentioned before can also be applied in this case.
In addition, when multiple spectral lines are available, density, temperature, and Differential Emission Measure DEM can be derived.
Differential Emission Measure. Even if the PCTR accounts for only a small fraction of the mass of the whole cool prominence body, the PCTR demands particular attention because of its interface role. To address this need, there were a few early attempts to build spectral atlases of these structures with the aim of providing as many identified emission lines as possible e. These provide the fingerprints of the structure and the physical parameters obtained through the inversion of this large quantity of lines have low uncertainties.
In addition, the authors built a similar atlas for the quiet Sun observed on the same day, providing additional material for comparison with quiet solar conditions. In terms of prominence plasma parameters, at a given time ideally we would like to determine the plasma distribution in space and temperature. At present, the best that can be done is to try to invert a large set of emission lines to derive the Differential Emission Measure DEM.
This gives the distribution of plasma in temperature, along the line of sight, under the assumption of a constant density. These quantities, among others, are elements used to test the energy balance in prominences.
The DEMs for two different prominences are plotted in the top half of Figure It would be interesting to further investigate whether this difference is systematic in prominences or not.
At present very few DEMs are available in the literature. When compared to the quiet sun QS DEM on the lower panel of Figure 13 , we notice the following differences: lower DEM values at transition region temperatures and a minimum located at lower temperatures. This is a general problem with solar observations of the transition region. Different results have been obtained in this context for other prominences Kucera and Landi, , and the issue is not yet solved.
The precision of the DEM distribution in this region could be improved by data in other wavebands obtained with multi-instrument and co-temporal observations. In addition, we point out that often a given instrument can provide co-temporal data only in a limited range of wavelengths, such as in the case of SUMER.
A full spectrum can be covered in up to 1—2 hours, depending on the exposure times. This means that the DEMs are built assuming the absence of structural changes in prominences. Therefore, we cannot rule out the possibility that the dynamic nature of these structures has a role in the different results obtained for the DEMs. Differential Emission Measure for two prominences top and the QS bottom.
It is generally believed that a prominence does not emit at coronal temperatures, and the coronal part of the DEMs of Figure 13 is the result of the foreground and background emission of the off-limb corona.
At the same time, the maximum temperature of emission of prominences is still unknown. This is another open issue in prominence physics, whose solution requires a detailed investigation of the emission of the environment around these structures. The difficulties of this task are discussed in Kucera and Landi The sensitivity of this channel peaks in the Fe IX line, which has its maximum emissivity at 0.
This would suggest that the PCTR contains more material at hot temperatures than previously thought. Electron density and pressure. A summary of the most-recent electron-density values derived from EUV observations of prominences can be found in Table 4 of Labrosse et al. Similar to measurements in the core, in the PCTR we obtain quite a large range of densities and, as expected, they tend to be lower than in the core. This range of values not only includes the variation of density from one prominence to another although there is no special difference between active and quiescent prominences , but also variations inside the same prominence.
This is not surprising considering that these measurements have always been made with limited spatial resolution at or above the known prominence fine scale. As will be discussed in Section 3 , in fact, prominences are composed of dynamic fine-scale components, smaller than the resolution of the EUV instruments used to observe the PCTR.
Observations at the limb in the optical, EUV and soft X-ray bands generally reveal variation of the flux with height, which traces a well-defined magnetic-field structure extending above prominences.
A darker area, the coronal cavity , extends above and around the prominence up to about 0. At present we recognize that the coronal cavity is how the coronal portion of the filament channel appears at the limb.
Over larger scales the arcade is generally part of a streamer structure. A historical review of the existing literature on cavity studies can be found in Gibson et al. An example of the different appearance of cavities is illustrated in Figure 14 , which shows the SW quadrant of the Sun where two prominences have been observed at three different wavebands. The prominences are seen in emission at chromospheric temperatures middle panels in the EUV.
The top panels show the corresponding dark cavity observed in white light WL. While the two prominences have a similar apparent dimension in the plane of the sky, the two cavities differ in size. The bottom panel shows the same area in EUV at coronal temperatures. While the small cavity is seen in absorption in the left panel, a large cavity is not visible on the right. In addition, their shape often suggests an elliptical cross section Fuller and Gibson, This has been interpreted as a reduced density and has also been confirmed by radio measurements e.
The density variation with height, however, results to be flatter in the cavity than in the arcade. Contrary to the traditional dark aspect of coronal cavities, the recent results from eclipse data by Habbal et al. This is illustrated in Figure 15 , which also shows a bright cavity at coronal temperature suggesting the high temperature of this region.
Analyzing cases of different prominence projections with respect to the plane of the sky, these authors suggest that the darkening appears mostly when the prominence axis is along the east-west direction. This may be caused by the longer line-of-sight LOS integration path in a reduced-density environment with respect to a prominence and then a cavity located along the north-south direction. This aspect has been further investigated by Gibson et al.
Top: detail of the eclipse studied by Habbal et al. Bottom row: spectral line intensities normalized to their corresponding maximum values y-axis vs.
The horizontal dashed lines correspond to the radial distances, where the normalized emission-line intensities are plotted. Image reproduced by permission from Habbal et al. In terms of thermodynamic parameters, as for prominences, the coronal environment of the cavity is still only partially understood. The faint emission of the cavities makes their study difficult and probably accounts for the variability of the measured parameters. Because of the lack of signal in EUV and X-ray, the data inversion for cavities is mostly limited to WL data, from which we may obtain the electron density.
The electron temperature and magnetic structures are generally deduced assuming a hydrostatic model, even though recent efforts have been able to abandon this assumption Kucera et al. Similar to Habbal et al. Habbal et al. Slightly lower values were found by Hudson et al. In some cases measurements imply a temperature substructure that might be due to the presence of fine structure in the magnetic field that cannot be observed directly Kucera et al.
Even though cavities have been observed for decades, only recently has it been possible to observe their fine scale dynamics. For example, Schmit et al.
Interestingly it is of similar amplitude as the axial motion measured inside prominences see Section 3. These authors noticed that the spinning direction is associated with the asymmetry of the magnetic-polarity concentration at one side of the PIL, and is directed from the strongest concentration to the weakest.
They interpret it in terms of siphon flow, which has already been postulated in the presence of asymmetric heating due in this case to the asymmetric magnetic flux at loop footpoints. In this picture, the flow highlights the magnetic helical flux rope of the cavity. Indeed, several authors interpret the coronal cavity and the filament channel as the location of a twisted magnetic structure the flux rope embedding the prominence e. Image reproduced by permission from Schmit et al.
Another question is the origin of hot plasma in the cavity. We need to better characterize it and understand whether it can really be hotter than the surrounding streamer.
Some continuous heating mechanism should be at work if the plasma is heated locally. Does it originate at the solar surface or within the cavity? A few explanations have been proposed. For example, Hudson et al. This is consistent with the Parenti et al. The origin of this hot plasma was suggested to be chromospheric evaporation due to asymmetric heating localized at the footpoints, as discussed in Antiochos and Klimchuk Further explorations of this scenario have reproduced the dynamic filamentary mixture of hot and cool plasmas in prominences and surrounding cavities.
For example, Luna et al. Their simulations imply that transition region and coronal temperature plasmas surround every prominence thread, both parallel and transverse to the field. This promising idea is still under investigation Berger et al. In addition, this model is based on observations of polar-crown prominences, which generally have significant vertical motions and structures see also Section 3.
It is still to be seen whether this instability could work in prominences characterized by different structural properties, and whether this instability could provide enough energy to initiate eruptions. Both sheared arcade and flux rope models can produce the cavity for example, see Figure 3. The presence of a detached flux rope can explain the observed flatter density gradient with height Fuller et al.
Fan and Gibson simulations of the evolution of a quasi-static emergence of a flux rope showed the formation of the cavity, where current sheets form at the interfaces of separatrix layers. These regions can potentially be seen as the location for energy dissipation and plasma heating. This configuration and the others mentioned above are all good candidates that need to be further investigated; in particular, the plasma distribution and evolution in these flux-rope filament-channel models have not been modeled with full thermodynamics, as in the Luna et al.
Clearly, better knowledge of the magnetic configuration is the key to resolving these issues. Therefore the discussion of the role of the magnetic field in massive prominences can still be debated.
Information on the magnetic field of prominences and their surroundings is sometimes deduced using the morphology and apparent dynamics of the plasma, including the apparent bulk flow thought to highlight the magnetic field lines of the fine structure.
However, the reduced emission of prominences at the limb limits the identification of magnetic fine structures, generally well highlighted in other solar structures e. The magnetic field strength and direction can also be inferred from measurements but with large uncertainties, both because of the faint emission of the prominence plasma and the low magnetic-field amplitude. These aspects make the determination of the global magnetic structure in prominences more difficult.
The ubiquitous presence of filaments and prominences on the Sun allows the accumulation of a good sample of data, which helps to identify certain morphological characteristics and to classify objects according to common properties. One of these is the magnetic chirality. Optical and EUV-X ray observations have revealed that the magnetic field of the filament environment filament, filament channel and coronal arcade follows a handedness rule called chirality , which is described in detail by Martin b and illustrated in Figures 17 and Opposite chilarity appears to be dominant in each hemisphere: negative in the north hemisphere and positive in the south.
However, there is no clear observational evidence yet for a change with the cyclic polarity reversal of the polar field e. One-to-one chirality relationships for 1 fibril patterns, 2 filament spines and barbs, and 3 overlying arcades of coronal loops, are shown in each column. The patterns in the left column are dominant in the northern hemisphere and those in the right column are dominant in the southern hemisphere.
Image reproduced by permission from Martin a , copyright by ASP. Upper right and lower left corners: Dextral and sinistral patterns. The small filament in the lower right corner exhibits both sinistral and dextral barbs.
Neidig; from work of A. Once the magnetic polarity of the filament channel is known from photospheric measurements, and we look at the filament from the positive side, the chirality is dextral if the filament axis is directed rightward, and sinistral if directed in the opposite way. This chirality rule is also valid for the chromospheric fibrils inside the filament channel.
We have already mentioned that they are almost aligned with the PIL and are rooted in small magnetic concentrations having the same polarity as the underlying photospheric network magnetic field. The loop arcade overlying a filament and its barbs also has a chirality pattern, but it is inverse to the filament: the arcade forms an angle which is generally almost perpendicular to the PIL, but the arcade skew is in the opposite sense to the filament chirality Figure Further information on the magnetic structure may be inferred from the sign of the magnetic helicity , Footnote 9 which is a measure of how many twists and turns there are in the magnetic field.
By definition the helicity is positive for a right-handed twist, and negative if in the opposite direction. In general, the solar magnetic field has negative helicity in the northern hemisphere and positive helicity in the southern hemisphere. Understanding whether this rule also applies to filaments will shed further light on the link they might have with the surrounding magnetic environment, for example, in terms of their formation. Filaments are also associated with coronal eruptions see Section 5.
Several theoretical models and simulations suggest that magnetic twist arises from the differential rotation, meridional flow, and diffusion, as well as direct transport during flux emergence e. However, some of these surface effects imply a dependence of the filament chirality on the PIL orientation, which is not supported by the observations Lim and Chae, Figure 19 shows an example of how the magnetic helicity in the different temperature regimes can be investigated.
In agreement with previous work, he showed that there were threads with positive helicity in sinistral filament channels, and negative helicity for dextral filament channels. The result supports the idea that filaments have the same helicity as their surroundings including the nearby active region , implying the same origin for these magnetic features. Observed thread crossings in an inverse S-shaped filament.
Note that the filament is dextral. The positive flux density levels are represented by white contours, and the negative ones by black contours.
Image reproduced by permission from Chae ; copyright by AAS. Spectro-polarimetry is the main method used to infer magnetic field amplitude and direction. The line emission processes in prominences are dominated by the scattering of photospheric radiation, which produces mainly linear polarization due to the anisotropy of the solar incident radiation e.
This polarization is modified if a magnetic field is present. The first polarimetric measurements were carried out by J. Leroy using the Pic du Midi Footnote 10 facility, starting from the s. For a review on the methodology and these first results see Tandberg-Hanssen and Paletou et al. In recent years the spectro-polarimetry method has evolved significantly.
In order to infer the magnetic-field amplitude and direction, an inversion method of line flux needs to be used. Because these lines are often optically thick in prominences e. In general, we need to remember that the inversion method relies on some assumption about the observed atmosphere; hence, we cannot say that we obtain a direct measurement of the magnetic field. Stokes parameters from a multi-line inversion of simultaneous and cospatial spectro-polarimetric observations of He I left and D3 right in a quiescent prominence, taken with THEMIS on 29 June Image reproduced by permission from Casini et al.
Modern instruments used to infer magnetic field values have a typical polarimetry sensitivity of about 10 3 or better, spatial resolution equal to or greater than half an arcsecond, and exposure times of 1—2 mins for prominence observations or coarser spatial resolution for faster exposures, e. Unfortunately, these are still low resolutions compared to the fine spatial and dynamical scales of filaments. In this case the inversion methods for the Stokes parameters cannot distinguish between the two opposite transverse solutions.
Several methods have been developed to resolve this ambiguity, and we refer to Metcalf et al. However, in filaments and filament channels the uncertainty in the measurements of the weak field, the assumptions about the observed atmosphere, and other factors make this issue harder to resolve than in other regions. Observation of prominence chirality may help solve this problem Martin et al. The most relevant results about the magnetic field include: the strength of the field from polarimetric measurements is between 8 and 10 G on average for quiescent filaments; magnetic field strengths deduced from filament thread oscillations are within this range, even though some strong assumptions e.
These differences appear to be associated with the recent inclusion of circular polarization into the inversion method. The low spatial resolution data provide only a lower limit on the magnetic field strength. The magnetic field strength can also be deduced from radio emission, by using the relation between temperature brightness, circular polarization, and optical depth Apushkinskij et al. The published results are generally in agreement with those from spectro-polarimetry. One recent measurement at the centimeter-wave radio emission using the Russian RATAN telescope from a quiescent prominence gives a strength of — G Golubchina et al.
This discrepancy highlights once more that much more effort should be made to derive reliable magnetic-field measurements in the corona. Leroy et al. This is apparently in contrast with Casini et al. This discrepancy as well could be attributed to the much lower spatial resolution data used by Leroy et al.
Polarimetric measurements also provide information on the magnetic field direction. However, these results are apparently in contradiction with the observation of the fine structure of some quiescent prominences as those belonging to the polar crown where vertical threads dominate. We will come back to discuss this point in Section 3. At the photospheric level, it is often possible to observe small areas of opposite polarity with respect to the dominant one, on either side of the PIL.
These minor polarities also called spurious or parasite are candidates for the location where filament barbs are rooted, as discussed in Section 2.
To this purpose, the 3D reconstruction of prominence morphology is certainly an important constraining element. When we try to infer the 3D morphology of prominences from their emission we are aware that the angle of view, amount of opacity prominence mass , and temperature of the structure with respect to the temperature sensitivity of the observing instruments all matter.
The observed differences among prominences may be partially due to these elements. For example, does a characteristic length, height, and width of prominences exist? We know that these parameters depend on how magnetically active the environment is where the prominences form. While efforts were made in the past to classify these different parameters, the most recent observations reveal a wide spectrum of dimensions with less definite limits between active, intermediate, and quiescent filaments.
This may suggest a continuity in spatial scales of the strength and topological complexity of the magnetic field. Clearly, if the 3D morphology of prominences were observable we could better infer the enveloping magnetic field structure for example, helical vs.
Unfortunately, due to the difficulty of isolating the emission of the lower portion of the structure or the full structure if it does not extend much in height from the surrounding bright environment, this reconstruction is, as of today, not easily accessible. In addition, we know that magnetic field measurements, which scan the structure in height, are also difficult. On the contrary, interesting results have been achieved in the case of erupting prominences where the contrast in brightness with the corona is stronger , as discussed in Section 5.
Concerning magnetic field measurements, in addition to the simple configuration where the magnetic field crossing the prominence has the same direction as the underlying photospheric field as in a simple arcade, called normal polarity , polarimetric measurements have revealed the presence of the inverse polarity of the prominence magnetic field in most cases. The inverse term refers to two directions: the magnetic field perpendicular to the filament axis has a direction opposite to that of the overlying prominence arcade; the field direction along the prominence axis is opposite to what is expected from photospheric magnetic-field extrapolations of the underlying bipolar field that is, directed from the negative photospheric polarity to the positive one, see Figure Detail of the vector photospheric magnetic field in regions around a filament channel observed on 11 December Arrows of equal length show the orientation of the horizontal component of the magnetic field vector.
The field is directed mainly along the filament channel running from lower left to upper right, but over most of its length the field has inverse configuration: the arrows have a component directed from negative polarity lower right toward positive polarity upper left. These enormous structures are called solar prominences. Although prominences appear to be very bright and hot, they are actually much cooler and denser than the surrounding plasma in the Sun's corona outermost atmosphere.
Prominences are shaped by the Sun's complex magnetic field, often forming loops with each end "anchored" to the Sun's surface photosphere. Prominences are enormous, extending for many thousands of kilometers miles. Cosmos: Origin and Fate of the Universe. Astronomy's Moon Globe. Galaxies by David Eicher. Astronomy Puzzles. Jon Lomberg Milky Way Posters. Astronomy for Kids.
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