Improvements in our understanding of their 3D plasma structure could be made by processing all relevant data provided by continuous observations from ground and space. We have now several automatic detection algorithms and long-term catalogs that use these data and can provide material for complementing the limited existing statistical studies. This is a challenge, because such algorithms should be first normalized in terms of selection criteria to identify a filament among the other solar structures. However, automated detection has the advantage of being less biased than manual selection, so this approach should be able to determine whether different categories exist or the filament population is made by a gradual continuum change of morphologies.
Progress in inferring the 3D morphology has been obtained from reconstruction using the STEREO data. However only a few data sets have been processed, and most of them relate to erupting prominences. The few results are probably due to the limitations imposed by the continuous changes of the relative position of the two satellites, because reconstruction with the STEREO data is only feasible for certain configurations. The experience learned from the STEREO mission should be further used in designing future missions to obtain observations from multiple points of view.
- Density. We have seen that the electron density measurements extend over a wide range of values and their uncertainties are still large. We must determine whether this result reflects the real range of variability of the parameter or depends on the data inversion methods. The uncertainties are due to the limited spatial resolution of the instruments and to the unknown emitting volume, among other causes. The uncertainties in the density values is relevant, for example, to the prominence’s plasma and magnetic pressures used in estimating the equilibrium conditions. Access to the whole prominence volume and that of its fine structure (filling factor) is a compelling goal, because it would enable an important step forward in prominence diagnostics.
- DEM. We mentioned that the investigation of the structure of the PCTR will greatly benefit
from improvements in spatial resolution of UV-EUV instruments. The inferred DEM does not
yield direct spatial information on the thermal structure of the PCTR, but establishing the
DEM variation (and hence the radiative losses), if any, over less than 1'' spatial scale (that is, the
actual resolution of the cool threads) will give us important hints. With this better constraint
to fine scale modeling, we could establish whether the PCTR surrounds each individual single
thread or is a common envelope to the whole structure.
At the same time it is urgent to obtain a more complete estimate of the radiative losses over the full core-to-PCTR temperature range. The hardest part is to estimate the core component via NLTE inversion, which has not been completely achieved. Once this temperature coverage is achieved and the small spatial scales are accessible, we could investigate the physical processes at that scale: for example, spatial variations of the radiative losses over fine scales suggest local (small scale) energy dissipation.
The recently launched NASA mission, IRIS20 will contribute to filling those gaps. With its 0.3'' resolution in the UV (in contrast to 0.1'' of optical instruments) it decreases the difference between the optical (for the prominence core) and UV data (for the PCTR). Similar or better improvements are also proposed for future instruments. Nevertheless, in the near future we also must improve data diagnostics and simulations that provide observable parameters of unresolved fine structures.
- Hot plasma in the cavity. Among the various properties of coronal cavities, the latest studies converge in suggesting that they contain hotter plasma than the prominence itself. One of the most appealing ideas is that this plasma originates under the prominences and escapes from them through magneto-thermal convection in the form of bubbles and plumes. The whole process needs to be better understood, but if confirmed, it will open completely new doors on physical processes that may influence prominence formation, dynamics, or stability. More generally, the origin of this hot plasma is not yet identified, although flux emergence and local reconnection have been suggested. If confirmed, these ideas also may contribute to our understanding of coronal heating.
- Internal dynamics. Another open question we have encountered is about the properties of the
various measured forms of plasma flows and their role in the prominence support, mass loading,
and energy balance. This is particularly important in the prominence core, where most of the
prominence mass is concentrated. In addition, by reducing the difference in spatial resolution
between optical and EUV data (as done for only few minutes by VAULT and Hi-C), we could
possibly link counter-streaming flows and EUV motions to infer the fine structure properties
and the filling factor also in the PCTR. This is important in order to understand how much
the PCTR flows contribute to in the whole kinetic energy budget of the prominence.
We also need to better interpret the observational inconsistency between the different flow patterns observed on disk and at the limb. In addition to what was previously mentioned on the importance of the stereoscopic observations, progress in the interpretation of the flow patterns (including resolving the full velocity vector) should be possible by co-temporal and multi-angular (on disk and at the limb) observations, to overcome line of sight and opacity effects. This could be possibly be achieved by the ground-based and SDO measurements together with the future observations of Solar Orbiter.
Finally, we need to better understand the origin of the unresolved motions measured from the non-thermal widths of EUV spectral lines: are they waves, turbulence, or superimposition of blue and red-shift motions along the line of sight? All these measurements and their interpretation should shed light on how important the plasma dynamics is to the prominence support.
In this review we have only briefly mentioned the modeling aspect of prominence investigation. However, we have pointed out the role of non-LTE modeling in inferring prominence core density, ionization state, optical depth, and element abundance, all of which are important for establishing the mass and producing synthetic spectra. As the latter are intended to be directly comparable to observations, we stress the importance of developing more sophisticated and realistic non-LTE modeling. Similarly, some models assume that the filament sits statically on a large scale magnetic structure, completely ignoring the flows which may play a role in the prominence evolution. Such important differences between modeling and observations should be overcome.
- Spatial distribution. Only direct vector magnetic-field measurements, coupled with plasma
diagnostics applied to the 3D inferred morphology, will allow us to recover the full information
on the spatial distribution of prominences. For example, these observations could confirm
the presence of multiple dips needed by some models to solve the paradox of vertical and
horizontal flow patterns. However, the magnetic field measurements and data inversion methods
available at present are not able yet to give a clear picture. Limitations include the low spatial
resolution of the data used for spectro-polarimetric inversion with respect to those for plasma
parameter determination. In addition, the few published inferred measurements are obtained
using emissions at the temperature of the cool core.
To date there are no magnetic field measurements in the PCTR. However, understanding the efficiency of the thermal conduction (which among other parameters depends on the magnetic field), is interesting as it controls the thickness of the PCTR by transporting the heating energy into the structure. In particular, the cross-field thermal conduction in typical prominence conditions is much less efficient than the field aligned component, which works along the thread, between the hot part and the cooler one. For this reason we expect a very thin PCTR in the cross-field direction.
However, few modeling works suggest that, under thermally usable conditions, cross-field thermal conduction in filaments may have a role in structuring its small scales and affecting its lifetime (e.g., van der Linden, 1993; Soler et al., 2011). More attention to this aspect would allow to better quantify its importance in the the prominence sub-structuring.
Finally, modeling studies have to make a stronger effort to address the small as well as the large scale structure at the same time, to reproduce the overall observed properties of prominence environment. Although this is a challenging goal, a model able to reproduce the richness of these details will fill significant gaps in our lack of knowledge on prominence support and stability.
- Temporal evolution. Knowing the temporal evolution of the magnetic field will help explain filament formation, stability, and destabilization. Not only flux emergence but also the interaction with pre-existing fields are important aspects that need to be better characterized by observations. The emergence of a flux rope has been suggested in a few observations of active region areas, while the concentration of helicity in pre-existing coronal magnetic field at PILs is the best candidate process for the formation of quiescent and intermediate filament magnetic structures. A definitive affirmation of these scenarios cannot yet be given. Optimal data able to confirm these results would require continuous magnetic field measurement over several or more hours at high resolution, from the photosphere to the corona, together with multi-temperature data to access plasma flows (which provide information on the photospheric magnetic flux transport or mass injection into the filament channel) and density variations at all layers.
The stability of a prominence and its magnetic envelope can vary due to nearby changes of physical conditions, either in the corona or in the underlying chromosphere and photosphere. Observations reveal multiple signatures of instability, but their timings with respect to the initial rearrangement of the magnetic configuration and prominence lift off are not clear. One reason is that we lack complete mapping of the connections between the prominence, the photosphere (if any), and the corona. Barb properties derived from observations are incomplete. For example, an open debate is on the distribution of the barb’s plasma along the field, and whether or not the barb reaches the photosphere. We have inferred that the magnetic field should be weak in barbs; this makes it more difficult to measure and localize the barbs rooting points and their relation to the minor polarities identified in the photosphere. Some argue that barbs play a key role in the loss of stability. However, more comprehensive and more detailed observations of barbs should allow a more rigorous evaluation of their role in the formation, stability, and eruption of prominences.
We have seen that prominence eruptions are large scale events which are often associated with flares and CMEs. The SDO mission, in conjunction with STEREO, is suitable for their study. The AIA multi-temperature images, together with the HMI data (photospheric magnetic field and velocity maps), are probably frequent enough (about 12 s cadence) to distinguish the signatures of the filament instabilities and those of the surrounding magnetic structure. This is helping, for example, to better map the relative timing between the EUV brightenings, the lifting-off of the prominence, and the CME acceleration and would, in turn, help discriminate among the proposed CME models.
Some of the issues that we have discussed here will be solved only when data with specific properties are available.
One new solar space mission now in preparation is the ESA Solar Orbiter,21 to be launched in 2017. This will be an encounter mission which will resolve about 150 km on the Sun with the UV-EUV imager. The EUV imager and EUV spectrometer will observe several hydrogen and helium lines, as well other chromospheric and coronal lines suitable for density and temperature diagnostics. As anticipated from the new IRIS mission, these cutting-edge EUV data, together with the improved temporal resolution, will reduce the discrepancies between the UV and optical spectroscopic data, and better access the prominence fine-scale morphology and thermodynamic properties. This high resolution will also be reached in the photosphere and help to clarify the dynamics of the plasma flows and their role in the magnetic flux transport as ingredients for prominence formation and instability. The localization and characterization of the minor polarities and flux emergence, as well as polarimetric measurements, will be possible with the high sensitivity instruments being developed for this mission. However, because of the limited pointing capabilities imposed by the mission profile, targeting prominences will be a challenge. The next few years will be dedicated to optimizing the scientific return of the mission.
New ground-based data will be available from the US Advanced Technology Solar Telescope (ATST)22 and the European Solar Telescope (EST).23 They will be optimized for the study of the photosphere-chromosphere connection with an angular resolution of 0.03''.
More diagnostic capabilities for prominences should also be obtained by the proposed Japanese Solar-C mission, for which sub-arcsec spectroscopic and imaging UV-EUV instruments are under consideration.