Partially ionized plasmas are found across the Universe in many different astrophysical environments. During recent years, the study of partially ionized plasmas has become a hot topic within the solar physics community because layers of the solar atmosphere (photosphere and chromosphere) as well as solar structures such as spicules, prominences, are made of partially ionized plasmas. Solar prominences are fascinating structures embedded in the solar corona whose peculiar properties and behavior, not yet well understood, cause them to be the subject of intense research. Furthermore, they are an integral part of major solar eruptions, therefore, greater understanding of their formation and evolution will contribute significantly to our understanding of the origin of space weather, a serious threat to our technology-dependent world. Due to their relatively low temperature prominence plasma is partially ionized, but the exact ionization degree is unknown and the reported ratio of electron density to neutral hydrogen density covers about two orders of magnitude (0.1-10). Partial ionization brings the presence of neutrals and electrons in addition to ions, thus collisions between the different species are possible and the effects on the prominence equilibrium and dynamics should be considered. We propose to set up an International Team of experts whose goal is to expand our understanding of the role of partial ionization in the formation, dynamics, and stability of solar prominences. The Team consists of twelve members coming from eight different Institutions in six different countries, including five ESA Member States. Each member of the team is an expert in different aspects of prominences; collectively, our research interests offer a combination of expertise in observations, prominence formation and dynamics, waves and instabilities, and numerical simulations. The primary goals of the team are to exchange ideas, establish collaborative links, and develop joint strategies for tackling current problems related to partially ionized prominence plasmas (PIPP). Therefore, we will meet at ISSI to discuss the current state-of-the-art in this field, to interpret existing ground- and space-based observations, and to decide on modelling and observational strategies to be carried out between the meetings. The expected outcome of the proposed research will be a better understanding of the impact of partial ionization on the physical properties and evolution of solar prominences.
Scientific rationale, goals and timeliness
Coupling between ions and neutrals in magnetized plasmas is of paramount importance in many aspects of solar physics and, during recent years, the study of the role played by partial ionization in the stability, energetics, and dynamical state of solar plasma is becoming a hot topic. Additional processes beyond ideal MHD, such as the Hall effect and ambipolar diffusion, must be taken into account, because they could significantly influence the evolving physical properties of the plasma (Khomenko & Collados, 2012; Zaqarashvili et al. 2013). Furthermore, the single-fluid MHD approximation fails in some circumstances, even when non-ideal effects are considered, in which case a multi-fluid approach must be adopted (Zaqarashvili et al. 2011). It is of great theoretical and observational importance to fully understand the differences introduced by the use of multi-fluid approaches versus the single-fluid approximation (Ballester et al. 2018a), which could lead to completely new physics whose visible signatures could be detected by near-future observational facilities.
Quiescent solar prominences are clouds of cool and dense plasma suspended against gravity by forces thought to be of magnetic origin. Since their temperature is typically of the order of 10000 K, the prominence plasma is partially ionized. In those conditions, ion-neutral coupling needs to be taken into account when describing the physics of prominences. Among their key properties that can be influenced by partial ionization, we highlight:
1.- Formation: Two key problems in prominence physics are the origin of their constituent plasma and how they are formed. Decades of research has clearly shown that the prominence mass must come from the chromosphere (Pikel’ner 1971; Mackay et al. 2010), because there is not enough plasma in the corona to form them. Three types of models have been proposed to explain how chromospheric plasma becomes prominence material. Injection models postulate that cool plasma is forced upwards in filament-channel flux tubes to reach the observed heights of prominences. Most injection models require reconnection of magnetic field lines in the low solar atmosphere, which expels material upward (Mackay et al. 2010; Karpen 2015). In levitation models, cool plasma supported by rising magnetic fields above the photospheric polarity inversion line is lifted transverse to the field (Mackay et al. 2010; Karpen 2015). Evaporation-condensation models require coronal heating to be localized at the footpoints of long, low-lying coronal loops straddling polarity inversion lines. Under these conditions, thermal nonequilibrium sets where the radiative losses from the buildup of evaporated material exceed the energy input at that location, leading to catastrophic cooling and condensation of the evaporated mass to create the prominence threads (Antiochos et al. 1999; Mackay et al. 2010; Luna et al. 2012a; Xia et al. 2014). The evaporation-condensation model is, nowadays, the most rigorously studied mechanism for prominence formation. Although all of these models acknowledge that partially ionized chromospheric plasma is the source of prominence mass, thus far these mechanisms have only been tested under fully ionized conditions. Therefore, the role of partial ionization in prominence formation and its consequences must be explored.
2.- Support: Another key problem in prominence physics, strongly linked to previous topic, is how these cool and dense structures are supported in the hotter, more rarefied solar corona. Although the ambient magnetic field is generally accepted as their structural foundation, the ability of the field to support partially ionized plasma (PIP) has not been adequately considered. In particular, the problem of how neutrals are supported in prominences is a matter of great interest. Bakhareva et al. (1992) studied the dynamic regimes of a partially ionized prominence in a 2D Kippenhahn-Schlüter magnetic configuration. After perturbing the equilibrium, they found that the system underwent amplified oscillations of density, magnetic field, and velocity, ultimately destroying the prominence. The instability is caused by the inability of the magnetic field to support the plasma’s neutral component against gravity. As the neutrals fall, ion-neutral collisions force the whole plasma to be dragged down, injecting an additional current in the prominence and altering the Lorentz force. Terradas et al. (2015) solved the two-fluid equations, including ion-neutral and charge-exchange collisions, to study the temporal behavior of a prominence plasma embedded in the solar corona. In their model, the prominence is represented by a large density enhancement above background, composed of 75% neutrals and 25% ions, and the magnetic configuration is quadrupolar with dips. The results demonstrated that partially ionized prominence plasma can be efficiently supported when the coupling between ions and neutrals is very strong (see also Gilbert et al. 2002, 2007). Although the ion-neutral drift velocity is very small, the friction coefficient is large; as a result, the upward frictional force can counterbalance the effect of gravity on neutrals. On the other hand, the ionized fluid is supported by the magnetic field (i. e., frozen in), but the inclusion of a neutral component enhances the deformation of the magnetic field in order to increase the restoring force. Finally, charge-exchange collisions increase the frictional forces, reducing the neutrals’ downward velocity and helping to sustain the prominence mass. In conclusion, the draining of neutrals from prominences is extremely slow when the fluids are strongly coupled.
These results should be fully validated by including additional physical processes that could substantially modify their outcome, such as non-adiabaticity, ionization- recombination, and weak ion-neutral coupling. In addition to explaining the longevity of prominence plasmas, we also need to explain their disappearance in the absence of eruption. For instance, the apparent lifetime of prominence threads is about 20 minutes which could be related to the cross-field diffusion of neutrals (Gilbert et al. 2007).
3.- Stability: High-resolution observations by Hinode satellite (Berger et al. 2008, 2010, 2011, 2017; Yang et al. 2018) have revealed a plethora of dynamical processes in prominences: counterstreaming, upflows, downflows, bubbles, plumes, etc. Most of these dynamical features have been interpreted and modeled in terms of different instabilities, e. g., Rayleigh-Taylor, Kelvin-Helmholtz, and thermal instabilities (Ryutova et al. 2010; Hillier et al. 2011, 2012, 2018a, 20128b; Berger et al. 2017). These studies primarily considered the plasma to be fully ionized, so complementary efforts have addressed the modifications in the instability thresholds and growth rates produced by partial ionization effects (Soler et al. 2012a; Soler et al. 2012b; Díaz et al. 2012; Khomenko et al. 2014; Díaz et al. 2014; Martínez-Gómez et al. 2015; Ballai et al. 2017; Ruderman et al. 2018). On the other hand, prominence oscillations have been observed for decades (Arregui et al. 2018), but are only beginning to be understood. In the case of large- amplitude oscillations, gravity seems to be the restoring force (Luna & Karpen 2012; Luna et al. 2012b) while small-amplitude oscillations have been interpreted in terms of standing or propagating MHD waves in a stationary background (Arregui et al. 2018). In rare circumstances these oscillations can grow, ending in eruption. Ballester et al. (2018a, b) have used single-fluid approach to obtain insight into partial ionization effects on MHD waves in both stationary and dynamic prominence plasmas. Much work remains, however, to incorporate partial ionization effects, in particular multi-fluid physics, into existing models of oscillatory phenomena and instabilities.
Building upon the progress made towards understanding the role of partial ionization in prominence physics, the main goal of this ISSI proposal is to tackle some of the remaining unsolved questions.
Research Topics
This proposal is focused around the three key problems highlighted in the above Scientific Rationale, which are of crucial importance for partially ionized solar structures, and in which the team members have proven expertise. The topics that we will address are:
1.- Role of partial ionization in prominence formation and support
During the cycle of evaporation-condensation, the chromospheric material forming prominences is partially ionized initially but becomes fully ionized during the evaporation. Later, during condensation, the plasma becomes partially ionized again. The changes in the ionization degree during the evaporation-condensation cycle alter plasma parameters such as mean atomic weight, resistivities, viscosity, thermal conduction coefficients, etc., thus influencing the dynamical, thermal, and magnetic processes taking place in the plasma. Therefore, the evaporation-condensation cycle should also be influenced by partial ionization effects that, in turn, affect prominence plasma formation. The team will discuss how to investigate the development of this process and how to include multi-fluid physics in some steps of the cycle, which will require advances in our numerical approaches. Regarding the support of the partially ionized prominence plasma, physical effects such as non-adiabaticity, ionization-recombination (which modifies the ionization degree), and viscosity should be included in models, following the initial studies by Terradas et al. (2015). Furthermore, we plan to consider 2D and 3D configurations that would bring the models closer to observed prominence structures. How to include non-ideal effects when multi-fluid physics is considered, the appropiate numerical implementation, and the development of 3D models will be the key problems discussed in team meetings.
2.- Role of partial ionization in prominence dynamics and stability
Present and past studies largely relied on static models to study the behavior of waves/instabilities excited in prominence plasmas. However, observations show that prominences contain highly dynamic plasmas that are far from a stationary equilibrium. Temperature and density inhomogeneities are also observed, suggesting the presence of temporal and spatial variations in heating and cooling. As a consequence, the prominence plasma parameters become time- and space-dependent. When waves/instabilities are excited/triggered in a dynamic, partially ionized, prominence plasma, the changing physical conditions can seriously modify wave motions as well as the development of instabilities. To advance our understanding of the behavior of waves/instabilities in prominence plasmas, it is necessary to improve our models by considering the dynamic background in which waves/instabilities develop and propagate. In our meetings, we plan to discuss how to best represent these important processes in our models, including multi-fluid physics, in order to make the models more realistic. The effects of neutrals on the development of key 3D instabilities, such as the magnetic Rayleigh-Taylor instability, will also be analysed. We further anticipate that the acquired knowledge will help us to lay the foundation of “real” prominence seismology, leading to improved characterization of prominence properties.
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