The phenomenon of accretion is involved in the process of formation of virtually every cosmic object in the Universe. Besides gravitation, another agent in this process is the thermal instability mechanism, in which radiative cooling locally overcomes the plasma heating processes leading to dynamics controlled by pressure gradients (Zanstra 1955, Field 1965, Cox 1972). In a low-β, magnetised plasma of stellar coronae, the phenomenon of thermal instability and accretion of material towards the star can be very complex since magnetic fields may dominate the dynamics. Adding to this complexity is the largely unknown organisation and evolution of the magnetic fields and the process of coronal heating. The dynamics of accretion, the thermal instability mechanism, coronal heating and the large and small scale magnetic field organisation are all linked together in the subject of partially ionised material accreting towards a stellar surface such as the Sun. This is the case of coronal rain, which corresponds to cool and dense blob-like structures occurring on a timescale of minutes in active regions due to thermal instability (in flaring and non-flaring conditions), observed to fall along loop-like paths (Parker 1953, Kawaguchi 1970, Leroy 1972).
Partially ionised plasma falling from coronal heights, such as coronal rain or prominence material falling back in quiescent or eruptive states, has been observed for over 40 years, but it is only recently that its investigation is receiving increased attention. One of the main reasons for this delay is the required high spatial resolution in order to resolve the bulk of the distribution (Antolin & Rouppe van der Voort 2012, Antolin et al. 2012). Another weighting factor is their multi-thermal nature, which allows fully visualising these events only if co-observing in multiple wavelengths spanning chromospheric to coronal temperatures (Schrijver 2001, Antolin et al. 2014). Recently, this range may have further expanded through coronal rain observation in post-flare loops. Rain-like material in such loops has been observed in photospheric wavelengths (Martinez-Oliveros et al. 2014, Saint-Hilaire et al. 2014). It is still unclear whether this photospheric emissions correspond to line emission or to scattering. Recent high resolution observations have estimated the fraction of the total coronal volume in a thermal non-equilibrium state to lie between 7% and 30% for the case of a decaying active region (Antolin & Rouppe van der Voort 2012). The global mass drainage rate in the form of coronal rain was found to be roughly 5 × 109 g/s, only a factor of three smaller than the estimated mass flux into the corona from spicules. Similar rates have been found for prominence drainage, a rather surprising fact leading to discussion on the heating and cooling role of magnetic fields (Liu et al. 2012, Berger et al. 2012). The recently launched IRIS mission provides for the first time high spatial, temporal and spectral resolution coverage of the transition region and chromosphere. It is therefore expected that better estimates of the coronal rain mass flux will be obtained, as well as a better assessment of its role in the chromosphere-corona mass cycle (Marsch et al. 2008, Berger 2011, McIntosh 2012).
Recently, very high redshifts (up to 200 km/s) have been discovered with IRIS within umbrae and penumbrae of sunspots linked to cool downflows from coronal heights, matching the characteristics of coronal rain (Kleint et al. 2014). The temporal and spatial occurrence characteristics of these events, reminiscent of flare ribbons, suggest a strong heating correlation of neighbouring coronal loops with footpoints within the umbra, thus raising interesting questions about the probable causes. Schad et al. (2014) have managed to acquire for the first time the coronal magnetic field along a loop through spectropolarimetry with FIRS of DST applied to a thermally unstable loop exhibiting coronal rain. By combining coronal wavelength observations from SDO/AIA, Liu et al. (2014) show that coronal rain can easily be triggered near what appear to be X-points of magnetic reconnection in the corona. Such a scenario is reminiscent of the coronal rain model suggested by Murawski et al. (2011), in which the entropy mode is triggered at a null point, leading to the formation of condensations. This model suggests the existence of a subcategory for partially ionised material in which blobs may actually be triggered by MHD waves. Along these lines, Low et al. (2012a,b) following early work by Heasley & Mihalas (1976) have shown how material in quiescent prominences can collapse into sheets through spontaneous formation and resistive dissipation of discrete currents, a novel application of the general Parker theory (Parker 1994).
A common characteristic in observational reports of cool condensations accreting towards the solar surface is that the downward acceleration is much lower than the effective gravity along curved magnetic structures (Loughhead & Bray 1984, Xu 1987, Heinzel et al. 1992, Wiik et al. 1996, Schrijver 2001, De Groof et al. 2004, Antolin et al. 2010, Antolin & Vissers 2011, Reale et al. 2013). Constant falling speeds and even changes of direction are reported. Although it is clear that gravity alone cannot explain such dynamics, it is yet unclear what can. Analytical and numerical work have shown that condensations can be far more sensitive to gas pressure variations within the loops (Müller et al. 2004, Antolin et al. 2010, Xia et al. 2011) or to the ponderomotive force from Alfvénic waves (Terradas & Ofman 2004, Antolin & Verwichte 2011) than to gravity. Recent numerical work by Oliver et al. (2014) shows that the formation of a dense condensation in the upper atmosphere generates downward propagating slow mode shocks which reset the pressure balance below along the blob’s path, stopping the downward acceleration and leading to a constant speed. Furthermore, the authors predict an increase of density in time for the falling blobs and the generation of leaky slow mode waves with specific periodicities related to the blobs characteristics (Terradas et al. 2005). Apart from offering unexplored attractive applications for coronal seismology, these results indicate a possible solution to the long standing problem of accretion stated above. Furthermore, they show that a falling condensation may have mixed flow and wave properties, reminiscent of a soliton in hydrodynamics or an entropy mode in MHD, a picture supporting the scenario suggested by Murawski et al. (2011).
Further on the numerical side Fang et al. (2013) have recently achieved 2.5D MHD simulations with AMR that provide a unique picture of the formation process of these condensations. Their work suggests an ever increasing number of small condensations at higher spatial resolution, suggesting that the current detection is just the tip of the iceberg. Such work reflects the high level of complexity in thermal instability, which may be coupled with other dynamical instabilities occurring in such conditions.
Coronal rain is seen in cool chromospheric spectral lines, which indicates the importance of partially ionised plasma effects. Collisions between ions and neutrals may significantly change the plasma dynamics which is traditionally studied in the fully ionised case (Braginskii 1965). Multi-fluid description of partially ionised plasma (Zaqarashvili et al. 2011a) and effects of neutral helium atoms (Zaqarashvili et al. 2011b) could be essential ingredients in modelling of coronal rain phenomena.
The highest spatial resolution of coronal structure has been achieved through observations in cool chromospheric lines of partially ionised plasma (Ofman & Wang 2008, Lin 2011, Antolin & Rouppe van der Voort 2012). This fact is especially relevant for the investigation of the small and large scale magnetic field. Accordingly, an increasing number of reports of small amplitude transverse MHD waves in the corona have appeared in recent years (Nakariakov & Verwichte 2005, De Moortel & Nakariakov 2012), a tendency that is expected to increase at higher spatial resolution. Apart from offering unique insights into the fundamental scales of the magnetic field in the solar corona, coronal rain further offers unique possibilities of wave detection, which suggests a far more precise application of coronal seismology (Antolin & Verwichte 2011). This calls for an extension of coronal seismology techniques into the regimes of thermally unstable loops.
Timeliness
As described above, major observational discoveries and significant progress in numerical modelling has been achieved in recent years, and is currently being achieved, in the topic of partially ionised material falling from coronal heights. The recent launch of the IRIS mission allows for the first time high spatial, temporal and spectral resolution observations of the transition region and chromosphere, connecting the photosphere and the corona. First results are highly successful in providing a new picture on the catastrophic cooling process produced by the thermal instability (Kleint et al. 2014, Antolin et al. 2014). Many of the team members are involved in on-going coordinated observational campaigns between different instruments, such as Hinode, SST, IRIS mission and DST. For instance, 10 such coordinated observational campaigns between SST, Hinode and IRIS are scheduled for this year. It is therefore expected that by the end of this first year of IRIS and coordinated observations major new results will appear and further enrich the material for discussion at ISSI. The expected impact of such discoveries and achievements, added to results from previous years guarantees an initial large impact on the scientific community. Pursuing the gained momentum through ISSI meetings at the end of the year and during next year will be essential to achieve the scientific goals of the team.
Acronyms
AIA: Atmospheric Imaging Assembly; AMR: Adaptive Mesh Refinement; ATST: Advanced Technology Solar Telescope; CRISP: CRisp Imaging SpectroPolarimeter; DST: Dunn Solar Telescope; EST: European Solar Telescope; FIRS: Facility Infrared Spectropolarimeter; IBIS: Interferometric BIdimensional Spectrometer; IRIS: Interface Region Imaging Spectrograph; SDO: Solar Dynamics Observatory; SOT: Solar Optical Telescope; SST: Swedish 1-m Solar Telescope.
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