Partially ionised cool and dense plasma falling from coronal heights towards the solar surface is receiving increasing attention in recent years. This is best manifested as coronal rain, a phenomenon of thermal instability occurring in coronal loops in active regions (Parker 1953, Field 1965, Kawaguchi 1970, Leroy 1972). Recent high resolution observations are providing a wealth of new information concerning the morphology, dynamics, and energetics of such falling cool material (Lin 2011, Mackay et al. 2010, Antolin & Rouppe van der Voort 2012, Antolin et al. 2012, Liu et al. 2012). Such observations have demonstrated that its chromospheric nature can be used as the highest resolution probe for the coronal magnetic field (Antolin & Verwichte 2011, Antolin & Rouppe van der Voort 2012). Numerical simulations have shown how it is deeply linked to the underlying coronal heating mechanism (Goldsmith 1971, Antiochos et al. 1999, Karpen et al. 2001, Antolin et al. 2010). Furthermore, the importance of such falling cool condensations has been highlighted in the chromosphere-corona mass cycle (Berger et al. 2011, Liu et al. 2012) and in the complex problem of mass accretion onto stars (Reale et al. 2013). The emerging picture is that falling partially ionised material (and thermal instability) in a corona may be far more common than previously thought.

The recently launched IRIS mission provides high spatial, temporal and spectral resolution coverage of the transition region and chromosphere, allowing for the first time proper visualisation of the thermal instability process leading to the formation of condensations such as coronal rain. By combining high resolution multi- wavelength observations with IRIS and other instruments, with state-of-the-art numerical codes, the proposed ISSI team will aim at thoroughly investigating coronal rain in each of the aforementioned topics. Through a series of publications and a major review, awareness from the scientific community will be raised on the importance of this partially ionised, cool and dense plasma. The team consists of experts in observations, numerical simulations, and theory whose joint fields of interest cover the different layers of the solar atmosphere and the different plasma regimes in which the phenomenon has been observed. A series of discussions are planned, which ultimately will help establish several unknown aspects of the coronal magnetic field and plasmas such as the fundamental length scales, the occurrence rate of thermal instability in the corona, the dynamics of accretion onto the solar surface, the interaction (through MHD waves) of coronal rain with the magnetic field, and the physical processes at the root of the rain morphology. Developing these aspects will allow setting constraints on coronal heating mechanisms, provide further understanding of the organisation of the coronal magnetic field at both small and large scales, develop considerably coronal seismology techniques by extending its applicability to thermally unstable loops, and help motivating instrumental requisites needed for future solar missions.