Understanding the chromosphere-corona interface in the Solar Orbiter and DKIST era

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After almost 20 years of hard work, planning and building, the two next-generation long-term major solar facilities will become operational in early 2020. These are DKIST and ESA's Solar Orbiter. They are expected to provide breakthrough observations of the solar chromosphere/corona.DKIST is the first large-scale solar telescope, with a 4 m off-axis mirror. It has been built in Maui, Hawaii, will have first light in 2019 and start operations in 2020 by performing on-disk and off-disk observations with unprecedented spatial resolution. DKIST has a wide range of on-disk instruments to observe the photosphere / chromosphere, but will also devote about half of the time to coronagraph spectral observations of forbidden coronal lines, out to 1.5 solar radii.We aim to study the heating of the solar corona by combining measurements of densities, temperatures and magnetic fields with DKIST.The transfer of energy and mass between the chromosphere and corona is still poorly understood. The modelling of this is extremely complex as it requires non-local transport and time-dependent non-equilibrium atomic processes coupled with MHD modelling. A key aspect of the proposal is to improve the modelling of atomic processes in the chromosphere/transition-region by adding density-dependent and photo-ionisation into the calculation of atomic rates.Solar Orbiter is a major ESA mission to be launched in February 2020 and is dedicated to the study of the inner parts of the heliosphere, the bubble where our solar system resides that is constantly modified by events on the Sun. Some of these events have relevance to Space Weather, i.e. affect significantly the outer layers of the Earth atmosphere, causing significant effects on satellites, human space flight and communication, for example.One of the main science goals of Solar Orbiter is to understand how and where the solar wind is released and accelerated from the solar surface into the heliosphere. To achieve this goal, the mission will have close encounters (to within 0.3 AU) with the Sun, and has a suite of in-situ and remote-sensing instruments. One of the main remote sensing instruments is SPICE, a UV spectrometer built at RAL (UK) that is able to measure Doppler flows, temperatures and chemical abundances, which we shall use to locate the source region of the solar wind observed in-situ and to study the formation of the solar wind.SPICE will mainly observe lines formed in the chromosphere-corona interface, so it will be suited to studies of the solar transition region, the thin layer of theatmosphere between the chromosphere and corona of the Sun. This region is very dynamic, and improved modelling of atomic processes, together with the inclusion of non-equilibrium effects will be required for the interpretation of the observations.We also aim to continue to provide the most fundamental atomic data for the interpretation of the observations of current and future solar missions. Our group, in particular Giulio Del Zanna, has been leading this field by continuously updating the CHIANTI atomic database, now also widely used to study astrophysical and laboratory plasma.Finally, we shall communicate our work to the public and to schools. Helen Mason, a well known communicator will play a key role in this regard.