Advanced models of the solar transition region and corona

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Current and upcoming solar missions are providing a wealth of novel observations of the upper chromosphere, transition region, and corona. They will be key in our efforts to address various science questions such as: how is energy transferred between the chromosphere and the corona? How is energy released in the corona and what is the connection with the solar wind?The observations can provide key plasma parameters such as densities, temperatures, chemical abundances and coronal magnetic fields. Measuring chemical abundances in the transition region and corona is important as their variations are likely related to the (yet unknown) plasma heating processes occurring in the corona and acting in the chromosphere. Also, as they can be used as a tracer for the solar wind. The coronal magnetic field dominates the energetics in the solar corona, yet its measurements have been elusive until recently. However, the full scientific exploitation of these observations requires a long-term effort on the theory and modelling side. The present project provides a significant contribution by building and providing to the community advanced models, with additional physics included. These models will also be used to improve measurements of the above key plasma parameters.Major facilities, with strong STFC support, are Solar Orbiter and DKIST. One of the main science goals of Solar Orbiter, which has now entered its science phase, is to understand how and where the solar wind is released and accelerated, by comparing in-situ with remote-sensing observations of the plasma parameters, during close encounters (to within 0.3 AU) with the Sun. SPICE, a UV spectrometer built in the UK, is mainly providing observations of the chromosphere and transition region.The advanced models will improve the SPICE measurements of temperatures and chemical abundances. They will also provide a building block for studies of the poorly understood connection between the chromosphere and the corona, which is very difficult to model as it requires complex non-local transport, time-dependent effects and non-LTE atomic physics. Studies on the formation mechanisms of helium and oxygen ions will be carried out. DKIST is the first large-scale solar telescope that will be performing unprecedented routine spectro-polarimetric observations of the chromosphere (on-disk) and the corona, mostly in the near-infrared (NIR). We will predict with the advanced atomic models the helium coronal emission, needed to establish its formation mechanism and to obtain the He coronal abundance and the magnetic field from DKIST observations. We will also improve our knowledge of the NIR with upcoming AIR-Spec and CORSAIR missions. Finally, we plan to revise recent measurements of active region coronal magnetic fields using existing Hinode EIS observations, by taking into account improvements in the instrument calibration and various physical effects (e.g. opacity and non-thermal electrons) not previously considered.