Unravelling the Dynamics of the Solar Atmosphere

The Sun is our star and governs the solar system. There is a long scientific tradition of observing and studying the Sun but many fundamental questions remain unclear. For example, we still lack detailed understanding on why the outer solar atmosphere, the corona, is so hot (millions of degrees) while the solar surface is only few thousands of degrees. Furthermore, it is largely unknown how the corona is filled with mass and what drives the solar wind that supplies our planetary system with plasma and magnetic fields. It has been realised that a small layer in the solar atmosphere, the interface layer between the surface and the corona, may hold the key to answers to these questions. This interface layer, consisting of what is known as the chromosphere and transition region, is very shallow, only a few thousand kilometers but also very dynamic. Its complex dynamics are dominated by magnetic fields and radiative effects and is very challenging to understand. NASA has launched a small explorer spacecraft in the summer of 2013, the Interface Region Imaging Spectrograph (IRIS), which is specifically designed to observe this part of the solar atmosphere with unprecedented capabilities. However, IRIS is mostly sensitive to this small part of the atmosphere. In this project, we have provided key data that are essentially missing in the IRIS' arsenal: high-quality observations of the atmosphere below. These observations were acquired through coordinated campaigns with the Swedish 1-m Solar Telescope (SST) on La Palma. In addition, we have used large-scale, state of the art, computer simulations of the solar atmosphere in combination with advanced numerical tools to interpret the complex observational data. The combination of coordinated IRIS and SST observations and advanced numerical simulations allows us to construct a comprehensive picture of the solar atmosphere. We have conducted 14 successful coordinated observing campaigns during the 2016 to 2022 observing seasons. Due to the Covid-19 pandemic we could not travel to La Palma during 2020 and the start of the 2021 observing season. During that period, we collaborated closely with the support astronomer in La Palma to optimise remote observations. In late July 2021, we could finally return to La Palma and have observers from our team conduct the observations at the SST. Unfortunately, the September 2021 campaign was severly affected by the volcanic eruption on La Palma. The 2022 observing season was normal again with 3 successful campaigns. The storage servers and computing nodes that have been purchased during the project are still fully used for the processing of the observations. Both postdocs have finished their contracts and have moved into other possitions. The first PhD student defended his PhD thesis in October 2021 and has moved on to a postdoc position in the US. The other PhD student is expected to defend her PhD thesis in 2023. In June 2017, we published a paper in Science where we compared new advanced simulations with coordinated IRIS and SST observations. In December 2017, we were the first to publish results from CHROMIS, the new instrument at the SST, in Astrophysical Journal Letters. In this study, we combined the CHROMIS observations with IRIS coordinated observations and numerical simulations. One of the postdocs published a letter in 2020 which was selected as highlight of the Astronomy & Astrophysics journal. During the summer of 2020, we published a paper describing the public release of a database of coordinated SST and IRIS observations. These datasets are open for analysis for all interested colleagues in the scientific community.

We find that our work is well received by the scientific community, as we see that our publications receive a relatively high number of citations and by invited and contributed presentations at international conferences. Our considerable effort in using observing time at the Swedish Solar Telescope (SST) and investments in methods and hardware for data processing has resulted in a significant number of unique and outstanding data sets. This has consolidated SST’s excellent reputation as one of the prime solar telescopes in the world and has counted towards continued efforts to expand and develop SST instrumentation. Presently, pioneering new instrumentation is being developed at the SST as preparation for the planned 4-meter European Solar Telescope (EST). The EST is a large international collaboration at the European level and is planned to be the new flagship telescope. The staff that was employed on temporary contracts through the project has moved on to new jobs in academia and the private sector. We see that their participation in the project has a positive impact on their careers.

The solar chromosphere and transition region, a seemingly feeble and insignificantly shallow region sandwiched between the brilliant solar surface and enigmatic corona, may hide the key to understanding what energises the outer solar atmosphere and drives the solar wind. Despite the importance of this interface region, it remains poorly understood due to its bewildering complexity and dynamics. The Interface Region Imaging Spectrograph (IRIS), NASA's small explorer spacecraft launched in June 2013, is specifically designed to address fundamental questions about the nature of the outer solar atmosphere through high spatial and temporal resolution UV spectroscopy and imaging. We plan to provide key data that are essentially missing in the IRIS' arsenal: high resolution photospheric and chromospheric polarimetry and spectral imaging under and around the IRIS spectrograph slit. These observations will be acquired through coordinated campaigns with the Swedish 1-m Solar Telescope (SST) on La Palma. In addition, we will use large-scale, state of the art, radiative magneto-hydrodynamic simulations of the solar atmosphere in combination with advanced 3D non-LTE radiative transfer codes to interpret the complex observational data. The combination of coordinated IRIS and SST observations and advanced numerical simulations will allow to construct a comprehensive picture of the solar atmosphere.