The Sun is a magnetically active star with violent eruptions that can hit Earth´s magnetosphere and cause important perturbations in our technology-dependent society. The objective of the Whole Sun project is to tackle in a coherent way for the first time key questions in Solar Physics that involve as a whole the solar interior and the atmosphere. Our star, the Sun, is a magnetically active celestial body. Its atmosphere undergoes violent eruptions, which are difficult to predict. The largest eruptions, after traversing the interplanetary space, can hit and deform Earth´s magnetosphere and cause important perturbations in our technology-dependent society.
The intense research in solar astrophysics in the past decades has produced important advancement in the knowledge of the solar structure and dynamics. Yet, there remain fundamental questions without a fully satisfactory answer, like: which processes in the interior lead to the generation of the solar magnetic field and why does the Sun have a magnetic activity cycle? What exactly is the mechanism leading to the giant magnetic eruptions seen in its atmosphere? What is the mutual relationship between the interior and the atmosphere? The objective of The Whole Sun project is to tackle these key questions that concern simultaneously the interior and the atmosphere as a coherent whole for the first time.
Until now, the research on the Sun had been carried out through the separate study of its interior, the low atmosphere and the corona, without a global, integrated vision of the complex dynamics that links the plasma in those regions. To understand and provide quantitative explanations for the physical processes in them one has to use advanced concepts of fluid dynamics, electromagnetism, kinetic theory and, additionally for the atmosphere, radiation-matter interaction; one has to apply refined techniques of theoretical and numerical modeling using massively parallel supercomputing installations; one must also carry out and interpret observations acquired in the advanced telescope installations on the ground and in space available at present. The Whole Sun project brings together five European institutions with leading solar physics research groups; we want to attain a deeper understanding of our star by linking the physics of its interior and atmosphere. To achieve that goal, we have to overcome important hurdles like: simultaneous consideration of very different space and time scales; challenging coupling of microphysics effects next to continuum physics and global effects; bringing together and coupling computer codes that were created separately with a specific region in mind. Our goal is to tackle these problems through the development of deep theoretical understanding of our star and the construction of the most advanced solar code of multiple space and time resolution attainable at present.
Solar atmosphere ejections.
There are still many open questions concerning key ejections in the solar atmosphere such as coronal jets, surges or spicules. We have addressed different aspects of these phenomena. For instance, we have showed that the characteristics of observed surges and kernels (plasmoids) that accompany coronal jets, as well as the detected double-chambered structures, show striking similarities with numerical jet models (Joshi et al. 2020). We have also studied through 2.5D numerical simulations, the impact of coronal jets on prominences, obtaining oscillation amplitudes and periods that are in general agreement with the observations (Luna & Moreno-Insertis, 2021). With respect to surges, we have characterized for the first time the chromospheric and transition region properties of these phenomena combining high-resolution observations and advanced techniques such as k-means, inversions and density diagnostics (Nóbrega-Siverio et al. 2021). In addition, we have involved in the understanding of the coronal and transition region responses to the recently reported chromospheric downflowing rapid red shifted excursions (RREs, Bose et al. 2021).
Nonequilibrium and partial ionization.
We have studied the effects of nonequilibrium and partial ionization in the dynamics and thermodynamics of new magnetized plasma emerging from the solar interior through numerical experiments (Nóbrega-Siverio et al. 2020a). To that end, it was necessary to implement a new Fortran module in the Bifrost code that calculates the ambipolar diffusion term in the Generalized Ohm's Law in an efficient way (Nóbrega-Siverio et al. 2020b). Recently, we have explored the ambipolar diffusion term from a more fundamental and mathematical perspective, finding new sets of self-similar solutions that can be used as demanding tests for MHD codes that include the ambipolar diffusion term, both in cylindrical coordinates (Moreno-Insertis et al. 2021, under review) and in cartesian ones (Moreno-Insertis et al. to be submitted).
We have contributed to the review of the Interface Region Imaging Spectrograph (IRIS, De Pontieu et al. 2021), focusing on the key aspects that this satellite has contributed to the better understanding of the formation and impact of spicules and other jets. We have also explored for the first time the chromospheric counterpart of Coronal Bright Points (CBPs), providing further understanding on the heating of the plasma confined in the small-scale loops of the CBPs (Madjarska et al. 2021).
In addition to that, we have contributed in two papers related to the Multi-slit Solar Explorer (MUSE), which is a proposed NASA MIDEX mission. Our role has been providing synthetic observables from our realistic simulations with the aim of showing the potential capabilities of diagnostics of the mission (see De Pontieu et al. 2021, accepted, and Cheung et al. 2021, under review).