WHOLE SUN · The Whole Sun Project: Untangling the complex physical mechanisms behind our eruptive star and its twins

We live around an active magnetic star, our Sun, that has a direct impact on our technological society through its variability and eruptive behaviour. Despite decades of intense research, fundamental questions such as How does the Sun work? Why does it possess a magnetic cycle and a dynamic hot atmosphere and how they are interrelated? remain mostly unanswered. In the Whole Sun project, we aim at tackling these key questions as a coherent whole for the first time. For too many years, the Sun has been split into internal and external solar physics topics lacking a global integrated view of its complex plasma dynamics. For instance, dynamo simulations seeking to answer the origin of the magnetic field and of its cyclic behaviour neglect surface physics and the existence of sunspots and likewise surface models often assume the magnetic field as a given input without the detailed knowledge of the nonlinear interplay between convection, rotation and magnetic fields in the Sun’s outer envelope. The time has come to gather European world leading solar/stellar physicists to build a deeper understanding of our star and to extend it to its twins. To do so many bottlenecks must be addressed: highly disparate spatial and temporal scales, physical interfaces of all solar layers, complex microphysics and global effects, strong dynamics and how parameters such as star’s metallicity, mass and rotation influence the outcome. By gathering physicists from each side of the solar surface we aim at tackling these challenging, beyond the state-of-the-art problems, by developing a deep theoretical understanding of our star and of its analogues and by building the most advanced multi-resolution solar code in order to jointly address global/macrophysics and local/microphysics aspects of the solar dynamics. The advent of Exa-scale computers makes such a challenge within our reach as do modern analysis methods to interpret observations and 4-D data cube that the project will produce or access.