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Understanding the eruptive activity of the Sun

How is the magnetic field generated inside the Sun? By which mechanisms does it create solar spots and eruptions of magnetised clouds and particles? To find out, the European consortium Wholesun, coordinated by the CEA, is developing numerical models of the star in its entirety using the most powerful supercomputers. The numerical models will be used in combination with observations from Solar Orbiter, an ESA satellite launched on 10 February 2020.

Published on 9 April 2020

​We live close to a star whose eminently variable magnetic activity poses a threat to our technological society. Despite decades of research, we still do not fully understand the workings of the Sun with its magnetic 11yr
cycle, the dark spots on its surface, and its very hot, turbulent atmosphere. 

"Until now, studies of the Sun have focused on either internal or external solar physics, to the detriment of an integrated view. For example, dynamo simulations explain the magnetic field and its cyclic behaviour but overlook surface physics and ignore the existence of solar spots. Similarly, surface models neglect the nonlinear interaction between the convection and rotation movements of plasma and the magnetic fields in the external envelope of the Sun. With Wholesun, we are looking to tackle these issues as a coherent whole for the first time", saids Dr Allan Sacha Brun, Astrophysicist and Research Director, CEA Institute of Research into the Fundamental Laws of the Universe (Irfu) and Project Coordinator.

Wholesun aims to consolidate work on the two major solar regions: (i) the Sun's interior, and (ii) its surface and atmosphere. "The detailed study of the thermodynamic and magnetic coupling between the core of the Sun, the solar surface and the highly stratified atmosphere is absolutely essential if we want to address the key and open problems of solar physics," argues Dr Antoine Strugarek, astrophysicist at CEA-Irfu.

Bringing together international experts from each party, the Wholesun project team will develop the most advanced simulations of the Sun in order to reconstruct the "lifecycle" of its magnetic field, from its formation and amplification inside the Sun by dynamo effect to its emergence at the surface and eruptive dynamics in the atmosphere.

The CEA – and particularly the Irfu – will be a major contributor to the project through its proficiency in the turbulent dynamics of solar-type stars and the origin of their magnetism, and will play an active role in new global solar code development through the expertise of its Maison de la Simulation. "The advent of exascale supercomputers capable of performing several billion billion arithmetic operations per second opens up the possibility of tackling the toughest astrophysical questions, such as solar magnetism," points out Dr Pierre Kestener, former Maison de la Simulation expert now at CEA-Irfu. "It is therefore essential that we use the best possible algorithms and numerical methods."

Finally, this integrated approach may be extended to twin stars of the Sun which have a different rotational speed and chemical composition. It will lead the way to better understanding of the magnetism and activity of stars in all their diversity, especially in preparation for the ESA's Plato (Planetary Transits and Oscillations of stars) mission which is due for launch at the end of the Wholesun project in 2026, and to which CEA-Irfu is supplying instrumentation.

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