SOUTENANCE DE THESE - 20/09/19 - THE MAGNETIC SUN AND SPACE WEATHER : STUDY OF THE NON-LINEAR COUPLING BETWEEN DYNAMO, MAGNETIC ACTIVITY AND SOLAR WIND - Barbara Perri

Soutenance de thèse du Département d’Astrophysique



 THE MAGNETIC SUN AND SPACE WEATHER : STUDY OF THE NON-LINEAR COUPLING BETWEEN DYNAMO, MAGNETIC ACTIVITY AND SOLAR WIND

 Barbara PERRI

 DAp



 Vendredi 20 septembre 2019, 10h30



 Salle Galilée (bâtiment 713)



 It has been more than a hundred years since we discovered that the Sun not only has a magnetic field, but that it displays intense activity evolving according to an eleven-year cycle. This field is most certainly originated by a fluid dynamo inside the star. Although the evolution of the dynamo field is purely internal to the star, this field crosses the star's surface to influence the corona and interact with the solar wind, which is a continuous flow of charged particles that reaches the Earth. This interaction is yet not well characterized and very difficult to model due to the disparity of space and time scales and the diversity of physical mechanisms at stake (plasma regime change, turbulence, heating...). However this coupling between the dynamo and the wind is a key topic for the upcoming space missions like Parker Solar Probe or Solar Orbiter, and constitutes a crucial knowledge to improve the space weather models to anticipate the impact of the solar wind on the Earth spatial environment.



  This is precisely this interaction between magnetic activity and solar wind that I have studied in detail thanks to a theoretical study based on numerical simulations and interpretation of solar data. In a first part, we present the fundamental concepts needed to understand the various layers of the Sun, including its internal ones with the generation of magnetic field, and its atmosphere where the solar wind is accelerated. In a second part, we focus on the coupling between dynamo and wind with two different approaches. First we study only the influence of the magnetic field on the wind, without feedback : using magnetic configurations extracted from a Babcock-Leighton dynamo with flux-transport, we perform 54 wind simulations to cover an activity cycle, and thus see the evolution of the corona according to the amplitude and topology of the magnetic field. This study shows the influence of the topology on the structure of the corona and on several integrated quantities such as the Alfvén radius or the angular momentum loss, with variations of respectively a factor 2 and 3.5. This study also includes the propagation of a north-south asymmetry from the dynamo field to the solar wind, and shows that the asymmetry is transmitted but reduced, which agrees with solar observations. Then we advanced to the next challenge by developing the first large scale coupling model between the internal dynamo and the dynamical atmosphere of the Sun within the same numerical simulation. This study quantifies the feedback from the wind on the magnetic field by influencing the boundary conditions; we then compare our results with more classical dynamo models with radial or potential boundary conditions. We also compare our results with our quasi-static study to quantify the impact of the dynamical coupling, especially on the self-consistent evolution of the various physical quantities. Finally, in a third part, we extend our study from 2.5D to 3D, and to the solar system and other solar-type stars. We question the impact on Earth with a space weather approach : with simulations reaching 1 AU, we study the influence of the magnetic topology on cosmic rays. This presentation ends with an overture to other solar-type stars : we investigate the dynamo in G and K type stars, its feedback on rotation and thus on the long-terme evolution of the star, and the consequences for the associated stellar wind.



 Cette soutenance sera en français