General information
Organisation
The French Alternative Energies and Atomic Energy Commission (CEA) is a key player in research, development and innovation in four main areas :
• defence and security,
• nuclear energy (fission and fusion),
• technological research for industry,
• fundamental research in the physical sciences and life sciences.
Drawing on its widely acknowledged expertise, and thanks to its 16000 technicians, engineers, researchers and staff, the CEA actively participates in collaborative projects with a large number of academic and industrial partners.
The CEA is established in ten centers spread throughout France
Reference
SL-DRF-26-0301
Direction
DRF
Thesis topic details
Category
Corpuscular physics and outer space
Thesis topics
Magneto-convection of solar-type stars: flux emergence and origin of starspots
Contract
Thèse
Job description
The Sun and solar-type stars possess rich and variable magnetism. In our recent work on turbulent convective dynamos in this type of star, we have been able to highlight a magneto-rotational history of their secular evolution. Stars are born active with short magnetic cycles, then slow down due to braking by their magnetized particle wind, their magnetic cycle lengthens to become commensurate with that of the Sun (lasting 11 years) and finally, for stars that live long enough, they end up with a loss of cycle and a so-called anti-solar rotation (slow equator/fast poles). The agreement with observations is excellent, but we are missing an essential element to conclude: What role do sunspots/starspots play in the organization of the magnetism of these stars, and are they necessary for the appearance of a stellar magnetic cycle, e.g. the so-called “paradox of spotty dynamos”? Indeed, our HPC simulations of solar dynamos do not have yet the angular resolution to resolve the spots, and yet we do observe cycles in our simulations of stellar dynamos for Rossby numbers < 1. So, are the spots simply a surface manifestation of an internal self-organization of the cyclic magnetism of these stars, or do they play a decisive role? Furthermore, how do the latitudinal flux emergence and the size and intensity of the spots forming on the surface evolve during the magneto-rotational evolution of these stars? To answer these key questions in stellar and solar magnetism in support of the ESA space missions Solar Orbiter and PLATO, in which we are involved, new HPC simulations of stellar dynamos must be developed, allowing us to get closer to the surface and thus better describe the process of magnetic flux emergence and the possible formation of sun/starspots. Recent tests showing that magnetic concentrations inhibiting local surface convection form in simulations with a higher magnetic Reynolds number and smaller-scale surface convection strongly encourage us to continue this project beyond the ERC Whole Sun project (ending in April 2026). Thanks to the Dyablo-Whole Sun code that we are co-developing with IRFU/Dedip, we wish to study in detail the convective dynamo, the emergence of magnetic flux, and the self-consistent formation of resolved spots, using its adaptive mesh refinement capability while varying global stellar parameters such as rotation rate, convective zone thickness, and surface convection intensity to assess how their number, morphology and latitude of emergence change and if they contribute or not to the closing of the cyclic dynamo loop.
University / doctoral school
Astronomie et Astrophysique d’Île de France (ED A&A)
Université de Paris
Thesis topic location
Site
Saclay
Requester
Position start date
01/10/2026
Person to be contacted by the applicant
BRUN Allan Sacha
sacha.brun@cea.fr
CEA
DSM/IRFU/DAp/LDE3
DRF/IRFU/DAp/LDE3
CEA/Saclay
Bat 709
91191 Gif-sur-Yvette, Cedex
+33 1 69 08 76 60
Tutor / Responsible thesis director
BRUN Allan Sacha
sacha.brun@cea.fr
CEA
DSM/IRFU/DAp/LDE3
DRF/IRFU/DAp/LDE3
CEA/Saclay
Bat 709
91191 Gif-sur-Yvette, Cedex
+33 1 69 08 76 60
En savoir plus
https://wholesun.eu
https://irfu.cea.fr/Phocea/Vie_des_labos/Ast/ast_groupe.php?id_groupe=971