Skip to Main content Skip to Navigation

Champs magnétiques générés par effet dynamo dans les objets astrophysiques en rotation

Abstract : The objective of this thesis is to investigate the generation of astrophysical magnetic fields via the dynamo effect. The impact of a global rotation and an average magnetic field imposed simultaneously, suitable configuration especially for planetary and stellar interiors, is the main issue of this study. Particular attention is paid to the temporal dynamics of the system, as well as to the topology of the generated field. In order to obtain an overview of the phenomenon, several numerical approaches were used, from the cubic model (TURBO code) focused on the study of local phenomena to the spherical model (PaRoDy code) allowing a more direct comparison with observations. These two types of geometry complement each other : the less constraining local approach makes it possible to observe precisely the consequences of a parameter, while the global approach is subject to more realistic constraints, such as thermal convection. The local model revealed an inverse cascade of hybrid helicity, the second invariant of the doubly anisotropic MHD, whose intensity depends on several parameters. The latter is more important when the polarization favored by the forcing corresponds to magnetostrophic waves (right polarization). Also, the tilt angle between the magnetic field and the rotation axes plays a major role, especially when θ≻35°, the value for which the process causing the cascade changes and weakens it. Finally, increasing the number of magnetic Prandtl (Pm) leads to a faster and more important transfer of magnetic energy to large scales. The global study also shows the key role of Pm in large-scale field generation. Indeed, the new values explored show that strong dipole fields can be maintained in more turbulent regimes. This specific behaviour corresponds to dynamos for which the Lorentz force is not negligible at large scales. The relative importance of inertia compared to magnetic forces is decisive in the topology of the observed field. Thus, the transition from a dipolar regime to a multipolar regime is delayed by the Lorentz force, the essential component of force balances in astrophysics. These results could be applied to various systems, including the geomagnetic field during reversals.
Complete list of metadatas

Cited literature [180 references]  Display  Hide  Download
Contributor : Abes Star :  Contact
Submitted on : Monday, February 24, 2020 - 2:35:21 PM
Last modification on : Sunday, October 18, 2020 - 4:13:07 PM


Version validated by the jury (STAR)


  • HAL Id : tel-02489431, version 1


Mélissa Menu. Champs magnétiques générés par effet dynamo dans les objets astrophysiques en rotation. Astrophysique [astro-ph]. Université Paris Saclay (COmUE), 2019. Français. ⟨NNT : 2019SACLX104⟩. ⟨tel-02489431⟩



Record views


Files downloads