arXiv subjects

Allan Sacha Brun

Publications and source records attributed to Allan Sacha Brun.

76 records · Page 5Linked to original sources

arXiv2003

Solar Turbulence and Magnetism Studied Within a Rotating Convective Spherical Shell

We discuss recent advances made in modelling the complex magnetohydrodynamics of the Sun using our anelastic spherical harmonics (ASH) code. We have conducted extensive 3--D simulations of compressible convection in rotating spherical shells with and without magnetic fields, to study the coupling between global-scale convection and rotation in seeking to understand how the solar differential rotation is established and maintained. Such simulations capable of studying fairly turbulent convection have been enabled by massively parallel supercomputers. The resulting convection within domains that capture a good fraction of the bulk of the convection zone is highly time dependent and intricate, and is dominated by intermittent upflows and networks of strong downflows. A high degree of coherent structures involving downflowing plumes can be embedded in otherwise chaotic flow fields. These vortical structures play a significant role in yielding Reynolds stresses that serve to redistribute angular momentum, leading to differential rotation profiles with pole to equator contrasts of about 30% in angular velocity $Ω$ and some constancy along radial line at mid latitudes, thereby making good contact with deductions from helioseismology. When a magnetic field is introduced, a dynamo regime can be found that does not destroy the strong differential rotation achieved in pure hydrodynamics cases. The magnetic fields are found to concentrate around the downflowing networks and to have significant north-south asymmetry and helicity.

astro-ph↗

arXiv2003

Looking Deep Within an A-type Star: Core Convection Under the Influence of Rotation

The advent of massively parallel supercomputing has begun to permit explicit 3--D simulations of turbulent convection occurring within the cores of early-type main sequence stars. Such studies should complement the stellar structure and evolution efforts that have so far largely employed 1--D nonlocal mixing length descriptions for the transport, mixing and overshooting achieved by core convection. We have turned to A-type stars as representative of many of the dynamical challenges raised by core convection within rotating stars. The differential rotation and meridional circulations achieved deep within the star by the convection, the likelihood of sustained magnetic dynamo action there, and the bringing of fresh fuel into the core by overshooting motions, thereby influencing main sequence lifetimes, all constitute interesting dynamical questions that require detailed modelling of global-scale convection. Using our anelastic spherical harmonic (ASH) code tested on the solar differential rotation problem, we have conducted a series of 3--D spherical domain simulations that deal with a simplified description of the central regions of rotating A-type stars, i.e a convectively unstable core is surrounded by a stable radiative envelope. A sequence of 3--D simulations are used to assess the properties of the convection (its global patterns, differential rotation, meridional circulations, extent and latitudinal variation of the overshooting) as transitions are made between laminar and turbulent states by changing the effective diffusivities, rotation rates, and subadiabaticity of the radiative exterior. We report on the properties deduced from these models for both the extent of penetration and the profile of rotation sustained by the convection.

astro-ph↗

arXiv2003

On Stellar Dynamo Processes and Differential Rotation

Many stars exhibit strong magnetic fields, some of which are thought to be of primordial origin and others a sign of magnetic dynamo processes. We briefly review the results of observational studies of solar-type stars seeking to evaluate the linkage between rotation rate and possible magnetic cycles of activity. Clearly turbulent convection and rotation within spherical shell geometries provide ingredients essential for dynamo action. However, intensive efforts over several decades in solar research have demonstrated that it is no easy matter to achieve cyclic magnetic activity that is in accord with observations. Helioseismology has revealed that an essential element for the global solar dynamo is the presence of a tachocline of shear at the base of the solar convection zone, leading to the likely operation of an interface dynamo. We review the crucial elements for achieving a cyclic magnetic activity. We then discuss some of our current 3--D MHD simulations of solar turbulent convection in spherical shells that yield differential rotation profiles which make good contact with some of the helioseismic findings. We show that such turbulent motions can amplify and sustain magnetic field in the bulk of the convective zone whose strength are sufficient to feed back both upon the convection and its global circulations.

astro-ph↗

arXiv2002

Turbulent Convection Under the Influence of Rotation: Sustaining a Strong Differential Rotation

The intense turbulence present in the solar convection zone is a major challenge to both theory and simulation as one tries to understand the origins of the striking differential rotation profile with radius and latitude that has been revealed by helioseismology. The differential rotation must be an essential element in the operation of the solar magnetic dynamo and its cycles of activity, yet there are many aspects of the interplay between convection, rotation and magnetic fields that are still unclear. We have here carried out a series of 3--D numerical simulations of turbulent convection within deep spherical shells using our anelastic spherical harmonic (ASH) code on massively parallel supercomputers. These studies of the global dynamics of the solar convection zone concentrate on how the differential rotation and meridional circulation are established. We have analyzed the transport of angular momentum in establishing such differential rotation, and clarified the roles played by Reynolds stresses and the meridional circulation in this process. We have found that the Reynolds stresses are crucial in transporting angular momentum toward the equator. The effects of baroclinicity (thermal wind) have been found to have a modest role in the resulting mean zonal flows. The simulations have produced differential rotation profiles within the bulk of the convection zone that make reasonable contact with ones inferred from helioseismic inversions, namely possessing a fast equator, an angular velocity difference of about 30% from equator to pole, and some constancy along radial lines at mid-latitudes.

astro-ph↗