Magnétohydrodynamic flows in Astrophysics and Geophysics using NumErical and Theoretical approaches – MAGNET

1 - Scientific background and objectives

The MAGNET project has two main objectives. The first objective is to pursue the development of existing magneto-hydrodynamics (MHD) numerical codes (ideal-MHD and kinetic codes) to make them freely accessible on line, once extensively tested and fully documented. Their increased versatility is intended to make them suitable to the treatment of a number of problems in astrophysics and geophysics where the magnetic field is known to play a major role. The second objective is to address the question of the generation, structure and influence of magnetic fields in the physics of galaxies, interstellar medium, accretion disks, stars and planets, on numerical and theoretical grounds. Our team comprises experts in domains which were rather separated hitherto and the issues addressed should greatly benefit from fruitful cross fertilizing exchanges within this new collaboration, providing original insights and tools.

2 - Description of the project and methodology

Three different kinds of codes have been developed by the partners of the team and will be further developed with the goal of making them usable in broader contexts: (1) two grid based MHD codes based on conservative Godunov schemes, one with adaptive mesh refinement, the other with a fix grid available in different metrics, (2) a spectral code dedicated to the study of the dynamo, based on spherical harmonics suitable for the study of incompressible (or weakly compressible) MHD in planets and stars, and (3) particle-in-cell (PIC) codes, allowing the investigation of non-ideal MHD. Once tested and fully documented, these versatile codes will be made freely accessible on line, at the end of the four year project.

Before their public release, several numerical studies will be conducted by the team. The generation of magnetic fields through the dynamo instability will be addressed in galactic disks, in the Sun and within the Earth. In the case of galactic disks, several mechanisms responsible for the growth of magnetic fields will be considered: the explosion of supernovae and large scale turbulence generated by the magneto-rotational instability. A comparative study of the growth rate of magnetic fields in planets and in the Sun will be conducted.

Numerical investigation of turbulence in a variety of media, in close relation with observationnal studies, will also be a major goal of the project. Emphasis will be given to turbulence in the diffuse interstellar medium and molecular clouds in the context of star formation. Simulations of the turbulence in the terrestrial magnetosheath will be developed. Theoretical and numerical efforts will be dedicated to the study of the role of turbulence in magnetic reconnection.

Last, the formation of accretion disks and MHD jets, as an outcome of the gravitational collapse of a dense magnetised core into a star, will be studied, numerically and theoretically, in close connection with available observations.

3 - Expected results

The development of versatile numerical MHD codes intended to be made freely accessible on line would be a major, and particularly influencial accomplishment. It would provide the French community in astrophysics and geophysics with tools that are not yet available in France.

All the studies will be conducted in collaboration between the three partners, and we anticipate original insights and major progress in the understanding of (1) the coupling between large and small scales in the galaxy evolution, (2) the role of magnetic fields in star formation, (3) micro-physical processes in interstellar turbulence, (4) field generation in astrophysics and geophysics, (5) the mechanisms at the origin of field reconnection in the magnetosheath. Interactions between specialists of collisionless media and hydrodynamicists should fill in gaps between these different approaches and bring to light generic aspects of turbulent in magnetized media.

Finally, the team will predict observable signatures (synthetic spectra, statistical properties) which will be used in the preparation and, later on, in the analysis of astrophysical data obtained on large existing (e.g. CLUSTER) and upcoming facilities, such as the Herschel Space Observatory or the ALMA interferometer.