Effect of a fossil magnetic field on the structure of a young Sun

TL;DR

This paper investigates how a fossil magnetic field influences the structure of a 500-million-year-old Sun, emphasizing the roles of magnetic pressure and tension, and provides a semi-analytical framework for future stellar evolution modeling.

Contribution

It introduces a novel non force-free mixed poloidal-toroidal magnetic field configuration and develops a general semi-analytical method to estimate magnetic effects on stellar structure.

Findings

Magnetic pressure and tension significantly affect stellar structure.

Small perturbations in the Sun's structure due to fossil magnetic fields.

Method enables future implementation in stellar evolution codes.

Abstract

We study the impact of a fossil magnetic field on the physical quantities which describe the structure of a young Sun of 500 Myr. We consider for the first time a non force-free field composed of a mixture of poloidal and toroidal magnetic fields and we propose a specific configuration to illustrate our purpose. In the present paper, we estimate the relative role of the different terms which appear in the modified stellar structure equations. We note that the Lorentz tension plays a non negligible role in addition to the magnetic pressure. This is interesting because most of the previous stellar evolution codes ignored that term and the geometry of the field. The solar structure perturbations are, as already known, small and consequently we have been able to estimate each term semi-analytically. We develop a general treatment to calculate the global modification of the structure and of the energetic balance. We estimate also the gravitational multipolar moments associated with the presence of a fossil large-scale magnetic field in radiative zone. The values given for the young Sun help the future implementation in stellar evolution codes. This work can be repeated for any other field configuration and prepares the achievement of a solar MHD model where we will follow the transport of such field on secular timescales and the associated transport of momentum and chemicals. The described method will be applied at the present Sun and the results will be compared with the coming balloon or space measurements.