New insights about the past, present and future of solar magnetism

Quentin Noraz

Hosted by Rosseland Centre for Solar Physics, University of Oslo, Norway on December 1, 2022

Abstract

The magnetic field of solar-type stars is generated and sustained through an internal dynamo process. This process is mostly determined by the combined action of turbulent convective motions and differential rotation. It can sometimes lead to magnetic cyclic variabilities, like the 11-years solar cycle. Evidence of magnetic cycles have been detected for other solar-type stars as well, ranging from a few years to a few tens of years. How are these cycles controlled? Observations and stellar evolution models show that solar-like stars spin-down during their main-sequence. In parallel, numerical simulations of these stars show that different regimes of differential rotation can be reach and are characterized with the Rossby number. In particular, anti-solar differential rotation (fast poles, slow equator) may exist for high Rossby numbers (slow rotators), which grows when the rotation spins-down. If this regime appears during the main sequence, we may wonder how the dynamo process will be impacted, especially if our Sun is in such a transition. In particular, can slowly rotating stars have magnetic cycles?

We performed a numerical multi-D parametric study with the STELEM and ASH codes to understand the magnetic field generation of solar-type stars under various differential rotation regimes. We particularly focused on the energy transfers powering these stellar dynamos, and on the existence of magnetic cycles for different stages of the main sequence. We find that short cycles are favoured for small Rossby numbers (fast rotators), and long cycles for intermediate (solar-like) Rossby numbers. We further assess that energy transfers can reach up to 3% of the stellar luminosity to sustains these dynamos, and ultimately powering surface eruptive events. Finally, we find that anti-solar rotating stars (high Rossby numbers) can only sustain magnetic cycles for specific dynamo processes. This led us to develop a theoretical criterion to select anti-solar candidates with the perspective to bring new constraints on our models, stellar evolution, and more particularly on the future of the Sun.