The Dynamo-Wind Feedback Loop: Characterizing How the Solar Wind Varies Along a Dynamo Cycle

Abstract:

Though generated deep inside the convection zone, the solar magnetic field has a direct impact on the Earth space environment via various mechanisms. In particular, it strongly modulates the solar wind in the whole heliosphere: observations have shown that the 11-year cycle created by the dynamo inside the Sun affects the latitudinal speed distribution of the solar wind over the years. However, the wind also influences the topology of the coronal magnetic field by opening the magnetic field lines in the coronal holes, which can affect the inner magnetic field of the star by altering the dynamo boundary conditions. This coupling is especially difficult to model because it covers a large variety of spatio-temporal scales. Quasi-static studies have begun to help us unveil this how the dynamical dynamo magnetic field shapes the wind. Nevertheless, the full interplay between the solar dynamo and the solar wind still eludes our understanding.

We use the compressible magnetohydrodynamical code PLUTO to compute simultaneously in 2.5D the generation and evolution of magnetic field inside the star via an alpha-omega dynamo process and the corresponding evolution of the corona over a dynamo cycle. A multi-layered internal boundary condition at the surface of the star connects the inner and outer stellar layers, allowing both to adapt and update in real time. We focus on young suns to test the parameter range in which the coupling is effective, with dynamo cycle periods between 5 weeks and 5 days. Our coupled dynamo-wind model allows us to characterize how the solar wind conditions change as a function of the cycle phase, and also to quantify the evolution of the Alfvén surface, mass and angular momentum losses with the changing dynamo field. We further assess for the first time the impact of the solar wind on the dynamo itself by testing different levels of feedback and comparing them to no feedback at all. Finally, we characterize the exchange of information between inner and outer stellar layers in terms of helicity.

This gives us a new tool to better understand the Sun-Earth connection at various moments of the solar cycle in a space weather perspective and to eventually extend it to other stars with different rotation and level of activity.