Towards a global picture of cool dwarf stellar wind models
Intervenant : Victor Reville
Solar-like stars, which possess a convective envelope, are believed to drive a stellar wind all along their lifespan on the main sequence. Stellar expanding atmospheres have been, since the discovery of the solar wind in the early 60’s, held responsible for the extraction of angular momentum of solar-like stars, that make them converge on the Skumanich law. Moreover, new observation techniques, such as Zeeman Doppler Imaging of the stellar surface magnetic field, or mass-loss measurements obtained through Lyman-alpha absorption of the stellar astrospheres of cool dwarfs urge theoreticians to capture the global picture of this ubiquitous phenomenon.
This talk will try to tackle this objective and derive properties of stellar winds starting with a simple model calibrated on the Sun and the solar wind characteristics. Magnetic field amplitude, topology, stellar rotation and stellar mass are all important parameters that need to be included in a relevant stellar wind model. We will see how this can be done within 3D MHD simulations as well as in much simpler semi-analytical models, and we will state on the necessary future improvements.
The geometry of the photospheric magnetic field strongly varies over the solar cycle, going from dipolar to multipolar between minimum and maximum of activity. The structure of the solar wind, particularly the spatial distribution of the slow and fast wind components shows a strong correlation with the structure of the magnetic field in the low-corona. For instance, the fast wind appears to be anti- correlated with the expansion factor, which is a consequence of the dynamics of the open and closed regions. We study the evolution of the solar coronae over the 22nd cycle with 3D MHD simulations. We constrain the photospheric magnetic field with synoptic magnetic maps obtained through observations at the Wilcox Observatory. We obtain 12 different simulations corresponding to quasi-static solutions of the solar coronal structure every year between 1989 and 2000. These simulations allow to compute expansion factor in a better fashion than the usual potential field source surface model and can be compared to remote and in-situ measurements of the solar wind, through Ulysses data for instance. We also compare our results to emission measurements made by the SOHO/EIT instrument thanks to synthetic images post-processed from our simulations.