Stirring the base of the solar wind: On heat transfer and vortex formation

Abstract

Context. Current models of the solar wind must approximate (or ignore) the small-scale dynamics within the solar atmosphere; however, these are likely important in shaping the emerging wave-turbulence spectrum that ultimately heats and accelerates the coronal plasma. Aims. This study strives to make connections between small-scale vortex motions at the base of the solar wind and the resulting heating and acceleration of the coronal plasma. Methods. The Bifrost code produces realistic simulations of the solar atmosphere which facilitate the analysis of spatial and temporal scales which are currently at, or beyond, the limit of modern solar telescopes. For this study, the Bifrost simulation is configured to represent the solar atmosphere in a coronal hole region, from which the fast solar wind emerges. The simulation extends from the upper-convection zone (2.5 Mm below the photosphere) to the low corona (14.5 Mm above the photosphere), with a horizontal extent of 24 Mm × 24 Mm. The network of magnetic funnels in the computational domain influence the movement of plasma, as well as the propagation of magnetohydrodynamic waves into the low corona. Results. The twisting of the coronal magnetic field by photospheric flows efficiently injects energy into the low corona. Poynting fluxes of up to 2 − 4 kWm−2 are commonly observed inside twisted magnetic structures with diameters in the low corona of 1–5 Mm. Torsional Alfvén waves are favourably transmitted along these structures, and subsequently escape into the solar wind. However, reflections of these waves from the upper boundary condition make it difficult to unambiguously quantify the emerging Alfvén wave-energy flux. Conclusions. This study represents a first step in quantifying the conditions at the base of the solar wind using Bifrost simulations. It is shown that the coronal magnetic field is readily braided and twisted by photospheric flows. Temperature and density contrasts form between regions with active stirring motions and those without. Stronger whirlpool-like flows in the convection, concurrent with magnetic concentrations, launch torsional Alfvén waves up through the magnetic funnel network, which are expected to enhance the turbulent generation of magnetic switchbacks in the solar wind.