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 and ultimately heating/accelerating 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/acceleration of coronal plasma. Methods. The Bifrost code produces realistic simulations of the solar atmosphere that 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 x 24 Mm. The network of magnetic funnels in the computational domain influence the movement of plasma, and 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 will 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.

Magnetic Tornado Properties: A Substantial Contribution to the Solar Coronal Heating via Efficient Energy Transfer

“…The merged MC exhibits a longer lifetime of 1650 s (20,460 s < t < 22,110 s) in comparison to the average lifetime of individual MCs, which is typically 300-500 s. This finding is consistent with prior observations demonstrating that the lifetimes of magnetic bright points (MBPs), which serve as observational signatures of MCs, are prolonged during merging events as compared to the lifetimes of individual MBPs (Keys et al 2011). Given that MCs can impede local magnetoconvection and prevent the collapse of photospheric vortices (Finley et al 2022), it is plausible that the prolonged lifetimes of MCs observed in this study may facilitate the development of long-lived photospheric vortices, which can, in turn, give rise to magnetic tornadoes featuring the large-scale chromospheric swirls. The detailed analysis of the condition for magnetic tornadoes to form is a line for future research.…”

“…to the corona. This finding is in line with previous investigations by Finley et al (2022). In this section, we further develop our discussion to address the underlying physical process for the enhancement of the coronal energy transport when the magnetic tornado is present, as compared to its absence.…”

“…Thus, in our specific setup, the vertical motion of the horizontal magnetic field plays minor or negative roles in the energy transfer. Although a similar trend is found in the previous work (Finley et al 2022), care needs to be taken in drawing a conclusion because the dynamics of flux emergence are significantly affected by the bottom boundary condition (Rempel & Cheung 2014). We need a more detailed analysis to clarify the role of flux emergence in coronal heating (Wang 2020, 2022) using, for example, a numerical model with sufficiently deep convection zone (e.g., Hotta et al 2019).…”

“…From the observational point of view, coronal heating is often discussed in terms of thermal response, in particular the differential emission measure (e.g., Aschwanden & Parnell 2002;Warren et al 2012;Shoda & Takasao 2021). Recently, large-scale three-dimensional simulations spanning from the convection zone to the corona have been conducted (e.g., Amari et al 2015;Hansteen et al 2015;Rempel 2017;Chen et al 2022;Finley et al 2022;Robinson et al 2022), which are often used to synthesize observational signatures (Peter 2015;Chen et al 2021;Breu et al 2022;Malanushenko et al 2022). Whereas, in exchange for reality, these simulations are often too complicated to reveal the underlying physics.…”

“…A recent numerical simulation by Wedemeyer-Böhm et al (2012) demonstrated that magnetic tornadoes are capable of transporting sufficient energy to heat the quiet Sun corona. Additionally, the merger of MCs can trigger magnetic tornadoes, which are efficient energy carriers (Finley et al 2022). In light of the fact that the merger of MCs frequently occurs on the photosphere (Berger & Title 1996;Berger et al 1998;Keys et al 2011;Iida et al 2012), magnetic tornadoes may have a vital role in coronal heating.…”