Volume 639, July 2020
|Number of page(s)||12|
|Section||The Sun and the Heliosphere|
|Published online||21 July 2020|
Vortices evolution in the solar atmosphere
A dynamical equation for the swirling strength
Istituto Ricerche Solari Locarno (IRSOL), Via Patocchi 57, 6605 Locarno-Monti, Switzerland
2 Center for Theoretical Astrophysics and Cosmology, Institute for Computational Science (ICS), University of Zurich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
3 Leibniz-Institut für Sonnenphysik (KIS), Schöneckstrasse 6, 79104 Freiburg i.Br., Germany
Accepted: 20 May 2020
Aims. We study vortex dynamics in the solar atmosphere by employing and deriving the analytical evolution equations of two vortex identification criteria.
Methods. The two criteria used are vorticity and the swirling strength. Vorticity can be biased in the presence of shear flows, but its dynamical equation is well known; the swirling strength is a more precise criterion for the identification of vortical flows, but its evolution equation is not known yet. Therefore, we explore the possibility of deriving a dynamical equation for the swirling strength. We then apply the two equations to analyze radiative magneto-hydrodynamical simulations of the solar atmosphere produced with the CO5BOLD code.
Results. We present a detailed review of the swirling strength criterion and the mathematical derivation of its evolution equation. This equation did not exist in the literature before and it constitutes a novel tool that is suitable for the analysis of a wide range of problems in (magneto-)hydrodynamics. By applying this equation to numerical models, we find that hydrodynamical and magnetic baroclinicities are the driving physical processes responsible for vortex generation in the convection zone and the photosphere. Higher up in the chromosphere, the magnetic terms alone dominate. Moreover, we find that the swirling strength is produced at small scales in a chaotic fashion, especially inside magnetic flux concentrations.
Conclusions. The swirling strength represents an appropriate criterion for the identification of vortices in turbulent flows, such as those in the solar atmosphere. Moreover, its evolution equation, which is derived in this paper, is pivotal for obtaining precise information about the dynamics of these vortices and the physical mechanisms responsible for their production and evolution. Since this equation is available, the swirling strength is now the ideal quantity to study the dynamics of vortices in (magneto-)hydrodynamics.
Key words: magnetohydrodynamics / Sun: atmosphere / Sun: magnetic fields / turbulence
© ESO 2020
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