Solar Vortex Tubes. II. On the Origin of Magnetic Vortices

Abstract

The solar atmosphere presents a wealth of dynamics due to a constant interplay between the plasma flows and magnetic fields. Twisted flux tubes are an essential magnetic structure, believed to be driven by rotational surface's motions and linked to plasma heating, jets and eruptive phenomena. Despite extensive investigations, twisted magnetic flux tubes lack a proper mathematical definition, precluding their automatic detection. This work addresses this issue by defining them as magnetic vortices and introduces a formal definition which is based on a recently developed magnetic vortices detection technique, the Integrated Averaged Current Deviation method. We applied this method and kinetic vortices identification technique to realistic magneto-convection simulations obtained from the MURaM code. The preferential site for these two types of vortices is the intergranular downflow, but while the magnetic vortices are found mostly in the small areas where plasma-β > 1, the rotational flow structures, the kinetic vortices, were detected in locations where plasma-β < 1. The magnetic vortices locally concentrate the magnetic field's vertical components and current, lasting on average around a minute. Two types of magnetic vortices are introduced based on their magnetic to kinetic energy ratio. For the first type, the magnetic energy prevails, and the magnetic vortices are mostly vertical. The second type of magnetic vortices presents distinct shapes and a lower magnetic to kinetic energy ratio. We have found that magnetic vortices may appear if two conditions are simultaneously present: (i) shear flow, (ii) plasma-β > 1. The presence of rotational motion is not necessary.

New approach for analysing dynamical processes on the surface of photospheric vortex tubes

“…The code has been used extensively in conjunction with simulated radiative diagnostics to explain a variety of small-scale photospheric phenomena, such as photospheric magnetic bright points (Shelyag et al 2004), photospheric absorption line profile shapes and asymmetries (Khomenko et al 2005;Shelyag et al 2007), analyse flow structures in photospheric magnetic reconnection (Shelyag et al 2018) and provide a link between photospheric reconnection events and Ellerman bombs (Nelson et al 2013). MURaM simulations have also been used to analyse the role of torsional motions in the solar atmospheric energy balance (Shelyag et al 2011b(Shelyag et al , 2012(Shelyag et al , 2013Yadav et al 2020Yadav et al , 2021 and in the analysis of physics and structure of photospheric vortical flows (Shelyag et al 2011a;Silva et al 2020Silva et al , 2021.…”

“…In simulations of quiet Sun regions vortices are found along intergranular lanes (Shelyag et al 2011a;Kitiashvili et al 2012;Moll et al 2012;Silva et al 2020). These structures have an average lifetime of around 80 s Silva et al (2021) and a radius between 40 and 80 km Shelyag et al (2013); Silva et al (2020). Solar kinetic vortex tubes (Silva et al 2021) act as a sink for magnetic field, creating magnetic flux tubes that expand with height (Kitiashvili et al 2012;Moll et al 2012;Silva et al 2020).…”

“…These structures have an average lifetime of around 80 s Silva et al (2021) and a radius between 40 and 80 km Shelyag et al (2013); Silva et al (2020). Solar kinetic vortex tubes (Silva et al 2021) act as a sink for magnetic field, creating magnetic flux tubes that expand with height (Kitiashvili et al 2012;Moll et al 2012;Silva et al 2020). The concentration of magnetic flux leads to a high magnetic field tension, which can prevent the magnetic field lines to be twisted by the rotational motion (Shelyag et al 2011b;Moll et al 2012;Nelson et al 2013;Silva et al 2021).…”

“…Solar kinetic vortex tubes (Silva et al 2021) act as a sink for magnetic field, creating magnetic flux tubes that expand with height (Kitiashvili et al 2012;Moll et al 2012;Silva et al 2020). The concentration of magnetic flux leads to a high magnetic field tension, which can prevent the magnetic field lines to be twisted by the rotational motion (Shelyag et al 2011b;Moll et al 2012;Nelson et al 2013;Silva et al 2021). In some cases, twisted magnetic flux tubes appear close enough to flow vortices, leading to magnetic and kinetic vortex structures closely co-existing in regions with high plasma-β (Wedemeyer & Steiner 2014;Rappazzo et al 2019;Silva et al 2021).…”

“…The concentration of magnetic flux leads to a high magnetic field tension, which can prevent the magnetic field lines to be twisted by the rotational motion (Shelyag et al 2011b;Moll et al 2012;Nelson et al 2013;Silva et al 2021). In some cases, twisted magnetic flux tubes appear close enough to flow vortices, leading to magnetic and kinetic vortex structures closely co-existing in regions with high plasma-β (Wedemeyer & Steiner 2014;Rappazzo et al 2019;Silva et al 2021). The vortical motions can still trigger perturbations along magnetic lines that could lead to wave excitation, e.g.…”