Volume 649, May 2021
|Number of page(s)||23|
|Section||The Sun and the Heliosphere|
|Published online||26 May 2021|
The Alfvénic nature of chromospheric swirls⋆
Istituto Ricerche Solari Locarno (IRSOL), Via Patocchi 57 – Prato Pernice, 6605 Locarno-Monti, Switzerland
2 Institute for Data Science (I4DS), STIX for Solar Orbiter Group, University of Applied Sciences Northwestern Switzerland (FHNW), 5210 Windisch, Switzerland
3 Institute for Particle Physics and Astrophysics (IPA), Solar Astrophysics Group, Swiss Federal Institute of Technology in Zurich (ETHZ), 8039 Zurich, Switzerland
4 Center for Theoretical Astrophysics and Cosmology, Institute for Computational Science (ICS), University of Zurich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
5 Institute for Solar Physics, Stockholm University, 10691 Stockholm, Sweden
6 Laboratory of Wave Engineering, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
7 Leibniz-Institut für Sonnenphysik (KIS), Schöneckstrasse 6, 79104 Freiburg i.Br., Germany
Accepted: 2 March 2021
Context. Observations show that small-scale vortical plasma motions are ubiquitous in the quiet solar atmosphere. They have received increasing attention in recent years because they are a viable candidate mechanism for the heating of the outer solar atmospheric layers. However, the true nature and the origin of these swirls, and their effective role in the energy transport, are still unclear.
Aims. We investigate the evolution and origin of chromospheric swirls by analyzing numerical simulations of the quiet solar atmosphere. In particular, we are interested in finding their relation with magnetic field perturbations and in the processes driving their evolution.
Methods. The radiative magnetohydrodynamic code CO5BOLD is used to perform realistic numerical simulations of a small portion of the solar atmosphere, ranging from the top layers of the convection zone to the middle chromosphere. For the analysis, the swirling strength criterion and its evolution equation are applied in order to identify vortical motions and to study their dynamics. As a new criterion, we introduce the magnetic swirling strength, which allows us to recognize torsional perturbations in the magnetic field.
Results. We find a strong correlation between swirling strength and magnetic swirling strength, in particular in intense magnetic flux concentrations, which suggests a tight relation between vortical motions and torsional magnetic field perturbations. Furthermore, we find that swirls propagate upward with the local Alfvén speed as unidirectional swirls driven by magnetic tension forces alone. In the photosphere and low chromosphere, the rotation of the plasma co-occurs with a twist in the upwardly directed magnetic field that is in the opposite direction of the plasma flow. All together, these are clear characteristics of torsional Alfvén waves. Yet, the Alfvén wave is not oscillatory but takes the form of a unidirectional pulse. The novelty of the present work is that these Alfvén pulses naturally emerge from realistic numerical simulations of the solar atmosphere. We also find indications of an imbalance between the hydrodynamic and magnetohydrodynamic baroclinic effects being at the origin of the swirls. At the base of the chromosphere, we find a mean net upwardly directed Poynting flux of 12.8 ± 6.5 kW m−2, which is mainly due to swirling motions. This energy flux is mostly associated with large and complex swirling structures, which we interpret as the superposition of various small-scale vortices.
Conclusions. We conclude that the ubiquitous swirling events observed in numerical simulations are tightly correlated with perturbations of the magnetic field. At photospheric and chromospheric levels, they form Alfvén pulses that propagate upward and may contribute to chromospheric heating.
Key words: magnetohydrodynamics (MHD) / Sun: atmosphere / Sun: magnetic fields
Movie associated to Fig. C.1 is available at https://www.aanda.org
© ESO 2021
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