The Alfvénic nature of chromospheric swirls

Tornado like plasma motions have been reported to abundantly exist in the chromospheric layers of quiet Sun regions by various solar observers in the recent past. They are considered a viable candidate mechanism for the heating of the outer solar atmosphere. However, their true nature and origin and their effective role in the transport of energy, waves, and mass are still unclear. A team of scientists, mainly based at the Leibniz Institute for Solar Physics (KIS) and the Istituto Ricerche Solari Locarno (IRSOL), have now studied the origin and evolution of chromospheric swirls in numerical simulations of the quiet solar atmosphere.

Time sequence of a single swirl event of a numerical simulation from times t = 5760 s to t = 6160 s. From top to bottom row: vertical component of the magnetic field strength on the solar surface at z = 0 (τ = 1), continuum intensity I, vertical component of the swirling vector λ_z at the base of the chromosphere at z = 700 km, vertical component of the magnetic swirling vector λ_z^B at z = 700 km, and the bin-5 intensity I_5 as a proxy of chromospheric diagnostics. Maps of λ_z and λ_z^B also show the streamlines of oppositely directed rotations of the velocity field and the magnetic field, projected into the horizontal plane at z = 700 km, respectively. The maps of I_5 show the evolution a chromospheric swirl.

Realistic numerical simulations of a small portion of the solar atmosphere, ranging from the top layers of the convection zone to the middle chromosphere were performed with the radiative magnetohydrodynamic simulation tool CO5BOLD. The simulations were carried out on the Piz Daint high performance computer at the "Centro Svizzero di Calcolo Scientifico" (CSCS).

For the analysis, the swirling strength criterion and its evolution equation (see picture of the month of Nov. 2020) were applied in order to identify vortical motions and to study their dynamics. As a new criterion, the magnetic swirling strength, was introduced for a measure of the torsional perturbation in the magnetic field.

Firstly, a strong correlation between swirling strength and magnetic swirling strength was found, particularly in intense magnetic flux concentrations. This suggests a tight relation between vortical motions and torsional magnetic field perturbations. Secondly, it was found that swirls propagate upward with the local Alfvén speed as unidirectional swirls driven by magnetic tension forces alone. Thirdly, the rotation of the plasma and the twist in the upwardly directed magnetic field proved to be of opposite direction. All together, these are clear characteristics of torsional Alfvén waves. Yet, the Alfvén waves were not oscillatory but took the form of unidirectional pulses. The novelty of the present work is that these Alfvén pulses naturally emerge from realistic numerical simulations of the solar atmosphere. At the base of the chromosphere, a mean net upwardly directed Poynting flux of 12.8 ± 6.5 kW m−2 was found, which is mainly due to swirling motions and mostly associated with large and complex swirling structures, which were interpreted as the superposition of various small-scale vortices.

From this work, it can be concluded that chromospheric swirls are the visible signature of torsional Alfvén pulses that propagate upward from the base of the photosphere to the chromosphere. Their origin in the surface layers of the convection zone and their fate in the upper chromosphere and the corona are subject of further investigations.

Reference: Andrea Francesco Battaglia, José Roberto Canivete Cuissa, Flavio Calvo, Aleksi Antoine Bossart, and Oskar Steiner: 2021. “The Alfvénic nature of chromospheric swirls”, Astronomy & Astrophysics  649, A121