Vortex flows and magnetic tornadoes on the Sun and cool stars

The detection algorithm for solar vortex flows was further improved, tested and applied to the previously calculated set of 3D magnetohydrodynamic simulations. The new algorithm combines several stages: (1) Determination of the velocity field. (2) Vortex detection (3) Detection post-processing (4) Event identification In the first stage, the line integral convolution (LIC) method is used to enhance streamlines and thus vortex flows, which are otherwise difficult to spot in the analysed time sequences. The LIC results are then used as input into a local correlation tracking (LCT) algorithm, which extracts the 2D velocity field in the view plane. The resulting velocity data is then passed on to the second stage, in which the algorithm looks for signatures of vortex flows. For this purpose, we have tested two different indicators for a variety of spatial/temporal scales. One is the enhanced vorticity method and the other is the vorticity strength method. Both methods were evaluated based on the systematic set of 3D model atmospheres created with CO5BOLD. The statistical analysis of the resulting vortex detections shows that the lifetime and the size of the detected events depend strongly on the selected method. We concluded that the vorticity strength method identifies vortex flows more accurately than the enhanced vorticity method and yields more complete and statistically meaningful detections than visual inspections. The detected vortices - as found as peak values in vorticity strength - were then carefully post-processed (stage 3) and followed through the model time sequences and accordingly connected into coherent vortex events (stage 4). A comprehensive article describing this new method will appear in Astronomy & Astrophysics in 2017.

The aim of the VORTEX project is the comprehensive study of vortex flows and magnetic tornadoes on the Sun and other stars. The project focuses on the small-scale magnetic tornadoes, which have been discovered recently and are thought to be abundant on ou r Sun. They are generated by vortex flows, which form due to the bathtub effect at the solar surface and force the footpoints of magnetic field concentrations to rotate. The magnetic fields extend through the atmospheric layers and thus mediate the rotati on upwards, resulting in a net energy transport into the upper layers. There, the energy is dissipated by yet unknown physical processes and thus contributes to the heating of the solar corona to temperatures in excess of a million degree Kelvin. The VOR TEX project addresses many fundamental and yet unknown aspects of this novel phenomenon. The chosen approach combines high-resolution observations with world-leading facilities like the ground-based solar telescope SST and the space-borne observatories SD O and IRIS and advanced numerical simulations with state-of-the-art 3-D radiative magnetohydrodynamics computer codes. The anticipated statistically significant sample will reveal the abundance, the spatial distribution, and the typical properties of magn etic tornadoes and related photospheric vortex flows, which is essential for an evaluation of the overall heating contribution across the Sun. The analysis and 3-D visualisation of the simulations allows for identifying the relevant physical processes beh ind the formation of magnetic tornadoes, the related energy transport and the dissipation in the upper layers. Similar 3-D simulations for red dwarf stars will be carried out, which constitute about 75 % of all stars in our galaxy. The gained knowledge wi ll be assembled into a comprehensive picture of the tornado phenomenon and its importance for the structure, dynamics, and heating of the outer layers of our Sun and stars in general.