Interaction of Magnetic Fields with a Vortex Tube at Solar Subgranular Scale

In a recent Letter to The Astrophysical Journal (Fischer et al., 2020) we report on cases of granular lanes showing magnetic activity.

The sequence of maps shows the continuum intensity close to the Fe I 617.3 nm line. Contours in red and blue mark positive and negative circular polarisation whereas green contours denote linear polarisation. The yellow arrow points to the location where the dark lane of the granular lane is occupied by a substantial patch of linear polarization. (Figure from Fischer et al., 2020)

The grayscale panel shows the synthesized continuum intensity. The contours in green outline the linear polarisation calculated from the synthesized Stokes profiles. The circular polarization is marked by red and blue contours. The 3D scene above displays the temperature and the flow field below the Rosseland optical depth τR = 1 (black curve). The magnetic field lines that cross the section plane are shown in pink color. The circular close-up view provides a clear look at the temperature, flow field, and magnetic field orientation at the location of the granular lane. (Figure made by G. Vigeesh with ParaView)

Granular lanes (GLs) can be readily seen in time series of continuum images as features presenting a bright rim followed by a dark lane traveling into their parent granule. They either disappear within the granule or retract back to the granule boundary from where they started from. GLs appear as archlike structures, evolve within minutes, and are omnipresent in the solar granulation. With the help of 3-D MHD simulations it was found several years ago that the observed intensity pattern is the visible signature of a vortex tube having its axis aligned parallel to the solar surface and forming beneath the visible solar surface (Steiner et al., 2010).

In a recent Letter to The Astrophysical Journal (Fischer et al., 2020), we now report on cases of granular lanes showing magnetic activity. With high-resolution observations with the Swedish 1-m Solar Telescope (Scharmer et al., 2003), we captured in few cases elongated linear polarization patches with the horizontal magnetic field aligned with the dark lane of the GL, flanked by circular polarization indicating vertical magnetic field (see the upper Figure showing observations of the close-up temporal evolution of a granule hosting a granular lane).

By comparison with CO5BOLD (Freytag et al., 2013) simulations, we show that GLs can transport magnetic field from intergranular lanes into granule interiors and back up into the solar atmosphere. The magnetic field shows several components (lane-aligned, turbulent, or twisted fields) dominating in different stages of the process and revealing a complex magnetic structure (see the lower Figure showing simulations of a granular lane). This process of “shallow recirculation” is thought to be an important component of the small-scale dynamo acting at the solar surface. We have therefore observed first direct indications of such a process taking place on the Sun.

 

C.E. Fischer, G. Vigeesh, P. Lindner, J.M. Borrero, F. Calvo, and O. Steiner

“Interaction of Magnetic Fields with Horizontal Vortex Tubes at Solar Subgranular Scales”, The Astrophysical Journal Letters, 2020, in press

 

Freytag, B., Steffen, M., Ludwig, H. G., et al. 2012, JCoPh, 231, 919

Scharmer, G. B., Bjelksjo, K., Korhonen, T. K., Lindberg, B., & Petterson, B. 2003, Proc. SPIE, 4853, 341

Steiner, O., Franz, M., Bello González, N., et al. 2010, ApJL, 723, L180