Fine structure in the photosphere and chromosphere

The physical parameters of a forming penumbra as observed with GFPI@VTT: Continuum intensity (normalized to quiet Sun), magnetic field strength (kG), LOS magnetic flux (1017 Mx), and LOS inclination (deg). From top to bottom, the maps are taken at 08:40, 08:50, 09:28, 10:13, 11:51, and 12:38 UT, respectively. Tick marks: arcsec.

This focus aims at an understanding of structure formation by cosmic magnetic fields at the tiniest cosmological scales. Radiatively driven magneto-convective processes under various conditions lead to a wealth of magnetic, convective and radiative phenomena on the Sun. In large-scale sunspots the strong magnetic fields suppress the regular convection, and convection occurs in different magneto-radiative modes to produce small-scale penumbral filaments, light bridges and umbral dots.

We study the evolution of magnetic flux on the solar surface, investigate its appearance and its decay: Emerging magnetic bipoles merge to form pores, merging pores and magnetic elements lead to the formation of penumbrae and sunspots. Eventually sunspots and active regions decay as the flux is spread out by diffusion and moving magnetic features. To understand the re-cycling of magnetic flux and local dynamo effects, the magnetic field of the so-called quiet Sun also deserves attention.

The structure of magnetic phenomena like sunspots which are stable relative to the dynamic time scale in the solar photosphere are in the focus of our research. It is of fundamental interest to learn about the various modes of magneto-convection taking place under various magnetic field configurations. We elaborate on the variation of the magnetic field topology from the photosphere into the chromosphere and further out as it is of fundamental importance to understand the interaction of the generation of magnetic fields in the interior of the Sun and space weather.

Our investigations are based on theoretical modelling as well as on observations. The measurements are performed with versatile methods: On the one hand, we study the magnetic imprints in spectral absorption lines which form at different layers in the solar photosphere atmosphere and on the other hand, we study the propagation of waves in the solar photosphere and chromosphere, to infer information about the magnetic fields.

The magnetic processes and structures need to be measured at highest possible spatial resolution with high spectral and polarimetric precision and at sufficient temporal cadence and coverage. Necessary compromises to advance our understanding are obtained by using various telescopes with different instrumentation (e.g., GREGOR, VTT, IBIS@DST, Hinode, SDO, IRIS, and SUNRISE). Such observations are compared with numerical simulations of radiative magneto-hydrodynamics in order to understand their physical nature.