Internal gravity waves (IGWs) are buoyancy-driven waves common in the Earth’s atmosphere and oceans. IGWs have also been observed in the Sun’s atmosphere and are thought to play an important role in the overall dynamics of the solar atmosphere.

Solar spectral lines have different shapes, strengths, asymmetries and wavelength shifts. These differences depend on the chemical element, and on the corresponding excitation state.

At the Vacuum-Tower-Telescope on Tenerife we conducted systematic observations of a number of spectral lines in the visible part of the solar spectrum, and measured the center-to-limb variation of the parameters mentioned above. To allow for a comparison of all the measurements, we used the Laser Absolute Reference Spektrograph.

The outer envelope of the Sun is highly magnetic and vigorously turbulent. We have employed concepts of phenomenological thermodynamics and information-theoretic entropy and discovered new, consistent patterns in evolution of solar magnetic fields.

The European Union's Programme on Research and Innovation, H2020, is funding the SOLARNET project for 4 years under the grant agreement number 824135. The project started on January 1st, 2019, and will end on December 31st, 2022.

The „Visible Tunable Filter“(VTF) is a spectro-polarimeter that is presently being built at KIS, and will be installed at the US-American Daniel K. Inouye Solar Telescope as a focal plane instrument in about two years from now. The VTF will be used to measure magnetic fields with high spatial resolution on the Sun. To this end, the VTF measures precisely the state and the degree of polarization of the incoming solar light.

The boundary between the umbra and the penumbra of sunspots has untill recently been defined by an intensity value only. Using Hinode satellite data, Jurcak et. al 2018 found an empirical law of the magnetic nature of the umbra-penumbra boundary in stable sunspots: an invariant vertical component of the magnetic field.

In the dark umbra of a sunspot, the convective motion is largely suppressed by the strong magnetic field. In the past, numerous studies have reported on negligible convective flows (e.g., Beckers 1977). Based on this, the umbra typically served as the zero-velocity reference for the calculation of Doppler velocities of an active region.

The presence of magnetic fields in the Sun lead to a plethora of magneto-dynamic phenomena that have triggered (and yet does) the interest of many solar physicist. The study of these physical phenomena relies on the accurate inference of the physical properties from the recorded spectra.

The Sun has a variable magnetic field. The most peculiar manifestation of this variability is the 11-year sunspot cycle. Besides the number of sunspots, the frequencies of the seismic waves that propagate through the solar interior are changing.

The magnetic topology in umbral dots has been studied by means of the inversion

of spectropolarimetric data recorded with the Hinode satellite and the GREGOR

solar telescope. The SP instrument attached to the 0.5-meter SOT telescope

on-board the Hinode satellite records spectropolarimetric data of two Fe I

lines at 630 nm, which are formed about 60-80 km higher than the three Fe I

lines at 1565 nm observed with the GRIS instrument attached to the 1.5-meter

ground solar telescope GREGOR.