Numerical simulations provide great insight into the various wave phenomena that occur in the solar atmosphere. However, a proper comparison with the observations of the real Sun require us to understand how these phenomena affect spectral lines. Synthetic observables derived from numerical simulation are becoming necessary as they help us to properly interpret observational data.

The stability of sunspots is one of the long-standing unsolved puzzles in the field of solar magnetism and the solar cycle. The thermal and magnetic structure of the sunspot beneath the solar surface is not accessible through observations, thus processes in these regions that contribute to the decay of sunspots can only be studied through theoretical and numerical studies.

For the past year, all departments of KIS worked together to carry out the biggest change of GREGOR since its inauguration. By replacing the relay optics and rearranging the AO and the majority of the instruments in the optics lab, we were able to significantly improve GREGOR's image quality and prepare the telescope for future instrumentation upgrades.

Magnetic flux cancelation refers to the interaction of opposite polarity magnetic elements leading to magnetic flux removal from the solar surface. If the interacting elements are previously unconnected, their cancelation is treated as an observational signature of magnetic reconnection.

Using high-resolution data obtained with the GREGOR telescope on Tenerife, we witnessed such a magnetic field reconnection event. The Integral Field Unit (IFU, built by Instituto de Astrofísica de Canarias, IAC) of the GREGOR Infrared Spectrograph (GRIS) recorded the Stokes profiles in the photospheric Si i 10827Å spectral line targeting the quiet-Sun at disk center.

On Monday, February 10, 2020 at 5:03 a.m. (CET), the ESA satellite Solar Orbiter was successfully launched with an Atlas 5 rocket from the Cape Canaveral Air Force Station in Florida. In less than 2 years the satellite will reach an orbit around the sun, where it can start scientific observations.

On Nov 11, 2019 KIS scientists together with C. Schmidt from Boston University observed and recorded the Mercury transit in front of the Sun with the GREGOR and Chrotel telescopes. Mercury transits are rare events and the next one will occur only in 2032. Such transits are great opportunities to study Mercury's exosphere. The first two-dimensional image of this exosphere was obtained by H. Schleicher at KIS at the occasion of the second to last transit in 2003, using the VTT telescope and the spectral line of sodium.

Inversion codes for the radiative transfer equation are tools that allow us to determine the temperature, magnetic field and plasma velocity in the solar atmosphere from spectro-polarimetric observations (i.e. Stokes vector) in spectral lines as a function of the optical depth.

Polarimetry is a sensitive technique for detecting magnetic fields, atmospheres, surfaces, atoms, molecules and dust grains, in astronomical objects such as the Sun, other distant stars, solar and exo-planets, asteroids, nebulae, and even black holes. The new, high-precision astronomical imaging polarimeter DIPOL-UF (Double Image Polarimeter – Ultra Fast) has been built in cooperation between KIS and a research group from the University of Turku (UTU), Finland; see the UTU press release here: www.utu.fi/en/news/news/new-astro-polarimeter-sees-first-light-at-the-nordic-optical-telescope.

GREGOR is the largest European Solar Telescope and recent enhancements in optics have allowed us to observe the Sun in extremely high resolution. The images show two snapshots, taken in August 2018 and the size of the Earth for comparison.

The controller of the Fabry-Perot Etalon for the Visible Tunable Filter instrument (VTF) has reached the demanding requirements for the controller precision and speed. The distance of the 20 kg Etalon plates can now be adjusted within a few milliseconds to the precision of a medium size atom. This is an important milestone towards the realization of the VTF. The multi-level controller was developed at KIS and has been verified on the basis of a full-size model of the Etalon.