Global magnetic activity

The complex joint magnetosphere of the binary system V4046 Sgr. The extrapolation of the magnetic field is based on Zeeman-Doppler imaging of both stars (Image credit: V. Holzwarth, KIS; S. G. Gregory, Univ. St Andrews)

This research focus deals with the physical origin, the evolution, and the energetics of solar and stellar magnetic fields, the flow field inside the Sun, and the interplay of the Sun and Earth. To place the Sun within its stellar family, magnetic fields of other stars and the influences on their planets are investigated within a wider context.

The investigation of the depth dependence of the meridional circulation is a central topic for understanding the solar dynamo. We developed advanced global and local helioseismic techniques that allow studying this flow in great depths in the solar interior.

Studying other stars expands our vision of the stellar interior and activity. Using Kepler space mission data and asteroseismic methods we determined important stellar parameters, such as mass and radius, which by other astronomical observations are not easily accessible. Furthermore, solar-like oscillations were detected on  Scuti stars that now can also be employed to determine the structure of these stars by asteroseismic analyses.

The magnetism of Sun-like stars evolved off the main sequence is important for understanding the evolution of global stellar magnetism and dynamos. The activity of such stars is greatly diminished and hard to detect. We developed a novel method for inferring very weak stellar magnetic fields. With an unprecedented sensitivity, the so far weakest stellar magnetic field was detected on the K-giant Arcturus, which suggests that a stellar dynamo seems still being active in red giants.

The effect of magnetic activity can also be seen in additional mixing of the nuclear fusion products from the stellar core into the envelope. We found that some red giants in binary components show signatures of such extra mixing in their chemical composition.

To understand magnetic fields at the final stage of stellar evolution, we investigate the magnetism of white dwarfs. Our detection of the second magnetic white dwarf with polarized CH molecule bands suggests that such objects may not be as rare as previously thought.

Stars can also be used as giant gravitational wave detectors. For the first time a direct upper bound of the stochastic gravitational wave background was determined based on helioseismic data.

Stellar radiation, activity and wind strongly influence planetary atmospheres. We showed that polarimetry is a powerful tool to study exoplanetary atmospheres. In particular, we found that hot Jupiters can have atmospheres which are strongly influenced by Rayleigh scattering.