# Prof. Dr. Thorsten Hohage appointed Max Planck Fellow

## 05.04.2017

**Mathematician and Max Planck Institute intensify their cooperation in the field of solar physics**

(mps) Applied Mathematician Prof. Dr. Thorsten Hohage from the University of Göttingen has been appointed as a Max Planck Fellow at the Max Planck Institute for Solar System Research (MPS) in Göttingen. For the next five years, Hohage will collaborate with the institute and other partners on the Göttingen Campus. One of the aims of the collaboration is to refine numerical methods to reconstruct interior properties of the Sun from observations of the Sun’s surface.

We cannot see directly into the interior of the Sun, and so observations at the surface must be used to infer what lies inside. Deriving information in this way is what mathematicians refer to as an inverse problem. In general, solving an inverse problem is a mathematical procedure to determine the cause of an observed effect, but they can be difficult to solve.

Methods to solve the inverse problem can be helpful for a wide variety of applications. Many medical imaging techniques, for example, such as magnetic resonance tomography, aim to reconstruct images of the interior of the human body. To image quickly developing processes, fast reconstructions are important. In cooperation with Prof. Dr. Jens Frahm from the Max Planck Institute for Biophysical Chemistry and Prof. Dr. Martin Uecker from the University Hospital Göttingen, Hohage has been able to develop the mathematical basis for real-time magnetic resonance imaging. In addition, Hohage cooperates with Nobel Laureate Prof. Dr. Stefan Hell from the MPIbpC in 4Pi microscopy.

As a Max Planck Fellow, Hohage now turns his focus to his biggest research object thus far: the Sun. Hohage has already established a working partnership with Prof. Dr. Laurent Gizon in the framework of the Collaborative Research Centre 963 "Astrophysical Flow Instabilities and Turbulence” to develop groundbreaking inversion techniques for helioseismology.

One of the major goals in solar physics, and a focus of research at the MPS is to understand the cyclic behaviour of the Sun’s magnetic field which increases and decreases in strength with a period of roughly 11 years. The flows inside the Sun are thought to be responsible for generating the magnetic field, and helioseismology is the only tool available to see these flows. This is only possible by solving the inverse problem. "The inverse problem for helioseismology is especially difficult to manage numerically”, explains Gizon. Small inaccuracies in the observations can have a dramatic impact on the result of the calculation. Gizon adds that “Prof. Hohage has already significantly improved and extended the analysis of the small inaccuracies in the observations required for an efficient inversion method”.

There is still the additional problem of there being many possible solutions for the interior flows. "This difficulty can be overcome by taking into account physical knowledge of properties of the solution and incorporating this knowledge into the calculations," adds Hohage. For example, during the Fellowship Hohage and Gizon will explore the effect of applying the assumption that the total mass of the system does not change.

Hohage’s interest in inverse problems was sparked early in his scientific career. After studying physics and mathematics at the universities of Marburg and Göttingen, Hohage chose a topic from this field for his diploma thesis. Following his doctorate at the Johannes Kepler University Linz, he worked for two years at the Konrad-Zuse-Zentrum in Berlin. In 2002 Hohage was appointed junior professor at the University of Göttingen; since 2007 he holds a full professorship there and heads the working group "Inverse Problems" at the Institute for Numerical and Applied Mathematics. With his appointment as a Max Planck Fellow at the MPS, the broader Göttingen science community will continue to benefit from his expertise.

The Max Planck Fellow program of the Max Planck Society promotes the close collaboration of outstanding university professors with scientists from a Max Planck Institute. The Max Planck Fellow is given the opportunity to lead a working group at the respective Max Planck Institute, for up to five years. Currently, 51 Max Planck Fellows are active within the Max Planck Society.

**Contact:**

Prof. Dr. Thorsten Hohage

University of Göttingen

Faculty of Mathematics and Computer Sciences

Institute for Numerical and Applied Mathematics

Lotzestraße 16-18, D-37083 Göttingen, Germany

Phone: +49 551 39 4509

Email: hohage@math.uni-goettingen.de

Web: ip.math.uni-goettingen.de/index.php?section=members&subsection=thohage&lang=en