CS Forum: Jörn Warnecke

Topic: "Understanding Solar and Stellar Activity with Numerical Simulations".

Place: Odeion, TUAS Building, Otaniementie 17, Espoo.

Abstract:
At the beginning of the cycle sunspots appear at high latitude, whereas at the end they appear close to the equator. This is associated with an underlying strong toroidal field, which migrates equatorward. For some years this behavior has been reproduced in global convective dynamo simulations. I will present results from our simulations of global convective dynamos. All of these simulations produce cyclic and migrating mean magnetic fields. Through detailed comparisons, we show that the migration direction can be clearly explained by an alpha-Omega dynamo wave following the Parker-Yoshimura rule. This leads to the conclusion, that the equatorward migration in this and other work is due to a positive (negative) alpha-effect in the northern (southern) hemisphere and a negative radial gradient of rotation outside the inner tangent cylinder of these models. Furthermore, I will present results for identifying various dynamo mechanisms in these simulations through obtaining transport coefficients: alpha effect, turbulent pumping and the turbulent magnetic diffusivity calculated with the test-field
method. These coefficients show a clear temporal variation with the activity cycle indicating a non-linear dynamo mechanism.

Bio:
Dr. Jörn Warnecke received his Ph.D. in Astronomy in 2013 at NORDITA, under the supervision of Prof. Axel Brandenburg working in Prof. Brandenburg's ERC Advanced Grant project 'ASTRODYN'. Jörn pursued his undergraduate studies in the Kiepenheuer-Institue for Solar Physics, Freiburg, under the supervision of Prof. Hardi Peter. Jörn's work concentrates on the numerical modeling of coronal mass ejections driven by dynamo action in the solar convection zone, involving complex magnetohydrodynamical simulations coupling the turbulent convection zone with a coronal layer. Such modeling constitutes a major high-performance computing challenge, but also produces massive amounts of data requiring intelligent techniques for its analysis. Jörn has also been working on the mechanisms to generate magnetic field concentrations in turbulent
flows, that relates to the formation of sun- and starspots and active  regions, a still poorly understood process in astrophysics. Recently, Jörn received the prestigious award of Marie Curie Fellowship to pursue his postdoctoral research at the Max Planck Institute for Solar System Research in Göttingen, in the Sun and Heliosphere Department.