This is my homepage. I am current a postdoc at the Max-Planck Institute for Solar System Research in Göttingen, Germany. I am there in Solar and Stellar Coronae and the SOLSTAR group, working on the interplay of dynamos, flux concentrations with coronal structures and activity.

In Summer 2013 finished my PhD Studies at Nordita and the Astronomy Department of Stockholm University under the supervision of Axel Brandenburg.

I am a core-developer of the open source PENCIL CODE, which is hosted by GitHub.

On the following pages, you can find my Curriculum Vitae, an overview of my research topics and a detail a list of publications and talks.



                                                                                                            December 2017

New paper submitted:

Dynamo cycles in global convection simulations

of solar-like stars

Warnecke, Jörn: 2017, ([ArXiv], [PDF])

Several solar-like stars exhibit cyclic magnetic activity similar to the Sun. We want to understand the rotational dependency of these activity cycle periods. We use three dimensional magnetohydrodynamical simulations of global convective dynamo models of solar-like stars to investigate the rotational dependency of dynamos. We further apply the test-field method to determine the $\alpha$ effect in these simulations. We find dynamo with clear oscillating mean magnetic fields for moderately and rapidly rotating runs. For slower rotation, the field is constant or exhibit irregular cycles. In the moderately and rapidly rotating regime the cycle periods increase weakly with rotation. This behavior can be well explained with a Parker-Yoshimura dynamo wave traveling equatorward. Even though the α effect becomes stronger for increasing rotation, the shear decreases steeper, causing this weak dependence on rotation. Similar as other numerical studies, we find no indication of activity branches as suggested by Brandenburg et al. 1998. However, our simulation seems to agree more with the transitional branch suggested by Distefano et al. (2017). If the Sun exhibit a dynamo wave similar as we find in our simulations, it would operate deep inside the convection zone.

Ratio of rotation period and cycle period Pcycl/Prot over the Coriolis number Co. The black asterisks indicate Pcycl. The dashed red line indicates the fit through our simulations and the dashed black line the transition from

anti-solar to solar like differential rotation. We include the simulations of Viviani et al. (2017) (blue squares, with cross for wedge runs) and Strugarek et al. (2017) (green diamond), the observational studies of Lehtinen et al. (2016) (purple crosses) andBrandenburg et al. (2017) (light grey triangles for K dwarfs, dark grey circles for

F, G dwarfs, including the Sun: yellow cross).

Cycle periods as a function of Coriolis number Co showing Pcycl (from Br and Bφ) in black and Pcycl (from Brms). The green dashed line indicates a power law fit of the Runs M4 to M15.

  1. a)Comparison of the cycle periods Pcycl (from Br and Bφ; black)  and Pcycl (from Brms; orange) with predicted cycle periods using a Parker-Yoshimura dynamo wave PPY (blue) for Runs M4 to M15. The dashed blue line indicates a power law fit to PPY.

  1. b)Contributions to the Parker-Yoshimura dynamo wave containing the radial shear (black line; left y-axis) and αφφ (red; right y-axis) for Runs M4 to M15. The dashed lines indicate the corresponding power law fits.