This is my homepage. I am current a Postdoc as Research Fellow of Max Planck Princeton Center for Plasma Physics at the Max-Planck Institute for Solar System Research in Göttingen, Germany. I am there in SOLSTAR group and collaborate with the Solar and Stellar Coronae 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.



                                                                                                            October 2019

Two papers submitted:

Paper I:

On the influence of magnetic helicity

on X-rays emission of solar and stellar coronae

Jörn Warnecke & Hardi Peter: 2019, ([ArXiv],[PDF])

Observation of solar-like stars show a clear relation between X-ray emission and their rotation. Higher stellar rotation can lead to a larger magnetic helicity production in stars. We aim to understand the relation between magnetic helicity on the surface of a star to their coronal X-ray emission.  We use 3D MHD simulations to model the corona of the solar-like stars. We take an observed magnetogram as in photospheric activity input, and inject different values of magnetic helicity. We use synthesis emission to calculate the X-ray emission flux of each simulation and investigate how this scales with injected magnetic helicity. We find that for larger injected magnetic helicities an increase in temperature and an increase in X-ray emission. The X-ray emission scaled cubicly with the injected helicity. We can related this to increase of horizontal magnetic field and therefore higher Poynting flux at the coronal base. Using typical scaling of magnetic helicity production with stellar rotation, we can explain the increase of X-ray emission with rotation only by an increase of magnetic helicity at the surface of a star.

Magnetic field lines configuration and X-ray and EUV emission for Runs with magnetic helicity of 0, 3e5, -3e5 and 1e6 G2Mm. The first row shows the vertical magnetic field at the photosphere (white outward, black inwards, between -100 and 100 G) together with traced magnetic field lines. The light blue lines show the close connecting magnetic field between the two polarities and the dark blue shows the larger arching fields connecting the two. The region of the seeds for the field line tracing are the same for each runs.  The bottom row show the synthesized X-ray emission using the Hinode/XRT Al-poly temperature response function as top view (xy) and side view (xz), respectively. The plots have been calculated from a six-hour snapshot of each

simulation. The emissions are plotted in units of DN pixel1.

Paper II:

Rotational dependence of turbulent transport coefficients

in global convective dynamo simulations of solar-like stars

Jörn Warnecke &Maarit J. Käpylä: 2019, ([ArXiv],[PDF])

For moderate and slow rotation, magnetic activity of solar-like stars is observed to strongly depend on rotation, while for rapid rotation, only a very weak or no dependency is detected.  These observations do not yet have a solid explanation in terms of dynamo theory. To work towards such an explanation, we numerically investigated the rotational dependency of dynamo drivers in solar-like stars, that is, stars that have a convective envelope of similar thickness as in the Sun. We ran semi-global convection simulations of stars with rotation rates from 0 to 30   times the solar value, corresponding to Coriolis numbers, Co, of 0 to 110. We measured the turbulent transport coefficients describing the magnetic field evolution with the help of the test-field method, and compared with the dynamo effect arising from the differential rotation, self-consistently generated in the models. The trace of the α tensor increases for moderate rotation rates with Co0.5 and levels off for rapid rotation. This behavior is in agreement with the kinetic α based on the kinetic helicity, if one takes into account the decrease of the convective scale with increasing rotation. The α tensor becomes highly anisotropic for Co > 1, αrr dominates for moderate rotation (1<Co<10), and αϕϕ for rapid rotation (Co > 10). The effective meridional flow, taking into account the turbulent pumping effects, is markedly different from the actual meridional circulation profile. Hence, the turbulent pumping effect is dominating the  meridional transport of the magnetic field. Taking all dynamo effects into account, we find three distinct regimes. For slow rotation, the  α and Rädler effects are dominating in presence of anti-solar differential rotation. For moderate rotation, α and Ω effects are dominant, indicative of αΩ or α2Ω dynamos in operation, producing equatorward-migrating dynamo waves with the qualitatively solar-like rotation profile. For rapid rotation, an α2 mechanism, with an influence from the Rädler effect, appears to be the most probable driver of the dynamo. Our study reveals the presence of a large variety of dynamo effects beyond the classical αΩ mechanism, which need to be investigated further to fully understand the dynamos of solar-like stars. The highly anisotropic α tensor might be the primary reason for the change of axisymmetric to non-axisymmetric dynamo solutions in the moderate rotation regime.

Scaling of X-ray emission with magnetic helicity. We show the total X-ray emission over injected magnetic helicity HinM. The red line is a power-law fit over the last five data points with corresponding slope in red. The X-ray emission is time-averaged over the relaxed stage (4.6 to 7 hours) and normalized by total emission of Run R. The errors are estimate from the time variation in the relaxed stage.

Rotational dependency of the diagonal α components showing anisotropy. We show their rms values with αrr (black lines), αθθ (red) and αϕϕ (blue) as a function of Coriolis number Co. The values of the magnetic runs are shown with a dashed line with diamonds. All values are normalized by α0 =urms/3. The zero rotation run have been moved to Co=10-4 to be visible in the lower panel.

Rotational dependence of the main dynamo mechanism driving the radial field (top panel), the latitudinal field (middle) and the azimuthal field (bottom) with the α effect (red), the turbulent diffusion (purple), the turbulent pumping (blue), the meridional circulation (black dashed), the Rädler effect (green) and the Ω effect (black solid). The vertical dashed lines indicate the transition from the anti-solar to solar-like differential rotation.