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To understand solar and stellar dynamos combining local and global numerical modelling with long-term observations is a challenging task: even with state of the art computational methods and resources, the stellar parameter regime remains unattainable. Our goal is to relax some approximations, in order to simulate more realistic systems, and try to connect the results with theoretical predictions and state-of-the-art observations. We present here the first test-field measurements from our higher-resolution runs with improved heat conduction description. They indicate significant changes in the profiles of the most crucial inductive effect related to solar and stellar dynamo mechanisms. Higher resolution runs, currently undertaken, will bring us into an even more turbulent regime, in which we will be able to study, for the first time, the interaction of small- and large-scale dynamos in a quantitative way., Originally published in "High Performance Computing Science and Engineering, Garching/Munich, 2020". Editors: P. Bastina, D. Kranzm\"uller, H. Br\"euchle, M. Brehm. ISBN: 978-3-9816675-4-7

openaire: EC/H2020/730897/EU//HPC-EUROPA3 | openaire: EC/H2020/818665/EU//UniSDyn Global and semi-global convective dynamo simulations of solar-like stars are known to show a transition from an antisolar (fast poles, slow equator) to solar-like (fast equator, slow poles) differential rotation (DR) for increasing rotation rate. The dynamo solutions in the latter regime can exhibit regular cyclic modes, whereas in the former one, only stationary or temporally irregular solutions have been obtained so far. In this paper we present a semi-global dynamo simulation in the transition region, exhibiting two coexisting dynamo modes, a cyclic and a stationary one, both being dynamically significant. We seek to understand how such a dynamo is driven by analyzing the large-scale flow properties (DR and meridional circulation) together with the turbulent transport coefficients obtained with the test-field method. Neither an ??dynamo wave nor an advection-dominated dynamo are able to explain the cycle period and the propagation direction of the mean magnetic field. Furthermore, we find that the ? effectis comparable or even larger than the ? effect in generating the toroidal magnetic field, and therefore, the dynamo seems to be of ?(2)? or ?(2) type. We further find that the effective large-scale flows are significantly altered by turbulent pumping.

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., 10 pages, 11 figures, submitted to A&A

Several solar-like stars exhibit cyclic magnetic activity similar to the Sun as found in photospheric and chromospheric emission. We want to understand the physical mechanism involved in rotational dependence of these activity cycle periods. We use three-dimensional magnetohydrodynamical simulations of global convective dynamos models of solar-like stars to investigate the rotational dependency of dynamos. We further apply the test-field method to determine the $��$ 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 $��$ 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) and Olspert 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., 11 pages, 10 figures, accepted for publication in A&A

We report on turbulent dynamo simulations in a spherical wedge with an outer coronal layer. We apply a two-layer model where the lower layer represents the convection zone and the upper layer the solar corona. This setup is used to study the coronal influence on the dynamo action beneath the surface. Increasing the radial coronal extent gradually to three times the solar radius and changing the magnetic Reynolds number, we find that dynamo action benefits from the additional coronal extent in terms of higher magnetic energy in the saturated stage. The flux of magnetic helicity can play an important role in this context., 4 pages, 1 figure, To appear in proceedings of IAUS 302: Magnetic fields throughout stellar evolution (August 2013, Biarritz, France)

We investigate the emergence of a large-scale magnetic field. This field is dynamo-generated by turbulence driven with a helical forcing function. Twisted arcade-like field structures are found to emerge in the exterior above the turbulence zone. Time series of the magnetic field structure show recurrent plasmoid ejections., 2 pages, 2 figures, conference proceeding

Magnetic buoyancy is believed to drive the transport of magnetic flux tubes from the convection zone to the surface of the Sun. The magnetic fields form twisted loop-like structures in the solar atmosphere. In this paper we use helical forcing to produce a large-scale dynamo-generated magnetic field, which rises even without magnetic buoyancy. A two layer system is used as computational domain where the upper part represents the solar atmosphere. Here, the evolution of the magnetic field is solved with the stress--and--relax method. Below this region a magnetic field is produced by a helical forcing function in the momentum equation, which leads to dynamo action. We find twisted magnetic fields emerging frequently to the outer layer, forming arch-like structures. In addition, recurrent plasmoid ejections can be found by looking at space--time diagrams of the magnetic field. Recent simulations in spherical coordinates show similar results., 4 pages, 8 figures, To appear in the proceedings of the IAU273 "Physics of Sun and Star Spots"

We present results from simulations of rotating magnetized turbulent convection in spherical wedge geometry representing parts of the latitudinal and longitudinal extents of a star. Here we consider a set of runs for which the density stratification is varied, keeping the Reynolds and Coriolis numbers at similar values. In the case of weak stratification we find quasi-steady solutions for moderate rotation and oscillatory dynamos with poleward migration of activity belts for more rapid rotation. For stronger stratification a similar transition as a function of the Coriolis number is found, but with an equatorward migrating branch near the equator. We test the domain size dependence of our results for a rapidly rotating run with equatorward migration by varying the longitudinal extent of our wedge. The energy of the axisymmetric mean magnetic field decreases as the domain size increases and we find that an m=1 mode is excited for a full 2π φ-extent, reminiscent of the field configurations deduced from observations of rapidly rotating late-type stars.