arXiv subjects

J. Warnecke

Publications and source records attributed to J. Warnecke.

12 recordsLinked to original sources

arXiv2021

Stellar X-rays and magnetic activity in 3D MHD coronal models

Observations suggest a power-law relation between the coronal emission in X-rays, $L_{\rm{X}}$, and the total (unsigned) magnetic flux at the stellar surface, $Φ$. The physics basis for this relation is poorly understood. We use three-dimensional (3D) magnetohydrodynamics (MHD) numerical models of the coronae above active regions, that is, strong concentrations of magnetic field, to investigate the $L_{\rm{X}}$ versus $Φ$ relation and illustrate this relation with an analytical model based on simple well-established scaling relations. In the 3D MHD model horizontal (convective) motions near the surface induce currents in the coronal magnetic field that are dissipated and heat the plasma. This self-consistently creates a corona with a temperature of 1 MK. We run a series of models that differ in terms of the (unsigned) magnetic flux at the surface by changing the (peak) magnetic field strength while keeping all other parameters fixed. In the 3D MHD models we find that the energy input into the corona, characterized by either the Poynting flux or the total volumetric heating, scales roughly quadratically with the unsigned surface flux $Φ$. This is expected from heating through field-line braiding. Our central result is the nonlinear scaling of the X-ray emission as $L_{\rm{X}}\propto Φ^{3.44}$. This scaling is slightly steeper than found in recent observations that give power-law indices of up to only 2 or 3. Assuming that on a real star, not only the peak magnetic field strength in the active regions changes but also their number (or surface filling factor), our results are consistent with observations. Our model provides indications of what causes the steep increase in X-ray luminosity by four orders of magnitude from solar-type activity to fast rotating active stars.

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arXiv2021

Hunting down the cause of solar magnetism

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.

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arXiv2020

The Pencil Code, a modular MPI code for partial differential equations and particles: multipurpose and multiuser-maintained

The Pencil Code is a highly modular physics-oriented simulation code that can be adapted to a wide range of applications. It is primarily designed to solve partial differential equations (PDEs) of compressible hydrodynamics and has lots of add-ons ranging from astrophysical magnetohydrodynamics (MHD) to meteorological cloud microphysics and engineering applications in combustion. Nevertheless, the framework is general and can also be applied to situations not related to hydrodynamics or even PDEs, for example when just the message passing interface or input/output strategies of the code are to be used. The code can also evolve Lagrangian (inertial and noninertial) particles, their coagulation and condensation, as well as their interaction with the fluid.

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arXiv2020

Rotational dependence of turbulent transport coefficients in global convective dynamo simulations of solar-like stars

For moderate and slow rotation, magnetic activity of solar-like stars is observed to strongly depend on rotation. These observations do not yet have a solid explanation in terms of dynamo theory. We aim to find such an explanation by numerically investigated the rotational dependency of dynamo drivers in solar-like stars. 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. The trace of the $α$ tensor increases for moderate rotation rates with Co$^{0.5}$ and levels off for rapid rotation. This behavior agrees with the kinetic $α$, if one considers the decrease of the convective scale with increasing rotation. The $α$ tensor becomes highly anisotropic for Co$\gtrsim 1$. Furthermore, $α_{rr}$ dominates for moderate rotation (1<Co<10), and $α_{ϕϕ}$ for rapid rotation (Co $\gtrsim 10$). 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 the presence of anti-solar differential rotation. For moderate rotation, $α$ and $Ω$ effects are dominant, indicative of $αΩ$ or $α^2Ω$ dynamos in operation, producing equatorward-migrating dynamo waves with a qualitatively solar-like rotation profile. For rapid rotation, an $α^2$ 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.

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arXiv2020

Stellar coronal X-ray emission and surface magnetic flux

Observations show that the coronal X-ray emission of the Sun and other stars depends on the surface magnetic field. Using power-law scaling relations between different physical parameters, we build an analytical model to connect the observed X-ray emission to the magnetic flux. The basis for our model are the scaling laws of Rosner, Tucker \& Vaiana (RTV) that connect the temperature and pressure of a coronal loop to its length and energy input. To estimate the energy flux into the upper atmosphere, we use scalings derived for different heating mechanisms, e.g. for field-line braiding or Alfven-wave heating. We supplement this by observed relations between active region size and magnetic flux and derive scalings of how X-ray emissivity depends on temperature. Based on our analytical model, we find a power-law dependence of the X-ray emission on the magnetic flux, $L_{\rm X}\propto Φ^m$, with a power-law index $m$ being in the range from about 1 to 2. This finding is consistent with a wide range of observations, from individual features on the Sun, e.g. bright points or active regions, to stars of different types and varying levels of activity. The power-law index $m$ depends on the choice of the heating mechanism, and our results slightly favour the braiding and nanoflare scenarios over Alfvén wave heating. In addition, the choice of instrument will have an impact on the power-law index $m$, which is because of the sensitivity of the observed wavelength region to the temperature of the coronal plasma. Overall, our simple analytical model based on the RTV scaling laws gives a good representation of the observed X-ray emission. This underlines that we might be able to understand stellar coronal activity though a collection of basic building blocks, i.e. loops, that we can study in spatially resolved detail on the Sun.

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arXiv2020

Shapes of stellar activity cycles

Magnetic activity cycles are an important phenomenon in both the Sun and other stars. The shape of the solar cycle is commonly characterised by a fast rise and a slower decline, but not much attention has been paid to the shape of cycles in other stars. Our aim is to study whether the asymmetric shape of the solar cycle is common in other stars as well, and compare the cycle asymmetry to other stellar parameters. We also study the differences in the shape of the solar cycle, depending on what activity indicator is used. The observations are also compared to simulated activity cycles. We use the chromospheric Ca II H&K data from the Mount Wilson Observatory HK Project. From this data set we identify 47 individual cycles from 18 stars. We use the statistical skewness of a cycle as a measure of its asymmetry, and compare this to other stellar parameters. A similar analysis has been done to magnetic cycles extracted from direct numerical magnetohydrodynamic simulations of solar-type convection zones. The shape of the solar cycle (fast rise and slower decline) is common in other stars as well, although the Sun has particularly asymmetric cycles. Cycle-to-cycle variations are large, but the average shape of a cycle is still fairly well represented by a sinusoid. We find only slight correlations between the cycle asymmetry and other stellar parameters. There are large differences in the shape of the solar cycle, depending on what activity indicator is used. In the simulated cycles, there is a difference in the symmetry of global simulations covering the full longitudinal range, hence capable of exciting non-axisymmetric dynamo modes, versus wedge simulations covering a partial extent in longitude, where only axisymmetric modes are possible. The former produce preferentially positive skewness, while the latter a negative one.

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arXiv2019

Stellar dynamos in the transition regime: multiple dynamo modes and anti-solar differential rotation

Global and semi-global convective dynamo simulations of solar-like stars are known to show a transition from an anti-solar (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 $αΩ$ 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 $α$ effect is comparable or even larger than the $Ω$ effect in generating the toroidal magnetic field, and therefore, the dynamo seems to be $α^2Ω$ or $α^2$ type. We further find that the effective large-scale flows are significantly altered by turbulent pumping.

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arXiv2018

Magnetic bipoles in rotating turbulence with coronal envelope

The formation of sunspots and starspots is not yet fully understood and is therefore one of the major open problems in solar and stellar physics. Magnetic flux concentrations can be produced by the negative effective magnetic pressure instability (NEMPI). This instability is strongly suppressed by rotation. However, the presence of an outer coronal envelope was previously found to strengthen the flux concentrations and make them more prominent. It also allows for the formation of bipolar regions (BRs). We want to know whether the presence of an outer coronal envelope also changes the excitation conditions and the rotational dependence of NEMPI. We use direct numerical simulations and mean-field simulations. We adopt a simple two-layer model of turbulence that mimics the jump between the convective turbulent and coronal layers below and above the surface of a star, respectively. The computational domain is Cartesian and located at a certain latitude of a rotating sphere. We investigate the effects of rotation on NEMPI by changing the Coriolis number, the latitude, and the box resolution. Rotation has a strong impact on the process of BR formation. Even rather slow rotation is found to suppress their formation. However, increasing the imposed magnetic field strength also makes the structures stronger and alleviates the rotational suppression somewhat. The presence of a coronal layer itself does not significantly alleviate the effects of rotational suppression.

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arXiv2018

Chromospheric activity catalogue of 4454 cool stars. Questioning the active branch of stellar activity cycles

Chromospheric activity monitoring of a wide range of cool stars can provide valuable information on stellar magnetic activity and its dependence on fundamental stellar parameters such as effective temperature and rotation.We compile a chromospheric activity catalogue of 4454 cool stars from a combination of archival HARPS spectra and multiple other surveys, including the Mount Wilson data that have recently been released by the NSO. We explore the variation in chromospheric activity of cool stars along the main sequence for stars with different effective temperatures. Additionally, we also perform an activity-cycle period search and investigate its relation with rotation. The chromospheric activity index, S-index, was measured for 304 main-sequence stars from archived high-resolution HARPS spectra. Additionally, the measured and archived S-indices were converted into the chromospheric flux ratio log R'HK. The activity-cycle periods were determined using the generalised Lomb-Scargle periodogram to study the active and inactive branches on the rotation-activity-cycle period plane. The global sample shows that the bimodality of chromospheric activity, known as the Vaughan-Preston gap, is not prominent, with a significant percentage of the stars at an intermediate-activity level around log R'HK = -4.75. Independently, the cycle period search shows that stars can lie in the region intermediate between the active and inactive branch, which means that the active branch is not as clearly distinct as previously thought. The weakening of the Vaughan-Preston gap indicates that cool stars spin down from a higher activity level and settle at a lower activity level without a sudden break at intermediate activity. Some cycle periods are close to the solar value between the active and inactive branch, which suggests that the solar dynamo is most likely a common case of the stellar dynamo.

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arXiv2016

Convection-driven spherical shell dynamos at varying Prandtl numbers

(abidged) Context: Stellar convection zones are characterized by vigorous high-Reynolds number turbulence at low Prandtl numbers. Aims: We study the dynamo and differential rotation regimes at varying levels of viscous, thermal, and magnetic diffusion. Methods: We perform three-dimensional simulations of stratified fully compressible magnetohydrodynamic convection in rotating spherical wedges at various thermal and magnetic Prandtl numbers (from 0.25 to 2 and 5, respectively). Results: We find that the rotation profiles for high thermal diffusivity show a monotonically increasing angular velocity from the bottom of the convection zone to the top and from the poles toward the equator. For sufficiently rapid rotation, a region of negative radial shear develops at mid-latitudes as the thermal diffusivity is decreased. This coincides with a change in the dynamo mode from poleward propagating activity belts to equatorward propagating ones. Furthermore, the cyclic solutions disappear at the highest magnetic Reynolds numbers. The total magnetic energy increases with the magnetic Reynolds number in the range studied here ($5-151$), but the energies of the mean magnetic fields level off at high magnetic Reynolds numbers. The differential rotation is strongly affected by the magnetic fields and almost vanishes at the highest magnetic Reynolds numbers. In some of our most turbulent cases we find that two regimes are possible where either differential rotation is strong and mean magnetic fields relatively weak or vice versa. Conclusions: Our simulations indicate a strong non-linear feedback of magnetic fields on differential rotation, leading to qualitative changes in the behaviors of large-scale dynamos at high magnetic Reynolds numbers. Furthermore, we do not find indications of the simulations approaching an asymptotic regime where the results would be independent of diffusion coefficients.

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arXiv2015

Limitations of force-free magnetic field extrapolations: revisiting basic assumptions

Force-free extrapolations are widely used to study the magnetic field in the solar corona based on surface measurements. The extrapolations assume that the ratio of internal energy of the plasma to magnetic energy, the plasma-beta is negligible. Despite the widespread use of this assumption observations, models, and theoretical considerations show that beta is of the order of a few percent to more than 10%, and thus not small. We investigate what consequences this has for the reliability of extrapolation results. We use basic concepts starting with the force and the energy balance to infer relations between plasma-beta and free magnetic energy, to study the direction of currents in the corona with respect to the magnetic field, and to estimate the errors in the free magnetic energy by neglecting effects of the plasma (beta<<1). A comparison with a 3D MHD model supports our basic considerations. If plasma-beta is of the order of the relative free energy (the ratio of the free magnetic energy to the total magnetic energy) then the pressure gradient can balance the Lorentz force. This is the case in the solar corona, and therefore the currents are not properly described. In particular the error in terms of magnetic energy by neglecting the plasma is of the order of the free magnetic energy, so that the latter can not be reliably determined by an extrapolation. While a force-free extrapolation might capture the magnetic structure and connectivity of the coronal magnetic field, the derived currents and free magnetic energy are not reliable. Thus quantitative results of extrapolations on the location and amount of heating in the corona (through current dissipation) and on the energy storage of the magnetic field (e.g. for eruptive events) are limited.

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arXiv2010

Surface appearance of dynamo-generated large-scale fields

Twisted magnetic fields are frequently seen to emerge above the visible surface of the Sun. This emergence is usually associated with the rise of buoyant magnetic flux structures. Here we ask how magnetic fields from a turbulent large-scale dynamo appear above the surface if there is no magnetic buoyancy. The computational domain is split into two parts. In the lower part, which we refer to as the turbulence zone, the flow is driven by an assumed helical forcing function leading to dynamo action. Above this region, which we refer to as the exterior, a nearly force-free magnetic field is computed at each time step using the stress-and-relax method. Twisted arcade-like field structures are found to emerge in the exterior above the turbulence zone. Strong current sheets tend to form above the neutral line, where the vertical field component vanishes. Time series of the magnetic field structure show recurrent plasmoid ejections. The degree to which the exterior field is force free is estimated as the ratio of the dot product of current density and magnetic field strength to their respective rms values. This ratio reaches values of up to 95% in the exterior. A weak outward flow is driven by the residual Lorentz force.

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