Late-type stars rotate differentially owing to anisotropic turbulence in their outer convection zones. The rotation is called solar-like (SL) when the equator rotates fastest and anti-solar (AS) otherwise. Hydrodynamic simulations show a transition from SL AS as influence of on reduced, but opposite occurs at different point parameter space. system bistable, i.e., profiles can both be stable. We study effect dynamo-generated magnetic field large-scale flows, particularly possibility bistable...

Solar magnetic activity shows both smooth secular changes, such as the Grand Modern Maximum, and quite abrupt drops that are denoted Minima. Direct numerical simulations (DNS) of convection driven dynamos offer one way examining mechanisms behind these events. In this work, we analyze a solution solar-like DNS has been evolved for roughly 80 cycles 4.9 years, during which epochs irregular behavior detected. The emphasis our analysis is to find physical causes behavior. employed semi-global...

We seek to understand the transition from nearly axisymmetric configurations at solar rotation rates nonaxisymmetric for rapid using 3D numerical simulations of turbulent convection and considering between 1 30 times value. find a axi- solutions around 1.8 rate. This coincides with change in profile antisolar- solar-like differential faster equator slow poles. In regime, field configuration consists an oscillatory accompanied by m=1 azimuthal mode (two active longitudes), which also shows...

We study the relation between strong and extreme geomagnetic storms solar cycle characteristics. The analysis uses an extensive index AA data set spanning over 150 yr complemented by Kakioka magnetometer recordings. apply Pearson correlation statistics estimate significance of with a bootstrapping technique. show that storm occurrence strength decreases from clear positive increasing magnitude toward negligible relationship. Hence, quieter Sun can also launch superstorms may lead to...

We present numerical simulations of hydrodynamic overshooting convection in local Cartesian domains. find that a substantial fraction the lower part zone (CZ) is stably stratified according to Schwarzschild criterion while enthalpy flux outward directed. This occurs when heat conduction profile at bottom CZ smoothly varying, based either on Kramers-like opacity prescription as function temperature and density or static similar shape. show subadiabatic layer arises due nonlocal energy...

(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 varying levels of viscous, thermal, magnetic diffusion. Methods: perform three-dimensional simulations stratified fully compressible magnetohydrodynamic in rotating spherical wedges various thermal numbers (from 0.25 to 2 5, respectively). Results: find that profiles for high diffusivity show a...

Debate over the existence of branches in stellar activity-rotation diagrams continues. Application modern time series analysis tools to study mean cycle periods chromospheric activity index is lacking. We develop such models, based on Gaussian processes, for one-dimensional and apply it extended Mount Wilson Ca H&K sample. Our main aim how previously commonly used assumption strict harmonicity cycles as well handling linear trends affects results. introduce three methods different...

We test the sensitivity of hydrodynamic and magnetohydrodynamic turbulent convection simulations with respect to Mach number, thermal magnetic boundary conditions, centrifugal force. find that varying luminosity, which also controls has only a minor effect on large-scale dynamics. A similar conclusion can be drawn from comparison two formulations lower condition either vanishing electric field or current density. The force an solutions, but if its magnitude acceleration due gravity is by...

Aims. According to previously published Doppler images of the magnetically active primary giant component RS CVn binary II Peg, surface star was dominated by one single longitude that clearly drifting in rotational frame system during 1994-2002; later imaging for 2004–2010, however, showed decreased and chaotic spot activity, with no signs drift pattern. Here we set out investigate from a more extensive photometric dataset whether this is persistent phenomenon, which case it could be caused...

The origin of the quiet Sun magnetism is under debate. Investigating solar cycle variation observationally in more detail can give us clues about how to resolve controversies. We investigate most magnetically regions and their surface gravity oscillation ($f$-) mode integrated energy ($E_f$). use 12 years HMI data apply a stringent selection criteria, based on spatial temporal quietness, avoid any influence active (ARs). develop an automated high-throughput pipeline go through all available...

We study LQ Hya photometry for 1982-2014 with the carrier fit (CF) -method and compare our results to earlier photometric analysis recent Doppler imaging maps. first utilize different types of statistical methods estimate various candidates period CF method. Secondly, a global whole data set local fits shorter segments are computed that is found be optimal one. The harmonic least-squares all available reveals short close 1.6 days as limiting value significant frequencies. interpret this...

The light curves of variable stars are commonly described using simple trigonometric models, that make use the assumption model parameters constant in time. This assumption, however, is often violated, and consequently, time series models with components vary slowly great interest. In this paper we introduce a class data analysis visualization methods which can be applied many different contexts star research, for example spotted stars, variables showing Blazhko effect, spin-down rapid...

Context. Period estimation is one of the central topics in astronomical time series analysis, which data often unevenly sampled. Studies stellar magnetic cycles are especially challenging, as periods expected those cases approximately same length datasets themselves. The contain trends, origin either a real long-term cycle or an instrumental effect. But these effects cannot be reliably separated, while they can lead to erroneous period determinations if not properly handled. Aims. In this...

Abstract Estimation of time delays from a noisy and gapped data is one the simplest analysis problems in astronomy by its formulation. But as history real experiments show, work with observed datasets can be quite complex evolved. By analyzing detail previous attempts to build delay estimation algorithms we try develop an automatic robust procedure perform task. To evaluate compare different variants use which have been objects past controversies. In this way hope select methods procedures...

This paper contributes to the field of modeling and hindcasting total solar irradiance (TSI) based on different proxy data that extend further back in time than TSI is measured from satellites. We introduce a simple method analyze persistent frequency-dependent correlations (FDCs) between series use these hindcast missing historical values. try avoid arbitrary choices free parameters model by computing them using an optimization procedure. The can be regarded as general tool for pairs sets,...

Observations from ground based telescopes are affected by the presence of Earth atmosphere, which severely perturbs them. The use adaptive optics techniques has allowed us to partly beat this limitation. However, image selection or post-facto reconstruction methods applied bursts short-exposure images routinely needed reach diffraction limit. Deep learning been recently proposed as an efficient way accelerate these reconstructions. Currently, deep neural networks trained with supervision, so...

Magnetic activity cycles are an important phenomenon in both the Sun and other stars. The shape of solar cycle is commonly characterised by a fast rise slower decline, but not much attention has been paid to Our aim study whether asymmetric common stars as well, compare asymmetry stellar parameters. We also differences cycle, depending on what indicator used. observations compared simulated cycles. use chromospheric Ca II H&K data from Mount Wilson Observatory HK Project. From this set we...

Estimation of time delays from a noisy and gapped data is one the simplest analysis problems in astronomy by its formulation. But as history real experiments show, work with observed sets can be quite complex evolved. By analysing detail previous attempts to build delay estimation algorithms we try develop an automatic robust procedure perform task. To evaluate compare different variants use which have been objects past controversies. In this way hope select methods procedures highest...

Many real world systems exhibit cyclic behavior that is, for example, due to the nearly harmonic oscillations being perturbed by strong fluctuations present in regime of significant non-linearities. For investigation such special techniques relaxing assumption periodicity are required. In this paper, we generalization one techniques, namely D <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> phase dispersion statistic, multidimensional...