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Karak B.B.,Indian Institute of Science | Karak B.B.,KTH Royal Institute of Technology | Kitchatinov L.L.,Institute for Solar Terrestrial Physics | Choudhuri A.R.,Indian Institute of Science
Astrophysical Journal | Year: 2014

We attempt to provide a quantitative theoretical explanation for the observations that Ca II H/K emission and X-ray emission from solar-like stars increase with decreasing Rossby number (i.e., with faster rotation). Assuming that these emissions are caused by magnetic cycles similar to the sunspot cycle, we construct flux transport dynamo models of 1 M ⊙ stars rotating with different rotation periods. We first compute the differential rotation and the meridional circulation inside these stars from a mean-field hydrodynamics model. Then these are substituted in our dynamo code to produce periodic solutions. We find that the dimensionless amplitude f m of the toroidal flux through the star increases with decreasing rotation period. The observational data can be matched if we assume the emissions to go as the power 3-4 of f m. Assuming that the Babcock-Leighton mechanism saturates with increasing rotation, we can provide an explanation for the observed saturation of emission at low Rossby numbers. The main failure of our model is that it predicts an increase of the magnetic cycle period with increasing rotation rate, which is the opposite of what is found observationally. Much of our calculations are based on the assumption that the magnetic buoyancy makes the magnetic flux tubes rise radially from the bottom of the convection zone. Taking into account the fact that the Coriolis force diverts the magnetic flux tubes to rise parallel to the rotation axis in rapidly rotating stars, the results do not change qualitatively. © 2014. The American Astronomical Society. All rights reserved..


Rudiger G.,Leibniz Institute for Astrophysics Potsdam | Kitchatinov L.L.,Institute for Solar Terrestrial Physics | Elstner D.,Leibniz Institute for Astrophysics Potsdam
Monthly Notices of the Royal Astronomical Society | Year: 2012

The helicity and α effect driven by the non-axisymmetric Tayler instability of toroidal magnetic fields in stellar radiation zones are computed. In the linear approximation, a purely toroidal field always excites pairs of modes with identical growth rates but with opposite helicity so that the net helicity vanishes. If the magnetic background field has a helical structure by an extra (weak) poloidal component then one of the modes dominates, producing a net kinetic helicity anticorrelated with the current helicity of the background field. The mean electromotive force is computed with the result that the α effect by the most rapidly growing mode has the same sign as the current helicity of the background field. The α effect is found to be too small to drive an α 2 dynamo, but the excitation conditions for an αΩ dynamo can be fulfilled for weak poloidal fields. Moreover, if the dynamo produces its own α effect by the magnetic instability, then problems with its sign do not arise. For all cases, however, the α effect shows an extremely strong concentration to the poles so that a possible αΩ dynamo might only work at the polar regions. Hence, the results of our linear theory lead to a new topological problem for the existence of large-scale dynamos in stellar radiation zones on the basis of the current-driven instability of toroidal fields. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Kitchatinov L.L.,Institute for Solar Terrestrial Physics
Astrophysical Journal | Year: 2014

Surfaces of constant pressure and constant density do not coincide in differentially rotating stars. Stellar radiation zones with baroclinic stratification can be unstable. Instabilities in radiation zones are of crucial importance for angular momentum transport, mixing of chemical species, and, possibly, for magnetic field generation. This paper performs linear analysis of baroclinic instability in differentially rotating stars. Linear stability equations are formulated for differential rotation of arbitrary shape and then solved numerically for rotation nonuniform in radius. As the differential rotation increases, r- and g-modes of initially stable global oscillations transform smoothly into growing modes of baroclinic instability. The instability can therefore be interpreted as stability loss to r- and g-modes excitation. Regions of stellar parameters where r- or g-modes are preferentially excited are defined. Baroclinic instability onsets at a very small differential rotation of below 1%. The characteristic time of instability growth is about 1000 rotation periods. Growing disturbances possess kinetic helicity. Magnetic field generation by the turbulence resulting from baroclinic instability in differentially rotating radiation zones is therefore possible. © 2014. The American Astronomical Society. All rights reserved.


Kitchatinov L.L.,Institute for Solar Terrestrial Physics
Astronomy Letters | Year: 2016

Helioseismology revealed an increase in the rotation rate with depth just beneath the solar surface. The relative magnitude of the radial shear is almost constant with latitude. This rotational state can be interpreted as a consequence of two conditions characteristic of the near-surface convection: the smallness of convective turnover time in comparison with the rotation period and absence of a horizontal preferred direction of convection anisotropy. The latter condition is violated in the presence of a magnetic field. This raises the question of whether the subphotospheric fields can be probed with measurements of near-surface rotational shear. The shear is shown to be weakly sensitive to magnetic fields but can serve as a probe for sufficiently strong fields of the order of one kilogauss. It is suggested that the radial differential rotation in extended convective envelopes of red giants is of the same origin as the near-surface rotational shear of the Sun. © 2016, Pleiades Publishing, Inc.


Olemskoy S.V.,Institute for Solar Terrestrial Physics | Kitchatinov L.L.,Institute for Solar Terrestrial Physics
Astrophysical Journal | Year: 2013

A solar-type dynamo model in a spherical shell is developed with allowance for random dependence of the poloidal field generation mechanism on time and latitude. The model shows repeatable epochs of a strongly decreased amplitude of magnetic cycles similar to the Maunder minimum of solar activity. Random dependence of dynamo parameters on latitude breaks the equatorial symmetry of generated fields. The model shows the correlation of the occurrence of grand minima with deviations in the dynamo field from dipolar parity. An increased north-south asymmetry of magnetic activity can, therefore, be an indicator of transitions to grand minima. Qualitative interpretation of this correlation is suggested. Statistics of grand minima in the model are close to the Poisson random process, indicating that the onset of a grand minimum is statistically independent of preceding minima. © 2013. The American Astronomical Society. All rights reserved.


Pipin V.V.,Institute for Solar Terrestrial Physics
Geophysical and Astrophysical Fluid Dynamics | Year: 2013

We study the effect of turbulent drift of a large-scale magnetic field that results from the interaction of helical convective motions and differential rotation in the solar convection zone. The principal direction of the drift corresponds to the direction of the large-scale vorticity vector. Thus, the effect produces a latitudinal transport of the large-scale magnetic field in the convective zone wherever the angular velocity has a strong radial gradient. The direction of the drift depends on the sign of helicity and it is defined by the Parker-Yoshimura rule. The analytic calculations are done within the framework of mean-field magnetohydrodynamics using the minimal τ-approximation. We estimate the magnitude of the drift velocity and find that it can be a few m/s near the base of the solar convection zone. The implications of this effect for the solar dynamo are illustrated on the basis of an axisymmetric mean-field dynamo model with a subsurface shear layer. The model shows that near the bottom of the convection zone the helicity-vorticity pumping results mostly from the kinetic helicity contributions. We find that the magnetic helicity contributions to the pumping effect are dominant at the subsurface shear layer. There the magnitude of the drift velocity is found to be a few cm/s. We find that the helicity-vorticity pumping effect can have an influence on the features of the sunspot time-latitude diagram, producing a fast drift of the sunspot activity maximum at the rise phase of the cycle and a slow drift at the decay phase of the cycle. © 2013 Copyright Taylor and Francis Group, LLC.


Kitchatinov L.L.,Institute for Solar Terrestrial Physics | Olemskoy S.V.,Institute for Solar Terrestrial Physics
Monthly Notices of the Royal Astronomical Society | Year: 2016

Recent data on superflares on Sun-like stars and radiocarbon data on solar activity in the past are both indicative of transient epochs of unusually high magnetic activity. We propose an explanation for the grand activity maxima in the framework of a solar dynamo model with fluctuating parameters. Solar-type dynamos are oscillatory because of the combination of the solar-type differential rotation with positive (in the Northern hemisphere) alpha-effect. An artificial reversal of the sign in the alpha-effect changes the dynamo to a steady regime with hundreds of times larger magnetic energy compared to the amplitude of the cyclic dynamo. Sufficiently large and durable fluctuations reversing the sign of the alpha-effect during the growth phase of a magnetic cycle can, therefore, cause a transient change to a steady dynamo with considerably increased magnetic energy. This qualitative scenario for grand activity maxima is supported by computations of the dynamo model with a fluctuating alpha-effect. The computed statistics of several thousand magnetic cycles gives examples of cycles with very high magnetic energy. Our preliminary estimations, however, suggest that the probability of solar superflares is extremely low. © 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Kitchatinov L.L.,Institute for Solar Terrestrial Physics | Olemskoy S.V.,Institute for Solar Terrestrial Physics
Monthly Notices of the Royal Astronomical Society | Year: 2012

Gyrochronology and recent theoretical findings are used to reduce the number of input parameters of differential rotation models. This eventually leads, after having fixed our turbulence model parameters, to a theoretical prediction for the surface differential rotation as a function of only two stellar parameters - surface temperature and rotation period - that can be defined observationally. An analytical approximation for this function is suggested. The tendency for the differential rotation to increase with temperature is confirmed. The increase is much steeper for late F stars compared to G and K dwarfs. Slow and fast rotation regimes for internal stellar rotation are identified. A star attains its maximum differential rotation at rotation rates intermediate between these two regimes. The amplitude of the meridional flow increases with surface temperature and rotation rate. The structure of the flow changes considerably between cases of slow and fast rotation. The flow in rapid rotators is concentrated in the boundary layers near the top and bottom of the convection zone, with very weak circulation in between. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Kitchatinov L.L.,Institute for Solar Terrestrial Physics | Olemskoy S.V.,Institute for Solar Terrestrial Physics
Solar Physics | Year: 2012

A combination of diamagnetic pumping and a nonlocal α-effect of the Babcock-Leighton type in a solar dynamo model is shown to reproduce observations of solar magnetic activity. The period of the solar cycle can be reproduced without reducing magnetic diffusivity in the bulk of the convection zone below the standard mixing-length value of 10 13 cm 2 s -1. The simulated global fields are antisymmetric about the equator, and the toroidal-to-poloidal field ratio is about one thousand. However, the time-latitude diagrams of magnetic fields in the model without meridional flow differ from observations. Only when the meridional flow is included and the α-effect profile peaking at mid-latitudes is applied, can the observed butterfly diagrams be reproduced. © 2011 Springer Science+Business Media B.V.


Kitchatinov L.L.,Institute for Solar Terrestrial Physics | Olemskoy S.V.,Institute for Solar Terrestrial Physics
Monthly Notices of the Royal Astronomical Society | Year: 2011

A new version of a numerical model of stellar differential rotation based on mean-field hydrodynamics is presented and tested by computing the differential rotation of the Sun. The model is then applied to four individual stars including two moderate and two fast rotators to reproduce their observed differential rotation quite closely. A series of models for rapidly rotating (Prot= 1d) stars of different masses and compositions are generated. The effective temperature is found convenient to parametrize the differential rotation: variations with metallicity, which are quite pronounced when the differential rotation is considered as a function of the stellar mass, almost disappear in the dependence of differential rotation on temperature. The differential rotation increases steadily with surface temperature to exceed the largest differential rotation observed to date for the hottest F-stars we considered. This strong differential rotation is, however, found not to be efficient for dynamos when the efficiency is estimated with the standard CΩ parameter of dynamo models. On the contrary, the small differential rotation of M-stars is the most dynamo-efficient. The meridional flow near the bottom of the convection zone is not small compared to the flow at the top in all our computations. The flow is distributed over the entire convection zone in slow rotators but retreats to the convection zone boundaries with an increasing rotation rate, to consist of two near-boundary jets in rapid rotators. The implications of the change of the flow structure for stellar dynamos are briefly discussed. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.

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