arXiv subjects

L. L. Kitchatinov

Publications and source records attributed to L. L. Kitchatinov.

At least 19 recordsLinked to original sources

arXiv2025

Tayler instability as possible reason for the period changes in Ap star 56 Ari

The physical mechanism responsible for the photometric period changes in chemically peculiar star 56 Ari was searched. It was previously shown that rate of the star's period increase is few orders of magnitude larger than the rates expected from the evolutionary changes of the angular momentum or due to magnetic braking. Also no secular changes were detected in the surface structure or visibility of chemical spots which are responsible for the rotational modulation of stellar brightness. We hypothesise that period changes in 56 Ari are caused by the drift of surface magnetic and associated abundance structures as a result of the kink-type (Tayler) instability of the background magnetic field in the radiative zone of the star. Results of the numerical simulation presented in the paper yield growth and drift rates of the most rapidly developing non-axisymmetric mode of the instability, consistent with the observed rate of period changes in 56 Ari. The surface geometry of the 56 Ari magnetic field is also reproduces in the calculations. The proposed mechanism may also be used to explain the character of period changes in other Ap/Bp stars demonstrating such an effect.

astro-ph.SR↗

arXiv2020

Longitudinal drift of Tayler instability eigenmodes as a possible explanation for super-slowly rotating Ap stars

Rotation periods inferred from the magnetic variability of some Ap stars are incredibly long, exceeding ten years in some cases. An explanation for such slow rotation is lacking. This paper attempts to provide an explanation of the super-slow rotation of the magnetic and thermal patterns of Ap stars in terms of the longitudinal drift of the unstable disturbances of the kink-type (Tayler) instability of their internal magnetic field. The rates of drift and growth were computed for eigenmodes of Tayler instability using stellar parameters estimated from a structure model of an A star. The computations refer to the toroidal background magnetic field of varied strength. The non-axisymmetric unstable disturbances drift in a counter-rotational direction in the co-rotating reference frame. The drift rate increases with the strength of the background field. For a field strength exceeding the (equipartition) value of equal Alfven and rotational velocities, the drift rate approaches the proper rotation rate of a star. The eigenmodes in an inertial frame show very slow rotation in this case. Patterns of magnetic and thermal disturbances of the slowly rotating eigenmodes are also computed. The counter-rotational drift of Tayler instability eigenmodes is a possible explanation for the observed phenomenon of super-slowly rotating Ap stars.

astro-ph.SR↗

arXiv2019

Evolution of the Sun's Polar-fields and the Poleward Transport of Remnant Magnetic Flux

Synoptic magnetograms and relevant proxy data were analyzed to study the evolution of the Sun's polar magnetic fields. Time-latitude analysis of large-scale magnetic fields demonstrates cyclic changes in their zonal structure and the polar-field buildup. The time-latitude distributions of the emergent and remnant magnetic flux enable us to examine individual features of recent cycles. The poleward transport of predominantly following polarities contributed much of the polar flux and led to polar-field reversals. Multiple reversals of dominant polarities at the Sun's poles were identified in Cycles 20 and 21. Three-fold reversals were caused by remnant flux surges of following and leading polarities. Time-latitude analysis of solar magnetic fields in Cycles 20--24 revealed zones which are characterized by a predominance of negative (non-Joy's) tilts and appearance of active regions which violate Hale's polarity law. The decay of non-Joy's and anti-Hale's active regions result in remnant flux surges which disturb the usual order in magnetic flux transport and sometimes lead to multiple reversals of polar fields. The analysis of local and large-scale magnetic fields in their causal relation improved our understanding of the Sun's polar field weakening.

astro-ph.SR↗

arXiv2018

Superflares on Giant Stars

The Kepler mission identified huge flares on various stars including some of solar type. These events are substantially more energetic than solar flares, and so they are referred to as superflares. Even a small probability of such a superflare on the Sun would be a menace to modern society. A flare comparable in energy with that of superflares was observed on 24th and 25th September on the binary HK Lac. Unlike the Kepler stars, there are observations of differential rotation for HK Lac. This differential rotation appears to be anti-solar. For anti-solar differential rotation, dynamo models can give magnetic activity waves of dipole symmetry as well as quasi-stationary magnetic configurations with quadrupole symmetry. The magnetic energy of such stationary configurations is usually about two orders of magnitude higher than that associated with activity waves. We believe that this mechanism could provide sufficient energy to produce superflares on late type stars, and present some simple models in support of this idea.

astro-ph.SR↗

arXiv2018

Modelling variability of solar activity cycles

Context. Solar activity cycles vary in amplitude and duration. The variations can be at least partly explained by fluctuations in dynamo parameters. Aims. We want to restrict uncertainty in fluctuating dynamo parameters and find out which properties of the fluctuations control the amplitudes of the magnetic field and energy in variable dynamo cycles. Methods. A flux-transport model for the solar dynamo with fluctuations of the Babcock-Leighton type $α$-effect was applied to generate statistics of magnetic cycles for our purposes. The statistics were compared with data on solar cycle periods to restrict the correlation time of dynamo fluctuations. Results. A characteristic time of fluctuations in the $α$-effect is estimated to be close to the solar rotation period. The fluctuations produce asymmetry between the times of rise and descent of dynamo cycles, the rise time being on average shorter. The affect of the fluctuations on cycle amplitudes depends on the phase of the cycle in which the fluctuations occur. Negative fluctuations (decrease in $α$) in the rise phase delay decay of poloidal field and increase the cycle amplitude in toroidal field and magnetic energy. Negative fluctuation in the decline phase reduces the polar field at the end of a cycle and the amplitude of the next cycle. The low amplitude of the 24th solar cycle compared to the preceding 23rd cycle can be explained by this effect. Positive fluctuations in the descent phase enhance the magnetic energy of the next cycle by increasing the seed poloidal field for the next cycle. The statistics of the computed energies of the cycles suggest that superflares of $\ge 10^{34}$ erg are not possible on the Sun.

astro-ph.SR↗

arXiv2017

Double Hall instability: A catalyzer of magnetic energy release

A pictorial explanation for shear-Hall instability is suggested and shows that the shear flow is not necessary for the instability because its role can be played by the Hall effect of an inhomogeneous background magnetic field. Linear stability analysis for a simple model of magnetic field varying periodically in space confirms such a 'double Hall' instability. Numerical computations show a considerable increase in Ohmic dissipation rate at the nonlinear stage of instability development. Field dissipation has a spiky character associated with magnetic reconnection in current sheets and X-points. Double Hall instability can be significant for magnetic field dissipation in neutron star crusts and, possibly, in the solar corona.

physics.plasm-ph↗

arXiv2017

Can superflares occur on the Sun? A view from dynamo theory

Recent data from the Kepler mission has revealed the occurrence of superflares in sun-like stars which exceed by far any observed solar flares in release of energy. A natural idea is that the dynamo mechanism in superflaring stars differs in some respect from that in the Sun. We search for a difference in the dynamo-related parameters between superflaring stars and the Sun to suggest a dynamo-mechanism as close as possible to the conventional solar/stellar dynamo but capable of providing much higher magnetic energy. Dynamo based on joint action of differential rotation and mirror asymmetric motions can in principle result in excitation of two types of magnetic fields. First of all, it is well-known in solar physics dynamo waves. The point is that another magnetic configuration with initial growth and further stabilisation is also possible for excitation. For comparable conditions, magnetic field strength of second configuration is much larger rather of the first one just because dynamo do not spend its efforts for periodic magnetic field inversions but use its for magnetic field growth. We analysed available data from the Kepler mission concerning the superflaring stars in order to find tracers of anomalous magnetic activity. Starting from the recent paper, we find that anti-solar differential rotation or anti-solar sign of the mirror-asymmetry of stellar convection can provide the desired strong magnetic field in dynamo models. We confirm this concept by numerical models of stellar dynamos with corresponding governing parameters. We conclude that the proposed mechanism can plausibly explain the superflaring events at least for some cool stars, including binaries, subgiants and, possibly, low-mass stars and young rapid rotators.

astro-ph.SR↗

arXiv2016

A joined model for solar dynamo and differential rotation

A model for the solar dynamo, consistent in global flow and numerical method employed with the differential rotation model, is developed. The magnetic turbulent diffusivity is expressed in terms of the entropy gradient, which is controlled by the model equations. The magnetic Prandtl number and latitudinal profile of the alpha-effect are specified by fitting the computed period of the activity cycle and the equatorial symmetry of magnetic fields to observations. Then, the instants of polar field reversals and time-latitude diagrams of the fields also come into agreement with observations. The poloidal field has a maximum amplitude of about 10 Gs in the polar regions. The toroidal field of several thousand Gauss concentrates near the base of the convection zone and is transported towards the equator by the meridional flow. The model predicts a value of about 10^37 erg for the total magnetic energy of large-scale fields in the solar convection zone.

astro-ph.SR↗

arXiv2016

Meridional circulation in the Sun and stars

Mean-field hydrodynamics advanced to clear explanations for the origin and properties of the global meridional flow in stellar convection zones. Qualitative arguments and analysis of basic equations both show that the meridional circulation is driven by non-conservative centrifugal and buoyancy forces and results from a slight disbalance between these two drivers. The deviations from the thermal wind balance are relatively large near the boundaries of convection zones. Accordingly, the meridional flow attains its largest velocities in the boundary layers and decreases inside the convection zone. This picture, however, is neither supported nor dismissed by the conflicting results of recent helioseismic soundings or 3D numerical experiments. The relevant physics of the differential temperature and its possible relation to the solar oblateness are briefly discussed.

astro-ph.SR↗

arXiv2016

Dynamo model for grand maxima of solar activity: can superflares occur on the Sun?

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.

astro-ph.SR↗

arXiv2015

Dynamo Saturation in Rapidly Rotating Solar-Type Stars

The magnetic activity of solar-type stars generally increases with stellar rotation rate. The increase, however, saturates for fast rotation. The Babcock-Leighton mechanism of stellar dynamos saturates as well when the mean tilt-angle of active regions approaches ninety degrees. Saturation of magnetic activity may be a consequence of this property of the Babcock-Leighton mechanism. Stellar dynamo models with a tilt-angle proportional to the rotation rate are constructed to probe this idea. Two versions of the model - treating the tilt-angles globally and using Joy's law for its latitude dependence - are considered. Both models show a saturation of dynamo-generated magnetic flux at high rotation rates. The model with latitude-dependent tilt-angles also shows a change in dynamo regime in the saturation region. The new regime combines a cyclic dynamo at low latitudes with an (almost) steady polar dynamo.

astro-ph.SR↗

arXiv2015

The effective magnetic Prandtl number in magnetized and differentially rotating stellar radiation zones

With application to inner stellar radiative zones, a linear theory is used to analyze the instability of a dipole-parity toroidal background field, in the presence of density stratification, differential rotation, and realistically small Prandtl numbers. The physical parameters are the normalized latitudinal shear $a$ and the normalized field amplitude $b$. Only the solutions for the wavelengths with the maximal growth rates are considered. If these scales are combined to the radial values of velocity, one finds that the (very small) radial velocity only depends slightly on $a$ and $b$, so that it can be used as the free parameter of the eigenvalue system. The resulting instability-generated tensors of magnetic diffusivity and eddy viscosity are highly anisotropic. The eddy diffusivity in latitudinal direction exceeds the eddy diffusivity in radial direction by orders of magnitude. Its latitudinal profile shows a strong concentration toward the poles which is also true for the effective viscosity which has been calculated via the angular momentum transport of the instability pattern. The resulting effective magnetic Prandtl number reaches values of $O(10^2)$, so that the differential rotation decays much faster than the toroidal background field, which is {the} necessary condition to explain the observed slow rotation of the early red-giant and sub-giant cores by means of magnetic instabilities.

astro-ph.SR↗

arXiv2014

North-south asymmetry of solar dynamo in the current activity cycle

An explanation is suggested for the north-south asymmetry of the polar magnetic field reversal in the current cycle of solar activity. The contribution of the Babcock-Leighton mechanism to the poloidal field generation is estimated using sunspot data for the current activity cycle. Estimations are performed separately for the northern and southern hemispheres. The contribution of the northern hemisphere exceeded considerably that of the southern hemisphere during the initial stage of the cycle. This is the probable reason for the earlier reversal of the northern polar field. The estimated contributions of the Babcock-Leighton mechanism are considerably smaller than similar estimations for the previous activity cycles. A relatively weak (<1G) large-scale polar field can be expected for the next activity minimum.

astro-ph.SR↗

arXiv2014

The solar dynamo: inferences from observations and modeling

It can be shown on observational grounds that two basic effects of dynamo theory for solar activity - production of the toroidal field from the poloidal one by differential rotation and reverse conversion of the toroidal field to the poloidal configuration by helical motions - are operating in the Sun. These two effects, however, do not suffice for constructing a realistic model for the solar dynamo. Only when a non-local version of the alpha-effect is applied, is downward diamagnetic pumping included and field advection by the equatorward meridional flow near the base of the convection zone allowed for, can the observed activity cycles be closely reproduced. Fluctuations in the alpha-effect can be estimated from sunspot data. Dynamo models with fluctuating parameters reproduce irregularities of solar cycles including the grand activity minima. The physics of parametric excitation of irregularities remains, however, to be understood.

astro-ph.SR↗

arXiv2014

Magnetic fields in fully convective M-dwarfs: oscillatory dynamos vs bistability

M-dwarfs demonstrate two types of activity: 1) strong (kilogauss) almost axisymmetric poloidal magnetic fields; and 2) considerably weaker nonaxisymmetric fields, sometimes including a substantial toroidal component. Dynamo bistability has been proposed as an explanation. However it is not straightforward to obtain such a bistability in dynamo models. On the other hand, the solar magnetic dipole at times of magnetic field inversion becomes transverse to the rotation axis, while the magnetic field becomes weaker at times far from that of inversion. Thus the Sun resembles a star with the second type of activity. We suggest that M-dwarfs can have magnetic cycles, and that M-dwarfs with the second type of activity can just be stars observed at times of magnetic field inversion. Then the relative number of M-dwarfs with the second type of activity can be used in the framework of this model to determine parameters of stellar convection near the surface.

astro-ph.SR↗

arXiv2013

Grand Minima and North-South Asymmetry of Solar Activity

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 strongly decreased amplitude of magnetic cycles similar to the Maunder minimum of solar activity. Random dependence of dynamo-parameters on latitude brakes 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.

astro-ph.SR↗

arXiv2013

Baroclinic Instability in Differentially Rotating Stars

A linear analysis of baroclinic instability in a stellar radiation zone with radial differential rotation is performed. The instability onsets at a very small rotation inhomogeneity. There are two families of unstable disturbances corresponding to Rossby waves and internal gravity waves. The instability is dynamical: its growth time of several thousand rotation periods is short compared to the stellar evolution time. A decrease in thermal conductivity amplifies the instability. Unstable disturbances possess kinetic helicity thus indicating the possibility of magnetic field generation by the turbulence resulting from the instability.

astro-ph.SR↗

arXiv2013

Fluctuations in the Alpha-Effect and Grand Solar Minima

Parameters of a special kind of α-effect known in dynamo theory as the Babcock-Leighton mechanism are estimated using the data of sunspot catalogs. The estimates evidence the presence of the Babcock-Leighton α-effect on the Sun. Fluctuations of the α-effect are also estimated. The fluctuation amplitude appreciably exceeds the mean value, and the characteristic time for the fluctuations is comparable to the period of the solar rotation. Fluctuations with the parameters found are included in a numerical model for the solar dynamo. Computations show irregular changes in the amplitudes of the magnetic cycles on time scales of centuries and millennia. The calculated statistical characteristics of the grand solar minima and maxima agree with the data on solar activity over the Holocene.

astro-ph.SR↗