A dynamo wave near the solar equator

The structure of a dynamo wave near the solar equator is considered in the Parker approximation. The results show that the principal dynamo wave, which travels in the northern hemisphere from middle latitudes toward the equator, penetrates slightly into the southern hemisphere. The wave that propagates in the southern hemisphere exhibits similar behavior. The angular distance the wave is able to penetrate into the neighboring hemisphere can reach about ten degrees of latitude. Possible observational manifestations of this effect are discussed. The growth rate of the dipolar magnetic-field configuration exceeds that of the quadrupolar configuration, and the difference between these growth rates is computed. A possible relation of this quantity to the time characteristics of the Maunder minimum is discussed.     

Near-polar starspots and polar dynamo waves

A possible mechanism for the formation of near-polar magnetic spots on some stars with convective envelopes is proposed. The mechanism is based on the idea that the maximum of the dynamo waves that are excited in thin convective shells by the dynamo mechanism is shifted appreciably from the maximum of the magnetic-field sources in the direction of motion of the dynamo wave. If there is no region of super-rotation near the equator for some reason (as a consequence of disruption due to tidal interaction with a companion in a binary system, for example) and the wave of stellar activity propagates toward the poles rather than toward the equator, this maximum will be in the near-polar regions.     

The anisotropy of kinetic and current helicity

A three-dimensional model for thermal convection with a dynamo in a rotating planar layer heated from below is used to investigate the behavior of the mean kinetic and current helicities. In spite of the presence of gravity and rotation, which introduce anisotropy into the system, the components of the helicity determined from the field components in the directions tangent and normal to the boundary have similar values. The existence of a separation by scale, when the current helicity has different signs on different spatial scales, is demonstrated. The number of regions where the sign of the helicity does not coincide with the sign of its mean value in that region is estimated (～43?45% of the total number of regions). The estimates presented are relevant for interpretations of observations of solar activity and analysis of the properties of rotating magnetohydrodynamical turbulence.     

Cosmological magnetic dynamo in the early Universe

We study the behavior of large-scale magnetic fields in the early Universe influenced by an instability associated with breaking of mirror symmetry in weak interactions. It is shown that the magnetic field, whose present scale reaches about 500 m, which is negligible for galactic sizes, increases considerably if we correctly take into account the dynamics of the Universe. We conclude that this magnetic field is unlikely to provide the seed field for galactic dynamos, nearly independent of the particular instability considered.     

Model of a multiscaled MHD dynamo

A simple model for a multiscaled MHD dynamo is suggested. The uppermost tier of the model controls the evolution of the large-scale magnetic field, while the lower tiers are responsible for the evolution of the small-scale velocity and magnetic fields. This approach makes it possible to reproduce, e.g., the evolution of the Galactic magnetic field for realistic magnetic Reynolds numbers, which cannot be done using direct, detailed simulations.     

The fast dynamo in interstellar turbulence

The α effect and coefficient of eddy diffusivity are calculated for the magnetic field in a random flow with recovery. Such a flow loses its memory abruptly at random times that form a Poisson flow of events. Interstellar turbulence sustained by supernova outbursts is one physical realization of such a flow. The growth rates and configurations of large-scale galactic magnetic fields for this situation are close to those predicted by simple galactic dynamo models. At the same time, the model of a flow with recovery makes it possible to trace the role of the effective “forgetting” of correlations. The presence of this forgetting distinguishes interstellar turbulence from other types of random flows.     

Magnetic and electric-current helicities in very simple models of the solar dynamo

The behavior of the electric-current and magnetic helicities in the course of the solar-activity cycle is studied in the framework of Parker’s very simple model for the solar dynamo. A polarity rule is formulated for the helicities, which generalizes the well-known Hale polarity rule.     

A small-scale flood plain

A miniature, 9 m-wide floodplain, developed along a gravel-washing effluent stream, shows features such as levées, crevasse splays and floodbasins which compare with their larger-scale counterparts. For sediments deposited overbank, median size decreases exponentially with distance from the channel whilst sorting increases, with coarser sediment on the outside of a meander bend. Overbank flows are only a few grain diameters in depth near the channel. This study shows potentially useful systematic relationships in floodplain sediment textures, but it involves only one of a possible variety of floodplain types dominated by overbank sedimentation. This suggests that further exploration of overbank depositional processes is desirable as an aid to field interpretation.     

A low-mode dynamo with meridional circulation and dipolar symmetry of the magnetic field

A dynamic system for the Parker dynamo including meridional circulation that is applicable to astrophysical objects is constructed. The meridional circulation is able to control the regimes for the generation of magnetic fields. If the meridional flows are weak, regimes with steady oscillations, dynamo bursts, fluctuations, and chaotic components are possible. When the meridional circulation is strengthened, the range of dynamo numbers required for fluctuations and dynamo bursts is reduced, and gradually vanishes; at the same time, the range required for oscillations is increased and raised to higher dynamo numbers. The latitude-time distributions of the toroidal and poloidal magnetic fields for steady oscillations are presented.     

The solar dynamo and integrated irradiance variations in the course of the 11-year cycle

Abstractthe effect of the large-scale magnetic fields generated by the solar dynamo on the radiation flux issuing from the convection zone is studied. A governing equation describing convective heat transfer is obtained in the framework of mean-field magnetohydrodynamics, with account for the influence of magnetic fields and differential rotation on the energy budget of the convection zone. The principal effects are illustrated using a one-dimensional numerical model. Calculations indicate that the influence of large-scale magnetic fields can modulate the solar irradiance with a relative amplitude of ～0.07%.     

小尺度断裂沿层曲率敏感性研究

沿层曲率作为地震属性的一种,不仅有数学几何意义更有着重要的地质意义。和相干体相比,沿层曲率在油气勘探中常被用于刻画地质构造形态及断裂精细解释,特别是在小尺度断裂发育的地区更为有效。而在实际工作中,有时利用沿层曲率所描述的断裂效果不太明显。为进一步明确对于刻画断裂结果的影响因素及其适用条件,以鄂尔多斯盆地Q区Tj9为研究对象,基于Geoeast软件对该区小尺度断裂系统从选取属性、方位角、频带三个方面展开分析。结果表明：（1）曲率比相干体更适用于刻画较小尺度的断层;（2）平面上从平行断裂的方位观测,断裂识别不清晰;从垂直断裂的方位观测,易于识别;（3）在反射层同相轴较强时,中高频段对断裂识别更清楚;在反射层同相轴较弱时,中频段对断裂识别更清楚。因此可以根据不同属性、不同方位角、不同频带对断裂的敏感程度,用不同的数据体解释小尺度断裂,提高对断层刻画的精度,为复杂油气勘探寻找有利区奠定基础。     

The effect of meridional circulation on dynamo waves approaching the solar poles

Dynamo waves approaching the solar poles are analyzed in the Parker approximation taking into account meridional circulation. Asymptotic solutions of the equations describing the generation of the magnetic field are constructed. It is shown in which cases the effect of meridional circulation results in traveling dynamo waves both incident on and reflected from the pole, or in a superposition of standing dynamo waves.     

Moons' magnetism: from Io's and Ganymede's present magnetic signatures to the ancient lunar dynamo

Intrinsic magnetic fields, corresponding to virtual axial dipole moments of the order of 10²⁰ Am², have recently been evidenced for the Jovian satellites Io and Ganymede. By reviewing the rock magnetic and palaeomagnetic properties together with the history of Io, the hypothesis of a moment due to induced or remanent magnetization of crustal rocks acquired within the ambient Jovian field is clearly eliminated. This demonstration is all the more valid for Ganymede, which experiences a much lower Jovian field. The demonstration of a present dynamo action for these Jovian moons, possibly sustained by Jupiter through tidal heating and background magnetic field, may be an actualistic model for the early lunar history. The hypothesis of a lunar dynamo, active at 3–4 Ga, seems to be strongly supported by this analogy.     

The structure of the global solar magnetic field excited by the turbulent dynamo mechanism

A mixing-length approximation is used to calculate Kλ for a Parker dynamo wave excited by the dynamo mechanism near the base of the solar convection zone (K is the wave number of the dynamo wave and λ the extent of the dynamo region). In a turbulent-dynamo model, this number characterizes the modes of the global magnetic field generated by a mechanism based on the joint action of the mean helical turbulence and solar differential rotation. Estimates are obtained for the helicity and radial angular-velocity gradient using the most recent helioseismological measurements at the growth phase of solar cycle 23. These estimates indicate that the dynamo mechanism most efficiently excites the fundamental antisymmetric (odd), dipole, mode of the poloidal field (Kλ≈?7) at low latitudes, while the conditions at latitudes above 50° are more favorable for the excitation of the lowest symmetric (even), quadrupole, mode (Kλ≈+8). The resulting north-south asymmetry of the poloidal field can explain the magnetic anomaly (“monopole” structure) of the polar fields observed near solar-cycle maxima. The effect of α quenching increases the calculated period of the dynamo-wave propagation from middle latitudes to the equator to about seven years, in rough agreement with the observed duration of the solar cycle.     

The helicity of the velocity field for cellular convection in a rotating layer

The helicity of a cellular convective flow in a horizontal layer of a compressible fluid (gas) heated from below and rotating about the vertical axis is studied using finite-difference numerical simulations. The medium is assumed to be polytropically stratified. A thermal perturbation that produces a system of Bénard-type hexagonal convection cells is introduced at the initial time. Next, the cells are deformed by the action of the Coriolis force; however, at some stage of the evolution, the flow is nearly steady (at later times, the cells break down). For given Rayleigh and Prandtl numbers, the velocity-field helicity for this stage averaged over the layer increases with decreasing polytrope index (i.e., with increasing the curvature of the static entropy profile) and has a maximum at a certain rotational velocity of the layer. Numerical simulations of such quasi-ordered convective flows should reduce the uncertainties in estimates of the helicity, a quantity important for the operation of MHD dynamos.     

Phase space of a two-dimensional model of a Parker dynamo

The dependences of the magnetic-field strength, variations of the magnetic field, and the multipole level on the amplitudes of the α and ω effects are considered using a two-dimensional model for a Parker dynamo in a spherical layer. Calculations have been carried out for both traditional spatial distributions of α and ω and geostrophic regimes obtained from three-dimensional modeling of thermal convection. Two-dimensional distributions of the dynamo-wave velocities in the zone where magnetic field is generated are presented. Comparisons with the solar and planetary dynamos are considered.     

Oppositely directed waves of stellar activity in simple dynamo models

Excitations of two oppositely directed waves of stellar activity generated by two dynamo-active layers located in a single stellar hemisphere are examined using simple dynamo models. The domains of model parameters corresponding to various types and directions of the activity waves are found. It is shown that oppositely directed waves of activity are generated if the dynamo numbers have the same order of magnitude, ~10⁵?10⁶, but opposite signs. How frequently this case can be observed among real stars remains open to question. The report of oppositely directed waves of stellar activity in the literature is especially valuable in this connection.