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Differential Rotation (differential + rotation)

Distribution by Scientific Domains

Physics and Astronomy	100%

Selected Abstracts

Convective dynamos in spherical wedge geometry

ASTRONOMISCHE NACHRICHTEN, Issue 1 2010
P.J. Käpylä
Abstract Self-consistent convective dynamo simulations in wedge-shaped spherical shells are presented. Differential rotation is generated by the interaction of convection with rotation. Equatorward acceleration and dynamo action are obtained only for sufficiently rapid rotation. The angular velocity tends to be constant along cylinders. Oscillatory large-scale fields are found to migrate in the poleward direction. Comparison with earlier simulations in full spherical shells and Cartesian domains is made (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Differential rotation and meridional circulation in global models of solar convection

ASTRONOMISCHE NACHRICHTEN, Issue 10 2007
M.S. MieschArticle first published online: 27 DEC 200
Abstract In the outer envelope of the Sun and in other stars, differential rotation and meridional circulation are maintained via the redistribution of momentum and energy by convective motions. In order to properly capture such processes in a numerical model, the correct spherical geometry is essential. In this paper I review recent insights into the maintenance of mean flows in the solar interior obtained from high-resolution simulations of solar convection in rotating spherical shells. The Coriolis force induces a Reynolds stress which transports angular momentum equatorward and also yields latitudinal variations in the convective heat flux. Meridional circulations induced by baroclinicity and rotational shear further redistribute angular momentum and alter the mean stratification. This gives rise to a complex nonlinear interplay between turbulent convection, differential rotation, meridional circulation, and the mean specific entropy profile. I will describe how this drama plays out in our simulations as well as in solar and stellar convection zones. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Dynamics of Dislocations in a 2D Plasma Crystal

CONTRIBUTIONS TO PLASMA PHYSICS, Issue 4-5 2009
V. Nosenko
Abstract Recent experimental results on the dislocation dynamics in a two-dimensional plasma crystal are reviewed. A single layer of micron-size microspheres was suspended in the sheath of a capacitively coupled rf discharge. The particles self-organized in a triangular lattice. Edge dislocations were created in pairs in this plasma crystal when the internal shear stress built up above a threshold, due to the crystal's slow differential rotation. Basic stages of dislocation nucleation were identified and studied, from gradual pile-up of shear strain in the crystal to eventual escape of free dislocations. After nucleation, dislocations moved supersonically with respect to the shear waves and generated shear-wave Mach cones (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Electromagnetic fields in jets

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007
B. D. Sherwin
ABSTRACT The magnetic fields and energy flows in an astronomical jet described by our earlier model are calculated in detail. Though the field distribution varies with the external pressure function p(z), it depends only weakly on the other boundary conditions. Individual field lines were plotted; the lines become nearly vertical at the bottom and are twisted at the top. An animation of a field line's motion was made, which shows the line being wound up by the accretion disc's differential rotation and rising as a result of this. The distribution of Poynting flux within the jet indicates that much of the energy flows up the jet from the inside of the accretion disc but a substantial fraction flows back down to the outside. [source]

Searching for mid-term variations in different aspects of solar activity , looking for probable common origins and studying temporal variations of magnetic polarities

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2007
E. Forgács-Dajka
ABSTRACT Several studies have examined the temporal variability of the solar activity, and many variations are reported in the literature. We also (re)analyse the statistical properties of the following kinds of data series of solar activity phenomena: magnetic synoptic charts, hemispherical relative sunspot numbers, solar flare index, coronal index, solar radio flux, interplanetary magnetic field and proton speed in the solar wind, in order to find common mid-term periods during solar cycles 21,23. As a new approach, we focus on the magnetic polarity relations and we define new quantities (e.g. magnetic positive,negative polarity asymmetry) to explore the connections between several aspects of the solar activity from different points of view. According to our survey, the mid-term periodicities (1,2 yr) are manifest in almost all data with the exception of the coronal index and the 10.7-cm solar flux data. In the case of these latter two we note that these surveys produce global data on the solar corona, so the Sun is studied on these bandwidths as a star. Besides these, with the accumulation of helioseismic data over the last 10 yr, it has become possible to study the temporal variation in the rotational rate residuals in tachocline region. In addition, we examine possible common origins of different activity markers and/or possible connections to differential rotation. [source]

LO Peg in 1998: star-spot patterns and differential rotation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2005
J. R. Barnes
ABSTRACT We present Doppler images of the young K5V,K7V rapid rotator LO Peg from seven nights of continuous spectroscopy obtained in 1998 from July 04 to July 10. The images reveal the presence of a strong polar cap with appendages extending to mid-latitudes, but no star-spots are seen below 15°. We briefly discuss the distribution of spots in light of recent flux transport simulations, which are able to reproduce the observed latitude dependence. With the full time series of spectra, of which 314 are useful, many phases are observed three times over the seven nights of observations. Using star-spots as tracers of a solar-like latitudinal differential rotation in our image reconstructions, we find that the equatorial regions complete one more rotation than the polar regions every 181 ± 35 d. LO Peg is the second coolest star for which such a measurement has been made using indirect imaging methods. The degree of latitudinal shear is less than that seen in G and early K dwarfs, suggesting a trend in which differential rotation decreases with stellar mass in (pre-)main-sequence objects. [source]

Dynamics of the solar tachocline , I. An incompressible study

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002
P. Garaud
Gough & McIntyre have suggested that the dynamics of the solar tachocline are dominated by the advection,diffusion balance between the differential rotation, a large-scale primordial field and baroclinicly driven meridional motions. This paper presents the first part of a study of the tachocline, in which a model of the rotation profile below the convection zone is constructed along the lines suggested by Gough & McIntyre and solved numerically. In this first part, a reduced model of the tachocline is derived in which the effects of compressibility and energy transport on the system are neglected; the meridional motions are driven instead by Ekman,Hartmann pumping. Through this simplification, the interaction of the fluid flow and the magnetic field can be isolated and is studied through non-linear numerical analysis for various field strengths and diffusivities. It is shown that there exists only a narrow range of magnetic field strengths for which the system can achieve a nearly uniform rotation. The results are discussed with respect to observations and to the limitations of this initial approach. A following paper combines the effects of realistic baroclinic driving and stratification with a model that closely follows the lines of work of Gough & McIntyre. [source]

The exceptional Herbig Ae star HD 101412: The first detection of resolved magnetically split lines and the presence of chemical spots in a Herbig star,

ASTRONOMISCHE NACHRICHTEN, Issue 4 2010
S. Hubrig
Abstract In our previous search for magnetic fields in Herbig Ae stars, we pointed out that HD 101412 possesses the strongest magnetic field among the Herbig Ae stars and hence is of special interest for follow-up studies of magnetism among young pre-main-sequence stars. We obtained high-resolution, high signal-to-noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD 101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting and several spectral lines observed in unpolarized light at high dispersion are resolved into magnetically split components. The measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8 kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The period determination was done using the Lomb-Scargle method. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period Prot = 13.86 d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non-statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current-driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate-mass stars could be an alternative to a frozen-in fossil field (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Tayler instability with Hall effect in young neutron stars

ASTRONOMISCHE NACHRICHTEN, Issue 1 2009
G. Rüdiger
Abstract Collapse calculations indicate that the hot young neutron stars rotate differentially so that strong toroidal magnetic field components should exist in the outer shell where also the Hall effect appears to be important when the Hall parameter = ,B, exceeds unity. The amplitudes of the induced toroidal magnetic fields are limited by the current-induced Tayler instability. An important characteristics of the Hall effect is its distinct dependence on the sign of the magnetic field. We find for fast rotation that positive (negative) Hall parameters essentially reduce (increase) the stability domain. It is thus concluded that the toroidal field belts in young neutron stars induced by their differential rotation should have different amplitudes in both hemispheres which later are frozen in. Due to the effect of magnetic suppression of the heat conductivity also the brightness of the two hemispheres should be different. As a possible example for our scenario the isolated neutron star RBS 1223 is considered which has been found to exhibit different X-ray brightness at both hemispheres (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Differential rotation and meridional circulation in global models of solar convection

Rapid rotation, active nests of convection and global-scale flows in solar-likestars

ASTRONOMISCHE NACHRICHTEN, Issue 10 2007
B.P. Brown
Abstract In the solar convection zone, rotation couples with intensely turbulent convection to build global-scale flows of differential rotation and meridional circulation. Our sun must have rotated more rapidly in its past, as is suggested by observations of many rapidly rotating young solar-type stars. Here we explore the effects of more rapid rotation on the patterns of convection in such stars and the global-scale flows which are self-consistently established. The convection in these systems is richly time dependent and in our most rapidly rotating suns a striking pattern of spatially localized convection emerges. Convection near the equator in these systems is dominated by one or two patches of locally enhanced convection, with nearly quiescent streaming flow in between at the highest rotation rates. These active nests of convection maintain a strong differential rotation despite their small size. The structure of differential rotation is similar in all of our more rapidly rotating suns, with fast equators and slower poles. We find that the total shear in differential rotation, as measured by latitudinal angular velocity contrast, ,,, increases with more rapid rotation while the relative shear, ,,/,, decreases. In contrast, at more rapid rotation the meridional circulations decrease in both energy and peak velocities and break into multiple cells of circulation in both radius and latitude. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Magnetic fields in halos of spiral galaxies

ASTRONOMISCHE NACHRICHTEN, Issue 5-6 2006
R.-J. Dettmar
Abstract Observations of magnetic fields in halos of edge-on disk galaxies are discussed in relation to the interstellar disk-halo interface in disk galaxies. The distribution of extra-planar diffuse ionized gas correlates on local and global scales with cosmic rays and magnetic fields as inferred from observations of the non-thermal radio continuum radiation and its polarisation. From the polarisation a large-scale and well-ordered magnetic field in these gaseous halos can be deduced. For several objects a significant poloidal component of the halo field is likely. These observations indicate the presence of physical processes which generate and maintain magnetic fields on galactic scales. The importance of differential rotation of the gaseous halos for such processes is briefly discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]