The motion of magnetic fronts in spiral galaxies     

D. Moss  A. Petrov  D. Sokoloff 《地球物理与天体物理流体动力学》2013,107(1-2):129-149

Abstract

We study the nonlinear asymptotic thin disc approximation to the mean field dynamo equations, as applicable to spiral galaxies. The circumstances in which sharp magnetic field structures (fronts) can propagate radially are investigated, and an expression for the speed of propagation derived. We find that the speed of an interior front is proportional to η//R _? (where η is the diffusivity and R_t the galactic radius), whereas an exterior front moves with speed of order , where γ is the local growth rate of the dynamo. Numerical simulations are presented, that agree well with our asymptotic results. Further, we perform numerical experiments using the 'no-z' approximation for thin disc dynamos, and show that the propagation of magnetic fronts in this approximation can also be understood in terms of our asymptotic results.  相似文献   

Erratum     

A. A. Ruzmaikin  D. D. Sokoloff 《Astrophysics and Space Science》1978,56(2):519-519

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Impact and response of southwest Florida mangroves to the 2004 hurricane season     

E. C. Milbrandt  J. M. Greenawalt-Boswell  P. D. Sokoloff  S. A. Bortone 《Estuaries and Coasts》2006,29(6):979-984

Although hurricane disturbance is a natural occurrence in mangrove forests, the effect of widespread human alterations on the resiliency of estuarine habitats is unknown. The resiliency of mangrove forests in southwest Florida to the 2004 hurricane season was evaluated by determining the immediate response of mangroves to a catastrophic hurricane in areas with restricted and unrestricted tidal connections. The landfall of Hurricane Charley, a category 4 storm, left pronounced disturbances to mangrove forests on southwest Florida barrier islands. A significant and negative relationship between canopy loss and distance from the eyewall was observed. While a species-specific response to the hurricane was expected, no significant differences were found among species in the size of severely impacted trees. In the region farthest from the eyewall, increases in canopy density indicated that refoliation and recovery occurred relatively quickly. There were no increases or decreases in canopy density in regions closer to the eyewall where there were complete losses of crown structures. In pre-hurricane surveys, plots located in areas of management concern (i.e., restricted connection) had significantly lower stem diameter at breast height and higher stem densities than plots with unrestricted connection. These differences partially dictated the severity of effect from the hurricane. There were also significantly lower red mangrove (Rhizophora mangle) seedling densities in plots with restricted connections. These observations suggest that delays in forest recovery are possible in severely impacted areas if either the delivery of propagules or the production of seedlings is reduced by habitat fragmentation.  相似文献   

Magnetic fronts in galaxies     

A. P. Petrov  D. D. Sokoloff  D. L. Moss 《Astronomy Reports》2001,45(7):497-501

Magnetic-field reversals observed in the Milky Way at one or more Galactocentric radii are interpreted as long-lived transient structures containing memory of the seed magnetic fields. These magnetic fronts can be described using the so-called no-z approximation for the nonlinear, mean-field dynamo equations, which are the topic of the paper. We obtain asymptotic estimates for the speed of propagation of discontinuities in the solution (internal fronts). These estimates agree well with numerical solutions of the no-z equations, and support support our interpretation of the magnetic-field reversals as long-lived transients.  相似文献   

Meridional circulation and dynamo-wave propagation     

E. P. Popova  M. Yu. Reshetnyak  D. D. Sokoloff 《Astronomy Reports》2008,52(2):157-163

The effects of meridional circulation on the solar dynamo wave are investigated in the Parker approximation using WKB techniques. The meridional circulation can substantially prolong the activity cycle; however, in the framework of the approximation considered, it cannot reverse the direction of propagation of the dynamo wave. If the circulation speed is too high, the solution is concentrated near the pole and can no longer be described in the Parker approximation.  相似文献   

A dynamo scenario for polar and whole-surface starspot generation     

Belvedère  G.  Lanza  A.F.  Sokoloff  D. 《Solar physics》1998,183(2):435-444

Rapidly rotating young (T Tauri, pre-Main-Sequence, and Main-Sequence) stars as well as subgiants seem to show starspots not only at low and middle latitudes, as the Sun, but also at high latitudes and even around the poles. We consider a simple nonlinear Parker migratory dynamo model working in a thin shell in order to investigate how high latitude and polar spots may be produced for different values of the dynamo layer radius and thickness and for various rotation rates. Simple assumptions on the angular velocity gradient and helicity distribution are made according to symmetry properties and recent solar and stellar observations. A recently proposed asymptotic WKB-type approach is used to solve the dynamo problem and its drawbacks and advantages in the solar and stellar contexts are discussed. As a general result, we find that a sizable toroidal field can be produced over a much more extended latitude range than in the Sun, thus explaining in a natural way the occurrence of activity from the poles to the equator in such stars. Our approach complements that proposed by Schüssler et al. (1996) which is focused on the instability and emergence of the azimuthal flux tubes, as well as the analyses based on a dynamo working over an extended part of the stellar convective envelope (Moss, Tuominen, and Brandenburg, 1991; Moss et al., 1995).  相似文献   

The scale and strength of the galactic magnetic field according to the pulsar data     

A. A. Ruzmaikin  D. D. Sokoloff 《Astrophysics and Space Science》1977,52(2):365-374

In accordance with the data on the Faraday rotation, angular coordinates, and dispersion measurements and distances of 38 pulsars, the strengthB=2.1±1.1 G and directionl=99°±24°,b0° of the large-scale galactic magnetic field and the mean electron density in the galactic discN _e=0.03±0.01 cm^–3 are determined. A comparison with the results of a study of the measures of rotation of extragalactic radio sources enabled us to estimate the characteristic half-width of the distribution of the electron density on the Z-coordinate (h400 ps). The characteristic size of galactic magnetic field flucturations is shown to be =100–150 ps.  相似文献   

Mean Square Geodesic Deviation in the Zeldovich Problem on Light Propagation in a Universe with Inhomogeneities     

Sokoloff  D. D.  Chikina  A. A.  Illarionov  E. A. 《Astronomy Reports》2021,65(5):362-369

Astronomy Reports - In 1964, Ya.B. Zeldovich formulated the problem of light propagation in the Universe under the influence of inhomogeneities. It is reduced to describing the divergence of two...  相似文献   

Erratum to: Mean Square Geodesic Deviation in the Zeldovich Problem on Light Propagation in a Universe with Inhomogeneities     

Sokoloff  D. D.  Chikina  A. A.  Illarionov  E. A. 《Astronomy Reports》2021,65(7):633-633

Astronomy Reports - An Erratum to this paper has been published: https://doi.org/10.1134/S1063772921340035  相似文献   

Statistical properties of polarized radio continuum emission and effects of data processing     

I. Mizeva  W. Reich  P. Frick  R. Beck  D. Sokoloff 《Astronomische Nachrichten》2007,328(1):80-91

Polarized intensity and polarization angles are calculated from Stokes parameters Q and U in a nonlinear way. The statistical properties of polarized emission hold information about the structure of magnetic fields in a large range of scales, but the contributions of different stages of data processing to the statistical properties should first be understood. We use 1.4 GHz polarization data from the Effelsberg 100‐m telescope of emission in the Galactic plane, near the plane and far out of the plane. We analyze the probability distribution function and the wavelet spectrum of the original maps in Stokes parameters Q, U and corresponding PI. Then we apply absolute calibration (i.e. adding the large‐scale emission to the maps in Q and U), subtraction of polarized sources and subtraction of the positive bias in PI due to noise (“denoising”). We show how each procedure affects the statistical properties of the data. We find a complex behavior of the statistical properties for the different regions analyzed which depends largely on the intensity level of polarized emission. Absolute calibration changes the morphology of the polarized structures. The statistical properties change in a complex way: Compact sources in the field flatten the wavelet spectrum over a substantial range. Adding large‐scale emission does not change the spectral slopes in Q and U at small scales, but changes the PI spectrum in a complex way. “Denoising” significantly changes the p.d.f. of PI and raises the entire spectrum. The final spectra are flat in the Galactic plane due to magnetic structures in the ISM, but steeper at high Galactic latitude and in the anticenter. For a reliable study of the statistical properties of magnetic fields and turbulence in the ISM based on radio polarization observations, absolute calibration and source subtraction are required. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献