The motion of magnetic fronts in spiral galaxies

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.     

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

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.     

Meridional circulation and dynamo-wave propagation

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.     

A dynamo scenario for polar and whole-surface starspot generation

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).     

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

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.     

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

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...     

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

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

Earth’s magnetic moment during geomagnetic reversals

The behavior of the dipole magnetic moment of the geomagnetic field during the reversals is considered. By analogy with the reversals of the magnetic field of the Sun, the scenario is suggested in which during the reversal the mean dipole moment becomes zero, whereas the instantaneous value of the dipole magnetic moment remains nonzero and the corresponding vector rotates from the vicinity of one geographical pole to the other. A thorough discussion concerning the definition of the mean magnetic moment, which is used in this concept, is presented. Since the behavior of the geomagnetic field during the reversal is far from stationary, the ensemble average instead of the time average has to be considered.     

Intermittency of the Solar Magnetic Field and Solar Magnetic Activity Cycle

Small-scale solar magnetic fields demonstrate features of fractal intermittent behavior, which requires quantification. For this purpose we investigate how the observational estimate of the solar magnetic flux density \(B\) depends on resolution \(D\) in order to obtain the scaling \(\ln B_{D} = - k \ln D +a\) in a reasonably wide range. The quantity \(k\) demonstrates cyclic variations typical of a solar activity cycle. In addition, \(k\) depends on the magnetic flux density, i.e. the ratio of the magnetic flux to the area over which the flux is calculated, at a given instant. The quantity \(a\) demonstrates some cyclic variation, but it is much weaker than in the case of \(k\). The scaling obtained generalizes previous scalings found for the particular cycle phases. The scaling is typical of fractal structures. In our opinion, the results obtained trace small-scale action in the solar convective zone and its coexistence with the conventional large-scale solar dynamo based on differential rotation and mirror-asymmetric convection.