首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 93 毫秒
1.
The property of inhomogeneous turbulence in conducting fluids to expel large‐scale magnetic fields in the direction of decreasing turbulence intensity is shown as important for the magnetic field dynamics near the base of a stellar convection zone. The downward diamagnetic pumping confines a fossil internal magnetic field in the radiative core so that the field geometry is appropriate for formation of the solar tachocline. For the stars of solar age, the diamagnetic confinement is efficient only if the ratio of turbulent magnetic diffusivity ηT of the convection zone to the (microscopic or turbulent) diffusivity ηin of the radiative interior is ηT/ηin ≥ 105. Confinement in younger stars requires larger ηT/ηin. The observation of persistent magnetic structures on young solar‐type stars can thus provide evidence for the nonexistence of tachoclines in stellar interiors and on the level of turbulence in radiative cores. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

2.
A combination of diamagnetic pumping and a nonlocal α-effect of the Babcock–Leighton type in a solar dynamo model is shown to reproduce observations of solar magnetic activity. The period of the solar cycle can be reproduced without reducing magnetic diffusivity in the bulk of the convection zone below the standard mixing-length value of 1013?cm2?s?1. The simulated global fields are antisymmetric about the equator, and the toroidal-to-poloidal field ratio is about one thousand. However, the time–latitude diagrams of magnetic fields in the model without meridional flow differ from observations. Only when the meridional flow is included and the α-effect profile peaking at mid-latitudes is applied, can the observed butterfly diagrams be reproduced.  相似文献   

3.
The first results of the solar dynamo model that allows for the diamagnetic effect of inhomogeneous turbulence and the nonlocal alpha-effect due to the rise ofmagnetic loops are discussed. The nonlocal alpha-effect is not subject to the catastrophic quenching related to the conservation of magnetic helicity. Given the diamagnetic pumping, the magnetic fields are concentrated near the base of the convection zone, although the distributed-typemodel covers the entire thickness of the convection zone. Themagnetic cycle period, the equatorial symmetry of the field, its meridional drift, and the polar-to-toroidal field ratio obtained in the model are in agreement with observations. There is also some disagreement with observations pointing the ways of improving the model.  相似文献   

4.
Results from kinematic solar dynamo models employing α ‐effect and turbulent pumping from local convection calculations are presented. We estimate the magnitude of these effects to be around 2–3 m s–1, having scaled the local quantities with the convective velocity at the bottom of the convection zone from a solar mixing‐length model. Rotation profile of the Sun as obtained from helioseismology is applied in the models; we also investigate the effects of the observed surface shear layer on the dynamo solutions. With these choices of the small‐ and large‐scale velocity fields, we obtain estimate of the ratio of the two induction effects, C α /C Ω ≈ 10–3, which we keep fixed in all models. We also include a one‐cell meridional circulation pattern having a magnitude of 10–20 m s–1 near the surface and 1–2 m s–1 at the bottom of the convection zone. The model essentially represents a distributed turbulent dynamo, as the α ‐effect is nonzero throughout the convection zone, although it concentrates near the bottom of the convection zone obtaining a maximum around 30° of latitude. Turbulent pumping of the mean fields is predominantly down‐ and equatorward. The anisotropies in the turbulent diffusivity are neglected apart from the fact that the diffusivity is significantly reduced in the overshoot region. We find that, when all these effects are included in the model, it is possible to correctly reproduce many features of the solar activity cycle, namely the correct equatorward migration at low latitudes and the polar branch at high latitudes, and the observed negative sign of B r B ϕ . Although the activity clearly shifts towards the equator in comparison to previous models due to the combined action of the α ‐effect peaking at midlatitudes, meridional circulation and latitudinal pumping, most of the activity still occurs at too high latitudes (between 5° … 60°). Other problems include the relatively narrow parameter space within which the preferred solution is dipolar (A0), and the somewhat too short cycle lengths of the solar‐type solutions. The role of the surface shear layer is found to be important only in the case where the α ‐effect has an appreciable magnitude near the surface. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

5.
A comparison of the equations for the magnetic field transfer and for the heat transfer by two-dimensional turbulent convection of a conducting compressible medium shows the magnetic field to be transported as a scalar admixture provided it is parallel to the convective rolls. At high magnetic Reynolds numbers the field strength in a convective zone varies proportionally to the density of the medium.A study of the distribution and amplification of the poloidal field in the two-dimensional convection zone of the Sun lying under the supergranulation, together with the processes of field pumping and amplification in other zones, reveals the importance of considering generation mechanisms of thesemi-dynamo type where the amplifying field is excited independently by weak e.m.f.'s of non-electric origin with no feedback which would otherwise produce MHD self-excitation of the field.An illustrative calculation of the solar poloidal field maintained by a weak Coriolis e.m.f. acting in a thin external layer of the convective envelope yields for the general near-polar field, if one somehow takes into account (1) field pumping by three-dimensional supergranulation, (2) field transfer and amplification by two-dimensional convection, and (3) ohmic diffusion of the field into a stable core, a value of the order of 10–1 gauss.  相似文献   

6.
The axisymmetric component of the large-scale solar magnetic fields has a pronounced poleward branch at higher latitudes. In order to clarify the origin of this branch we construct an axisymmetric model of the passive transport of the mean poloidal magnetic field in the convective zone, including meridional circulation, anisotropic diffusivity, turbulent pumping and density pumping. For realistic values of the transport coefficients we find that diffusivity is prevalent, and the latitudinal distribution of the field at the surface simply reflects the conditions at the bottom of the convective zone. Pumping effects concentrate the field to the bottom of the convective zone; a significant part of this pumping occurs in a shallow subsurface layer, normally not resolved in dynamo models. The phase delay of the surface poloidal field relative to the bottom poloidal field is found to be small. These results support the double dynamo wave models, may be compatible with some form of a mixed transport scenario, and exclude the passive transport theory for the origin of the polar branch.  相似文献   

7.
Using a perturbated (noised) dipole model of a sunspot magnetic field structure we simulated the influence of background noise or apparent noise (unresolved small-scale magnetic field structure) on sunspot magnetic field parameters. We evaluated mean values of the vertical and horizontal electric current densities |j|∥ and |j|⊥, respectively, of the force-free parameter α and of the Lorentz force |F|. For comparison we estimated |j⊥| and |F| of a standard sunspot magnetic field model (return-flux model, OSHEROVICH 1982). Furthermore, we compared our results with those from observations resulting in estimated values of |j|∥ for quiet sunspots. Our investigation led to the following results: the estimated values of 〈|F|〉 show clearly that due to the noise the axisymmetric magnetic dipole model is clustered into several subsystems of fluxbundles. The latter are connected with a system of electric current densities of the order of |j|∥ ∼ 10−3 Am−2 and |j|⊥ = 10−1 Am−2, i.e., this system is a noise-generated nonaxisymmetric magnetohydrostatic model.  相似文献   

8.
A new pumping mechanism – methanol masers without population inversion is presented in this paper. It can be used to explain the formation of a series of J 0 → J -1 E methanol masers, while the 21 → 30 A + methanol masers are regarded as a driving coherent micrwave field. In the new mechanism, the intensities of J 0 - J -1 E methanol masers are increased with the decreasing transition frequencies (or with rotational number J, approximately). These results agree with Slysh et al. (1995) and Slysh et al. (1999) J ≤ 5 observations for G3345.01+1.79 and W48, in which both J 0 → J -1 E and 21 → 30 A + methanol masers are detected coincidentally. Other astronomical conditions, such as magnetic field, 21 → 30 A + coherent radition, incoherent pumping rate by thermal radition and so on are also discussed. The new mechanism can operate as a complement to other ordinary maser pumping mechanisms for some class II methonal maser sources. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

9.
Tilted Bianchi Type I cosmological model for perfect fluid distribution in presence of magnetic field, is investigated. To get a determinate solution, it has been assumed that the universe is filled with stiff perfect fluid distribution together with A=(BC) n where A,B,C are metric potentials and n is a constant. The behaviour of the model in presence and absence of magnetic field is discussed. The various physical and geometrical aspects of the model, is also discussed. It has been shown that tilted nature of the Bianchi Type I model is preserved due to magnetic field. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

10.
A two-dimensional potential energy surface of an H 2 + molecular ion is calculated for the case of the strong magnetic field of the neutron starB=1011–1013 G. It is shown that the dependence of the potential energy from the angle between the magnetic field direction and the internuclear axis becomes very sharp as the magnetic field increases. The obtained potential energy surfaces can be used for studying the vibrational-rotational structure of the H 2 + spectrum in a strong magnetic field and the development of the observational methods for the determination of the magnetic field of a neutron star.  相似文献   

11.
Perpendicular diffusion due to either field line wandering or random gradient and curvature effects makes a substantial contribution to the radial transport of particles at distances of about 5 AU and beyond when lines of the interplanetary magnetic field (IMF) become almost azimuthal. Here test particle trajectories are followed in a field model which uses a magnetic field sample taken by Voyager 2 at 14.8 AU. Techniques previously developed (Moussas et al., 1982a) are employed to calculate the perpendicular diffusion coefficient for 100 MeV protons and arising from random gradient and curvature effects. The result K=(2.8±0.9)×1021 cm2 s–1 shows a substantial increase above the value determined previously at 5 AU and, assuming a power law radial dependence Kr , implies 0.8–1.1. This result is consistent with observations of the cosmic-ray radial gradient.  相似文献   

12.
Following some of the recent articles on hole super-conductivity and related phenomena by Hirsch (Phys. Lett. A 134:451, 1989; Phys. Rev. B 68:184502, 2003a; Phys. Rev. B 71:184521, 2005a and Phys. Lett. A 345:453, 2005b) a simple model is proposed to explain the observed low surface magnetic field of the expected quark stars. It is argued that the diamagnetic moments of the electrons circulating in the electro-sphere induce a magnetic field, which forces the existing quark star magnetic flux density to become dilute. For the sake of completeness, we have also included the analyses of instability at the normal-super-conducting interface due to excess accumulation of magnetic flux lines. The instability at the interface has also been studied numerically.   相似文献   

13.
In the presence of a strong magnetic field (such as those believed to be characteristic of neutron stars:B-1012 Gauss) positronium may annihilate through the emission of a single photon, the magnetic field providing the photon momentum. We report on calculations of the one-photon and two-photon annihilation rates for the ground state of positronium, for magnetic fields in the range (1–44)×1012 Gauss, and give, in the two-photon case, the minimum energy half-width of the emission line due to the momentum contributions from the magnetic field. We find that unless neutron stars have magnetic fields in excess of 1013 Gauss, it is unlikely that the one-photon process will be observable.Research supported in part by the National Research Council of Canada.  相似文献   

14.
Nonlinear Alfvén waves, which in the infinitely conducting plasma are noncompressive and have a constant magnetic field strength (B 2=const), propagate in a turbulent plasma. The latter is characterized by a big (but finite) electrical conductivity eff due to micro-instabilities. The Alfvén wave in such a medium is governed by the diffusion equation. It is shown that an initial periodic perturbation (withB 2=const) while still being incompressive, decays due to dissipation.  相似文献   

15.
Y. Liu  J. Zhao 《Solar physics》2009,260(2):289-298
Using one-minute cadence time-series full disk magnetograms taken by the SOHO/MDI, we have studied the magnetic field elements at high latitude (poleward of 65° in latitude). It is found that an average lifetime of the magnetic field elements is 16.5 h during solar minimum, much longer than that during solar maximum (7.3 h). During solar minimum, number of the magnetic field elements with the dominant polarity is about 3 times as that of the opposite polarity elements. Their lifetime is 21.0 h on average, longer than that of the opposite polarity elements (2.3 h). It is also found that the lifetime of the magnetic field elements is related with their size, consistent with the magnetic field elements in the quiet sun at low latitude found by Hagenaar et al. (Astrophys. J. 511:932, 1999). During solar maximum, the polar regions are equally occupied by magnetic field elements with both polarities, and their lifetimes are roughly the same on average. No evidence shows there is a correlation between the lifetime and size of the magnetic field elements. Using an image cross-correlation method, we also measure the solar rotation rate at high latitude, up to 85° in latitude. The rate is ω=2.914−0.342sin 2 φ−0.482sin 4 φ μrad s−1 sidereal. It agrees with previous studies using the spectroscopic and image cross-correlation methods, and also agrees with the results using the element tracking method when the sample of the tracked magnetic field elements is large. The consistency of those results strongly suggests that this rate at high latitude is reliable.  相似文献   

16.
A realistic model of the interplanetary magnetic field (IMF) is constructed based on measurements taken by Pioneer 10 magnetometer at 5 AU. Energetic particle (0.1–100 MeV) propagation in this field is studied by a computer simulation of its motion in order to calculateK , the perpendicular diffusion coefficient, and V D the average drift velocity of an ensemble of particles. Determinations ofK lie in the range 3×1019–8×1020 cm2 s–1 for the energies considered and they show that perpendicular diffusion may be an important process at these heliodistances when compared with parallel diffusion results obtained by similar techniques, contrary to what was previously thought. Drift velocity calculations are very close to predictions of guiding centre theory (within 30%) suggesting that this theory can be applied in the IMF. This result shows that gradient and curvature drifts can be present even in a highly perturbed field and thus they can have some influence in cosmic ray modulation.  相似文献   

17.
A new method using an integral equation has been proposed in this paper for calculating the magnetic field enclosed within a perfect diamagnetic material. A magnetic field is assumed to exist inside an arbitrary cavity in a perfect diamagnetic material. Self-consistent magnetic fields inside the cavity and currents on its surface are calculated, resulting as it should in zero magnetic field outside the cavity. This method has been tested in a special case and the results have been compared to the analytical solutions. This method should be also applicable to any case concerning the interaction between magnetic fields and perfect diamagnetic materials, e.g., to cases where a perfect diamagnetic material is surrounded by a magnetic field.  相似文献   

18.
T. Takakura 《Solar physics》1987,107(2):283-297
Numerical simulation for the dynamics of a coronal filamentary magnetic loop has been made under the assumption that the field is initially force-free and an electric resistivity suddenly increases at a given moment due to an appearance of ion sound waves, which can be excited due to a high current density if a characteristic radius r 0 of the magnetic loop is about 3 km or less in a magnetic field B 0 of 1000 G. During the resistive decay of the magnetic field a strong field-aligned electric field is created and maintained for a sufficient time to acceleratie both electrons and protons to a high energy, which is proportional to B 0/r 0 and can be 100 MeV if r 0 = 10 km and B 0 = 1000 G. If the coronal magnetic tube is composed of many such filamentary loops, the total number of accelerated electrons is consistent with the observations.  相似文献   

19.
T. Hirayama 《Solar physics》1992,137(1):33-50
Joule heating in a slender magnetic flux tube is investigated. The distribution of the magnetic field and electric sheet current encircling a vertical cylindrical magnetic tube is determined by equating the converging magnetic flux, which results from the converging and downward flow of the granulation, and the dissipative expanding magnetic flux due to Ohmic decay. Here, to ensure the mass flux conservation, an overshooting convective flow pattern resembling recent simulations was assumed. Even with the electrical resistivity from neutral hydrogen, the width of the current sheet was found to be 2 km, being much smaller than the tube diameter of 150 km, either from an exact or approximate (Gaussian) field distribution.The resultant energy flux density due to Joule heating averaged over the cylindrical cross sectional area, is 1 × 109 erg cm-2 s-1 for an assumed photospheric magnetic field of 1500 G. This amount may supply enough energy to heat the temperature minimum region of the flux tube by T = 300 K in accord with observations, though our estimation of the excess radiation loss which should be supplied by the Joule heating to keep T = 300 K is rather uncertain.A possible role of the Joule heating on spicule formation is briefly discussed together with discussions on the slab geometry, general flow patterns, and non-constant field distributions inside the flux tube.  相似文献   

20.
Pneuman  G. W. 《Solar physics》1984,94(2):387-411
Adopting the point of view that a coronal transient is a defined magnetic structure, it must be diamagnetic with respect to the external ambient magnetic field, i.e., the external field lines cannot penetrate the structure. If this is so, an integral approach involving only external forces can be very useful for studying the conditions for acceleration and large-scale dynamical behavior of the transient.After a discussion of a suggested transient configuration based upon observations of prominences, flare loops, and transient - filament relative orientations observed by Trottet and MacQueen (1980), we demonstrate the diamagnetic approach to this problem through a particularly simplified model. Necessary conditions for upward acceleration of the transient are discussed in some detail. One such plausible initiation mechanism is shown to be a constriction of the structure near its base by the external forces. This mechanism not only can provide the upward acceleration for the transient but is also compatible with the observation of hot rising flare loops during two-ribbon flare which show evidence for magnetic reconnection.We have studied the equilibrium conditions and dynamical behavior of the transient using this mechanism for two limiting cases - that in which the gas pressure in the structure dominates over the magnetic pressure and that in which the magnetic pressure dominates. For both cases, the required equilibrium conditions are compatible with observed coronal parameters. The dynamical behavior upon inward constriction, however, resembles the observed characteristics for transients best for the magnetically dominated case. For example, in the pressure-dominated case, the required temperatures for acceleration appear somewhat high being in excess of about 1.9 × 106 K. If, in addition, the internal temperature declines adiabatically during the outward motion, the structure does not reach inifinity unless its initial temperature exceeds about 3 × 106 K but stops a some radial distance, returns to the Sun only to be accelerated outward again in the same fashion. The rather stringent requirements on internal temperature for the pressure-dominated case in addition to the expectation that pressure-dominated transients should evolve into a thin pencil shape instead of maintaining an approximately self-similar profile as observed are strong arguments in favor of the magnetically dominated case.Based upon the above results, we suggest that the reconnection process evidenced in two-ribbon flares may not necessarily be the result of the relaxation of a locally open field configuration produced by the transient as described by Kopp and Pneuman (1976) but, instead, that the acceleration of the transient and the two-ribbon flare both may be produced by a common force, namely that provided by the constricting effect of the external magnetic field displaced by the presence of the structure.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号