共查询到20条相似文献，搜索用时 31 毫秒
1.
Abstract In this paper a method for solving the equation for the mean magnetic energy <BB> of a solar type dynamo with an axisymmetric convection zone geometry is developed and the main features of the method are described. This method is referred to as the finite magnetic energy method since it is based on the idea that the real magnetic field B of the dynamo remains finite only if <BB> remains finite. Ensemble averaging is used, which implies that fields of all spatial scales are included, small-scale as well as large-scale fields. The method yields an energy balance for the mean energy density ε ≡ B 2/8π of the dynamo, from which the relative energy production rates by the different dynamo processes can be inferred. An estimate for the r.m.s. field strength at the surface and at the base of the convection zone can be found by comparing the magnetic energy density and the outgoing flux at the surface with the observed values. We neglect resistive effects and present arguments indicating that this is a fair assumption for the solar convection zone. The model considerations and examples presented indicate that (1) the energy loss at the solar surface is almost instantaneous; (2) the convection in the convection zone takes place in the form of giant cells; (3) the r.m.s. field strength at the base of the solar convection zone is no more than a few hundred gauss; (4) the turbulent diffusion coefficient within the bulk of the convection zone is about 1014cm2s?1, which is an order of magnitude larger than usually adopted in solar mean field models. 相似文献
2.
E. N. Parker 《地球物理与天体物理流体动力学》2013,107(3-4):195-218
Abstract This paper treats the dynamical conditions that obtain when long straight parallel twisted flux tubes in a highly conducting fluid are packed together in a broad array. It is shown that there is generally no hydrostatic equilibrium. In place of equilibrium there is a dynamical nonequilibrium, leading to neutral point reconnection and progressive coalescence of neighboring tubes (with the same sense of twisting), forming tubes of larger diameter and reduced twist. The magnetic energy in the twisting of each tube declines toward zero, dissipated into small-scale motions of the fluid and thence into heat. The physical implications are numerous. For instance, it has been suggested that the subsurface magnetic field of the sun is composed of close-packed twisted flux tubes. Any such structures are short lived, at best. The footpoints of the filamentary magnetic fields above bipolar magnetic regions on the sun are continually shuffled and rotated by the convection, so that the fields are composed of twisted rubes. The twisting and mutual wrapping is converted directly into fluid motion and heat by the dynamical nonequilibrium, so that the work done by the convection of the footpoints goes directly into heating the corona above. This theoretical result is the final step, then, in understanding the assertion by Rosner, Tucker, and Valana, and others, that the observed structure of the visible corona implies that it is heated principally by direct dissipation of the supporting magnetic field. It is the dynamical nonequilibrium that causes the dissipation, in spite of the high electrical conductivity. It would appear that any bipolar magnetic field extending upward from a dense convective layer into a tenuous atmosphere automatically produces heating, and a corona of some sort, in the sun or any other convective star. 相似文献
3.
《Journal of Atmospheric and Solar》1999,61(1-2):101-108
This review presents some of the new developments in the understanding of coronal magnetic fields in flares and coronal mass ejections. The modelling of the coronal magnetic field based on observed photospheric field permits to understand the location of energy release. Various flare observations are consistent with a model where magnetic reconnection occurs between two magnetic fields of different connectivity. Because magnetic helicity is almost conserved, the stored energy cannot be fully released in confined flares. The corona gets rid of the helicity injected by the convection zone only by ejecting part of the magnetic field. A severe physical constraint (open-field limit) on these ejections has been firmly established for force-free fields. It is, however, possible to open partially the field or to eject a twisted flux-tube keeping the energy of the field behind the open-field limit. New results show that in simply connected fields this happen after a finite time without loss of equilibrium, while in more complex topology a loss of equilibrium can still be present. 相似文献
4.
The topology and dynamics of the three-dimensional magnetic field in the solar atmosphere govern various solar eruptive phenomena and activities, such as flares, coronal mass ejections, and filaments/prominences. We have to observe and model the vector magnetic field to understand the structures and physical mechanisms of these solar activities. Vector magnetic fields on the photosphere are routinely observed via the polarized light, and inferred with the inversion of Stokes profiles. To analyze these vector magnetic fields, we need first to remove the 180° ambiguity of the transverse components and correct the projection effect. Then, the vector magnetic field can be served as the boundary conditions for a force-free field modeling after a proper preprocessing. The photospheric velocity field can also be derived from a time sequence of vector magnetic fields.Three-dimensional magnetic field could be derived and studied with theoretical force-free field models, numerical nonlinear force-free field models, magnetohydrostatic models, and magnetohydrodynamic models. Magnetic energy can be computed with three-dimensional magnetic field models or a time series of vector magnetic field. The magnetic topology is analyzed by pinpointing the positions of magnetic null points, bald patches, and quasi-separatrix layers. As a well conserved physical quantity,magnetic helicity can be computed with various methods, such as the finite volume method, discrete flux tube method, and helicity flux integration method. This quantity serves as a promising parameter characterizing the activity level of solar active regions. 相似文献
5.
A family of force-free magnetic fields, describing multipolar magnetic arcades is described. The family allows for twisted magnetic ropes, where magnetic field lines have a helical shape. These helical structures are specific to filament channels of active regions. In their structure, arcades belong to normal-polarity fields and correspond to prominences of active regions according to available observational data. A potential field is found that is external with respect to an arcade. The constructed force-free fields can be used to further develop the MHD model of filaments that takes into account the gas pressure gradient and gravity. 相似文献
6.
Nigel Weiss FRAS 《Astronomy& Geophysics》2001,42(3):3.10-3.17
Nigel Weiss recounts his Presidential Address 2001, given to the RAS A&G Ordinary Meeting on 9 February 2001.
Recent high-resolution observations, from the ground and from space, have revealed the fine structure of magnetic features at the surface of the Sun. At the same time, advances in computing power have at last made it possible to develop models of turbulent magnetoconvection that can be related to these observations. The key features of flux emergence and annihilation, as observed by the MDI experiment on SOHO, are reproduced in kinematic calculations, while three-dimensional numerical experiments reveal the dynamical processes that are involved. The pattern of convection depends on the strength of the magnetic field: as the mean field decreases, slender rising plumes give way to a regime where magnetic flux is separated from the motion and then to one where locally intense magnetic fields nestle between broad and vigorously convecting plumes. Moreover, turbulent convection is itself able to act as a small-scale dynamo, generating disordered fields near the solar surface. 相似文献
Recent high-resolution observations, from the ground and from space, have revealed the fine structure of magnetic features at the surface of the Sun. At the same time, advances in computing power have at last made it possible to develop models of turbulent magnetoconvection that can be related to these observations. The key features of flux emergence and annihilation, as observed by the MDI experiment on SOHO, are reproduced in kinematic calculations, while three-dimensional numerical experiments reveal the dynamical processes that are involved. The pattern of convection depends on the strength of the magnetic field: as the mean field decreases, slender rising plumes give way to a regime where magnetic flux is separated from the motion and then to one where locally intense magnetic fields nestle between broad and vigorously convecting plumes. Moreover, turbulent convection is itself able to act as a small-scale dynamo, generating disordered fields near the solar surface. 相似文献
7.
Abstract The static equilibrium of a thin vertical magnetic tube embedded in the solar atmosphere is shown to be dynamically unstable against the fundamental mode of perturbation having no nodes in the vertical displacement. The instability has its origin in the convection zone, and the eigenfunction is extended further up in the stable upper layers by the magnetic field which guides the displacement mainly in the longitudinal direction. It is suggested that the downdraft observed in the solar network structure is a finite amplitude consequence of this instability. The overtone modes are found to be stable. 相似文献
8.
We discuss the results of a study of microwave radiation from three flare-active regions??NOAA 10300, 10930, and 11158??with powerful eruptive events (X-class flares and coronal mass ejections) recorded on July 15, 2002; December 13, 2006; and February 15, 2011, when the regions were in the central part of the disk. There exists evidence of a ??-configuration in the structure of the photospheric magnetic field formed one or two days prior to the eruptive process as a result of the emergence of a new magnetic flux and shifting movements of the sunspots and accompanied by changes in the spectral characteristics of the microwave radiation of the active regions (ARs), which suggests the development of a peculiar radio source. The analysis of these regions continues a series of studies of eruptive events carried out at RATAN-600 in the 1980s?C1990s and gives a reason to conclude that early detections of peculiar sources in the microwave radiation of ARs, which are essentially areas of high energy release in the solar atmosphere, can be used as a factor in predicting powerful eruptive (geoeffective) processes on the Sun. 相似文献
9.
The generation and evolution of the Sun’s magnetic field and other stars is usually related to the dynamo mechanism. This mechanism is based on the consideration of the joint influence of the α effect and differential rotation. Dynamo sources can be located at different depths of the convection zone and can have different intensities. Based on such a system, the dynamical system in the case of the stellar dynamo in a two-layer medium has been constructed with regard to meridional fluxes in order to model the double cycle that corresponds to the simultaneous presence of 22-year and quasi-biennial magnetic field oscillations. It has been indicated that the regime of mixed oscillations can originate because a dynamo wave moves oppositely to the meridional flows in the upper layer of the convection zone. This results in the deceleration of the toroidal field propagation and in the generation of slow oscillations. In deeper layers, the directions of a dynamo wave and meridional flows coincide with each other, as a result of which fast magnetic fields originate. Therefore, the total contribution of two oscillations with different frequencies corresponds to the appearance of quasi-biennial cycles against 22-year cycles. It has been indicated that the beating regime, which can be related to the secular oscillations of solar magnetic activity, originates in the system when the meridional flows are weak. 相似文献
10.
V.V. Pipin 《地球物理与天体物理流体动力学》2013,107(1-2):185-206
We study the effect of turbulent drift of a large-scale magnetic field that results from the interaction of helical convective motions and differential rotation in the solar convection zone. The principal direction of the drift corresponds to the direction of the large-scale vorticity vector. Thus, the effect produces a latitudinal transport of the large-scale magnetic field in the convective zone wherever the angular velocity has a strong radial gradient. The direction of the drift depends on the sign of helicity and it is defined by the Parker–Yoshimura rule. The analytic calculations are done within the framework of mean-field magnetohydrodynamics using the minimal τ-approximation. We estimate the magnitude of the drift velocity and find that it can be a few m/s near the base of the solar convection zone. The implications of this effect for the solar dynamo are illustrated on the basis of an axisymmetric mean-field dynamo model with a subsurface shear layer. The model shows that near the bottom of the convection zone the helicity–vorticity pumping results mostly from the kinetic helicity contributions. We find that the magnetic helicity contributions to the pumping effect are dominant at the subsurface shear layer. There the magnitude of the drift velocity is found to be a few cm/s. We find that the helicity–vorticity pumping effect can have an influence on the features of the sunspot time–latitude diagram, producing a fast drift of the sunspot activity maximum at the rise phase of the cycle and a slow drift at the decay phase of the cycle. 相似文献
11.
Hadi Alkahby 《地球物理与天体物理流体动力学》2013,107(4):335-346
In this article, we examine reflection, dissipation and attenuation of vertically propagating waves in an isothermal atmosphere under the combined effect of Newtonian cooling, thermal conduction and viscosity with a weak horizontal magnetic field. We consider the case in which the combined effect of viscosity and magnetic field is dominated by that of the thermal conduction and for small values of the Newtonian parameter. As a result, the atmosphere can be divided into three distinct regions that are connected by two transition regions. The lower and middle regions are connected by a semi-transparent barrier and the middle and upper regions are connected by an absorbing and reflecting barrier. In the connecting barriers the reflection and transmission of the waves takes place. The presence of Newtonian cooling effects on the adiabatic region, produces attenuation in the amplitudes of the waves and reduces the energy absorption in the transition regions. The reflection coefficient is determined in the lower and middle regions and the results are discussed in the context of the heating of the solar atmosphere. 相似文献
12.
The problem of the transport and transformation of magnetic fields from the generation zone to the photosphere is studied in this paper. For this purpose, the temporal variations of parameters of bipolar magnetic regions are analyzed based on the magnetic synoptic maps of the Wilcox Solar Observatory (WSO) for the declining phase of cycle 22. A 150-day modulation of the magnetic flux value in bipolar regions and a variation in their rotation velocity with a duration of 80–100 days have been found. Such variations in the parameters are interpreted as a result of action of supergiant and giant convection cells. The magnetic flux from the generation zone emerges through the local channels formed by the supergiant convection cells. From the level of 0.95 RSun, the flux is redistributed by giant cells, which form bipolar magnetic regions on the photosphere. 相似文献
13.
We determine the nonlinear drift velocities of the mean magnetic field and nonlinear turbulent magnetic diffusion in a turbulent convection. We show that the nonlinear drift velocities are caused by three kinds of the inhomogeneities; i.e., inhomogeneous turbulence, the nonuniform fluid density and the nonuniform turbulent heat flux. The inhomogeneous turbulence results in the well-known turbulent diamagnetic and paramagnetic velocities. The nonlinear drift velocities of the mean magnetic field cause the small-scale magnetic buoyancy and magnetic pumping effects in the turbulent convection. These phenomena are different from the large-scale magnetic buoyancy and magnetic pumping effects which are due to the effect of the mean magnetic field on the large-scale density stratified fluid flow. The small-scale magnetic buoyancy and magnetic pumping can be stronger than these large-scale effects when the mean magnetic field is smaller than the equipartition field. We discuss the small-scale magnetic buoyancy and magnetic pumping effects in the context of the solar and stellar turbulent convection. We demonstrate also that the nonlinear turbulent magnetic diffusion in the turbulent convection is anisotropic even for a weak mean magnetic field. In particular, it is enhanced in the radial direction. The magnetic fluctuations due to the small-scale dynamo increase the turbulent magnetic diffusion of the toroidal component of the mean magnetic field, while they do not affect the turbulent magnetic diffusion of the poloidal field. 相似文献
14.
15.
M. Schüssler 《Annales Geophysicae》1999,17(5):578-582
Two aspects of solar MHD are discussed in relation to the work of the MHD simulation group at KIS. Photospheric magneto-convection, the nonlinear interaction of magnetic field and convection in a strongly stratified, radiating fluid, is a key process of general astrophysical relevance. Comprehensive numerical simulations including radiative transfer have significantly improved our understanding of the processes and have become an important tool for the interpretation of observational data. Examples of field intensification in the solar photosphere (‘convective collapse’) are shown. The second line of research is concerned with the dynamics of flux tubes in the convection zone, which has far-reaching implications for our understanding of the solar dynamo. Simulations indicate that the field strength in the region where the flux is stored before erupting to form sunspot groups is of the order of 105 G, an order of magnitude larger than previous estimates based on equipartition with the kinetic energy of convective flows. 相似文献
16.
Bashir W. Sharif 《地球物理与天体物理流体动力学》2013,107(6):493-511
In this article we study the linear instability of the two-dimensional strongly stratified model for global MHD in the diffusive solar tachocline. Gilman and Fox [Gilman, P.A. and Fox, P., Joint instability of the latitudinal differential rotation and toroidal magnetic fields below the solar convection zone. Astrophys. J., 1997, 484, 439–454] showed that for ideal MHD, the observed surface differential rotation becomes more unstable than is predicted by Watson's [Watson, M., Shear instability of differential rotation in stars. Geophys. Astrophys. Fluid Dyn., 1981, 16, 285–298] nonmagnetic analysis. They showed that the solar differential rotation is unstable for essentially all reasonable values of the differential rotation in the presence of an antisymmetric toroidal field. They found that for the broad field case B φ～sinθcosθ, θ being the co-latitude, instability occurs only for the azimuthal m?=?1 mode, and concluded that modes which are symmetric (meridional flow in the same direction) about the equator onset at lower field strengths than the antisymmetric modes. We study the effect of viscosity and magnetic diffusivity in the strongly stably stratified case where diffusion is primarily along the level surfaces. We show that antisymmetric modes are now strongly preferred over symmetric modes, and that diffusion can sometimes be destabilising. Even solid body rotation can be destabilised through the action of magnetic field. In addition, we show that when diffusion is present, instability can occur when the longitudinal wavenumber m?=?2. 相似文献
17.
K. G. Ivanov 《Geomagnetism and Aeronomy》2010,50(6):695-710
A more detailed scenario of one stage (August–November 2004) of the quasibiennial MHD process “Origination ... and dissipation
of the four-sector structure of the solar magnetic field” during the decline phase of cycle 23 has been constructed. It has
been indicated that the following working hypothesis on the propagation of an MHD disturbance westward (in the direction of
solar rotation) and eastward (toward the zone of active longitudes) with the displacement of the large-scale open solar magnetic
field (LOSMF) from this zone can be constructed based on LOSMF model representations and data on sunspot formation, flares,
active filaments, and coronal ejections as well as on the estimated contribution of sporadic energy release to the flare luminosity
and kinetic energy of ejections: (1) The “explosion” of the LOSMF singularity and the formation in the explosion zone of an
anemone active region (AR), which produced the satellite sunspot formation that continued west and east of the “anemone,”
represented a powerful and energy-intensive source of MHD processes at this stage. (2) This resulted in the origination of
two “governing” large-scale MHD processes, which regulated various usual manifestations of solar activity: the fast LOSMF
along the neutral line in the solar atmosphere, strongly affecting the zone of active longitudes, and the slow LOSMF in the
outer layers of the convection zone. The fronts of these processes were identified by powerful (about 1031 erg) coronal ejections. (3) The collision of a wave reflected from the zone of active longitudes with the eastern front of
the hydromagnetic impulse of the convection zone resulted in an increase in LOSMF magnetic fluxes, origination of an active
sector boundary in the zone of active longitudes, shear-convergent motions, and generation and destabilization of the flare-productive
AR 10696 responsible for the heliospheric storm of November 3–10, 2004. 相似文献
18.
文中选了５ 个典型活动区, 分析了这些活动区的磁场, 与活动区相应的ＣＭＥｓ, 太阳爆发事件和太阳质子事件我们发现, 对于Ｅ ≥１０ｍｅＶ 的太阳质子事件有相应的源活动区, 源耀斑和ＣＭＥ; 活动区矢量磁场有剪切, 磁场剪切越强质子事件越强; 多数在质子耀斑发生前出现磁流浮现; 太阳１０ｃｍ 射电爆发持续时间长文中结果还佐证了Ｓｈｅａｌｙ 等的结果： Ｘ 射线耀斑的长持续时间与ＣＭＥ 的发生正相关另外,在５ 个活动区中, 有三个大耀斑发生前没有明显的磁剪切作为它们的先兆, 它们是非质子源耀斑这是Ｍｏｏｒｅ, Ｈａｇｙａｒｄ 和Ｄａｖｉｓ 的磁场强剪切是耀斑产生的必要条件的反例 相似文献
19.
The velocity field of large-scale magnetic structures during fast reorganizations of the global solar magnetic field structure
has been analyzed. Some characteristic features of the velocity field have been found during these periods. At that time,
a considerable part of the solar surface is occupied by regions with low horizontal velocities, which correspond to the regions
of positive and negative velocity field divergence during the solar activity growth and decline phases, respectively. Such
character of changes in the velocity field during these periods agrees with the previously proposed scenario of magnetic field
variations during global reorganizations of the magnetic field structure. The average horizontal velocities during a Carrington
rotation and their divergence have been calculated for Carrington rotations from 1646 to 2006. Relatively slow regular variations
in these parameters as well as their abrupt changes, observed during different solar cycle phases, have been revealed. An
increase in the average horizontal velocity during the solar activity growth phase is most probably caused by relative motions
of the regions with a new emerging magnetic flux. We assume that abrupt increases in the average horizontal velocity divergence
are related to fast reorganizations of the magnetic field structure. 相似文献
20.
A. A. Spivak 《Izvestiya Physics of the Solid Earth》2010,46(4):327-338
Instrumental measurements of geophysical fields in several regions of the Earth’s crust with a complex structure and tectonics
are analyzed. The observed geophysical fields include the electric field in the boundary layer of the atmosphere and in the
subsurface crust, the ground magnetic field, and the fields formed by microseismic vibrations and natural radon emanation.
It is shown that the fault zones are characterized by noticeably higher (compared to the middle segments of crustal blocks)
variations in the geophysical fields, a stronger response to the faint external impacts in the form of lunisolar tides, and
baric variations in the atmosphere, as well as by higher intensity relaxation processes. Energy transformations between the
geophysical fields of different origins are observed predominantly in the fault regions. 相似文献