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1.
The emergence of magnetic flux 总被引：1，自引：0，他引：1
Cornelis Zwaan 《Solar physics》1985,100(1-2):397-414
This paper first summarizes the morphology and dynamics of emerging flux regions and arch filament systems and then discusses detailed observations of a particular active region with emerging magnetic flux.The central part of the growing active region shows abnormal granulation and a weak magnetic field that, locally, is transverse. In the border zone, strong downward flows occur in the chromopshere and photosphere (small features with strong magnetic fields (faculae, pores) are formed here.) Near the leading and following edge, sunspots are formed by the coalescence of such small magnetic elements.The observational data are interpreted by means of a heuristic model of an emergent magnetic loop-shaped bundle consisting of many flux tubes. In this model we incorporate the theory of convective collapse and the buoyancy of flux tubes. The observed complexity in the structure and dynamics, including strong transverse fields and velocity shear, is attributed to the emergence of several flux regions within the active region at different orientations. 相似文献
2.
P. J. Bushby S. M. Houghton M. R. E. Proctor N. O. Weiss 《Monthly notices of the Royal Astronomical Society》2008,387(2):698-706
Kilogauss-strength magnetic fields are often observed in intergranular lanes at the photosphere in the quiet Sun. Such fields are stronger than the equipartition field B e , corresponding to a magnetic energy density that matches the kinetic energy density of photospheric convection, and comparable with the field B p that exerts a magnetic pressure equal to the ambient gas pressure. We present an idealized numerical model of three-dimensional compressible magnetoconvection at the photosphere, for a range of values of the magnetic Reynolds number. In the absence of a magnetic field, the convection is highly supercritical and characterized by a pattern of vigorous, time-dependent, 'granular' motions. When a weak magnetic field is imposed upon the convection, magnetic flux is swept into the convective downflows where it forms localized concentrations. Unless this process is significantly inhibited by magnetic diffusion, the resulting fields are often much greater than B e and the high magnetic pressure in these flux elements leads to their being partially evacuated. Some of these flux elements contains ultraintense magnetic fields that are significantly greater than B p . Such fields are contained by a combination of the thermal pressure of the gas and the dynamic pressure of the convective motion, and they are constantly evolving. These ultraintense fields develop owing to non-linear interactions between magnetic fields and convection; they cannot be explained in terms of 'convective collapse' within a thin flux tube that remains in overall pressure equilibrium with its surroundings. 相似文献
3.
4.
A number of independent arguments indicate that the toroidal flux system responsible for the sunspot cycle is stored at the
base of the convection zone in the form of flux tubes with field strength close to 105 G. Although the evidence for such strong fields is quite compelling, how such field strength can be reached is still a topic
of debate. Flux expulsion by convection should lead to about the equipartition field strength, but the magnetic energy density
of a 105-G field is two orders of magnitude larger than the mean kinetic energy density of convective motions. Line stretching by
differential rotation (i.e., the “Ω effect” in the classical mean-field dynamo approach) probably plays an important role, but arguments based on energy considerations show that it does not seem feasible
that a 105-G field can be produced in this way. An alternative scenario for the intensification of the toroidal flux system in the overshoot
layer is related to the explosion of rising, buoyantly unstable magnetic flux tubes, which opens a complementary mechanism for magnetic-field intensification.
A parallelism is pointed out with the mechanism of “convective collapse” for the intensification of photospheric magnetic
flux tubes up to field strengths well above equipartition; both mechanisms, which are fundamentally thermal processes, are
reviewed. 相似文献
5.
The process of active region formation was researched by analyzing the densities of electric current and electric current helicity in the photosphere. The observational data were obtained with the vector magnetograph of the Sayan observatory. The appearance (as the sunspot developed) of the part of current helicity which is determined by the vertical components of the magnetic field and electric current density was studied. It is concluded that the loop-like magnetic flux tube which is responsible for the active region emergence contains thinner tubes with the same structure. The electric current system in a sunspot is simplified as the sunspot forms perhaps because the thinner flux tubes are merged together. 相似文献
6.
D. W. Hughes S. A. E. G. Falle & P. Joarder 《Monthly notices of the Royal Astronomical Society》1998,298(2):433-444
Sunspots are caused by the eruption of magnetic flux tubes through the solar photosphere: current theories of the internal magnetic field of the Sun suggest that such tubes must rise relatively unscathed from the base of the convection zone. In order to understand how the structure of the magnetic field within a buoyant flux tube affects its stability as it rises, we have considered the quasi-two-dimensional rise of isolated magnetic flux tubes through an adiabatically stratified atmosphere. The magnetic field is initially helical; we have investigated a range of initial field configurations, varying the distribution and strength of the twist of the field. 相似文献
7.
Magnetic buoyancy is thought to play an important role in the dynamical behavior of the Sun's magnetic field in the convection zone. Magnetic buoyancy is commonly thought to cause inescapable rapid loss of toroidal flux from much of the convection zone, thereby suppressing effective operation of a solar dynamo. This paper re-examines the detailed character of magnetic buoyancy, especially as it is influenced by the magnetic field's effect on heat transport and temperature gradients in the convection zone. It is suggested that suppression of convective heat transport across strong magnetic flux tubes can alter the temperature within the tubes and can subdue, or even reverse, the effect of magnetic buoyancy. 相似文献
8.
9.
Helen Valentine Will Saunders Andy Taylor 《Monthly notices of the Royal Astronomical Society》2000,319(2):L13-L17
We study the influence of different magnetic boundary conditions on the generation of magnetic fields by turbulent convection. It is found that the structure and strength of the generated field in the vicinity of the boundary is strongly dependent on the choice of boundary conditions. In the convective interior, however, the solutions remain largely insensitive to the boundary conditions. In all cases the overall efficiency of the dynamo process remains high with a steady state magnetic energy density between 12 and 25 per cent of the turbulent kinetic energy, and peak field values exceeding the equipartition level. These results support the idea that the solar granulation may constitute a dynamo source for magnetic fields in the quiet photosphere. 相似文献
10.
R. Muller 《Solar physics》1985,100(1-2):237-255
The observed properties of the small-scale features visible in the quiet photosphere — the granulation, of convective origin, and the network bright points, associated with kG magnetic fields — are described. The known properties of the magnetic flux tubes associated with network bright points are also presented. Empirical models derived from the observations are discussed, as well as a few theoretical models of particular importance for the understanding of the origin of the small-scale features of the quiet photosphere. Finally, the observational evidences showing that the structure of the granulation and of the photospheric network are varying over the solar cycle are reported. 相似文献
11.
We review recent progress and define unanswered scientific questions in five related topics: granulation- to supergranulation-scale convection and magnetic structures; global convection and circulation; the rise of magnetic flux tubes to the photosphere, and their injection into the base of the convection zone; tachocline fluid dynamics and MHD; and the solar dynamo. We close with a set of observational `targets' for helioseismologists to aim for. 相似文献
12.
We present a series of numerical simulations of the quiet-Sun plasma threaded by magnetic fields that extend from the upper
convection zone into the low corona. We discuss an efficient, simplified approximation to the physics of optically thick radiative
transport through the surface layers, and investigate the effects of convective turbulence on the magnetic structure of the
Sun’s atmosphere in an initially unipolar (open field) region. We find that the net Poynting flux below the surface is on
average directed toward the interior, while in the photosphere and chromosphere the net flow of electromagnetic energy is
outward into the solar corona. Overturning convective motions between these layers driven by rapid radiative cooling appears
to be the source of energy for the oppositely directed fluxes of electromagnetic energy. 相似文献
13.
We present a method to include stereoscopic information about the three-dimensional structure of flux tubes into the reconstruction of the coronal magnetic field. Due to the low plasma beta in the corona we can assume a force-free magnetic field, with the current density parallel to the magnetic field lines. Here we use linear force-free fields for simplicity. The method uses the line-of-sight magnetic field on the photosphere as observational input. The value of is determined iteratively by comparing the reconstructed magnetic field with the observed structures. The final configuration is the optimal linear force-free solution constrained by both the photospheric magnetogram and the observed plasma structures. As an example we apply our method to SOHO MDI/EIT data of an active region. In the future it is planned to apply the method to analyse data from the SECCHI instrument aboard the STEREO mission. 相似文献
14.
We discuss the dynamical interpretation of evidence for an azimuthal tilt of the global magnetic field from the radial direction at the photosphere. We point out that the Reynolds stresses of supergranular convective motions might produce the required small tilt of intense flux tubes, without implying an unacceptably large momentum flux across the photospheric surface into the solar wind. Our calculations lead us to conclude that there is little reason, at present, to infer (Duvall et al., 1979) a separate low intensity constituent of the global magnetic field, from the observational evidence for an azimuthal tilt. More precise measurements of the vertical component of supergranular motions would be useful in determining the actual torque exerted by the Reynolds stresses on the magnetic field. 相似文献
15.
We present a combined model for magnetic field generation and transport in cool stars with outer convection zones. The mean toroidal magnetic field, which is generated by a cyclic thin-layer α Ω dynamo at the bottom of the convection zone is taken to determine the emergence probability of magnetic flux tubes in the photosphere. Following the nonlinear rise of the unstable thin flux tubes, emergence latitudes and tilt angles of bipolar magnetic regions are determined. These quantities are put into a surface flux transport model, which simulates the surface evolution of magnetic flux under the effects of large-scale flows and turbulent diffusion. First results are discussed for the case of the Sun and for more rapidly rotating solar-type stars. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
16.
William J. Merryfield 《Solar physics》1990,128(1):305-320
The continual emergence of magnetic flux in solar active regions suggests that a substantial reservoir of flux is present somewhere beneath the photosphere. It has been proposed that this flux could be stored in an azimuthal field of order 3000 G residing in the lower portion of the convection zone. Such a field may be large enough to substantially influence the dynamics of the convection: linear stability analyses indicate that donut-like convective rolls having azimuthal symmetry might then be preferred to banana cells aligned with the rotation axis. Observational detections of such azimuthal rolls have been claimed.The problem of pattern selection by convection in the presence of rotation and a horizontal magnetic field is examined here in a model system consisting of a planar Boussinesq fluid layer. Nonlinear solutions are obtained numerically. It is found that solutions consisting solely of donut cells can exist even at parameter values at which linear theory suggests that banana cells should be preferred instead. However, when the horizontal field decays below a critical value, banana cells may then grow. This leads to the destruction of the horizontal field and a permanent transition to banana cells. 相似文献
17.
K. Marubashi 《Planetary and Space Science》1979,27(5):603-615
Dynamic behavior of the coupled ionosphere-protonosphere system in the magnetospheric convection electric field has been theoretically studied for two plasmasphere models. In the first model, it is assumed that the whole plasmasphere is in equilibrium with the underlying ionosphere in a diurnal average sense. The result for this model shows that the plasma flow between the ionosphere and the protonosphere is strongly affected by the convection electric field as a result of changes in the volume of magnetic flux tubes associated with the convective cross-L motion. Since the convection electric field is assumed to be directed from dawn to dusk, magnetic flux tubes expand on the dusk side and contract on the dawn side when rotating around the earth. The expansion of magnetic flux tubes on the dusk side causes the enhancement of the upward H+ flow, whereas the contraction on the dawn side causes the enhancement of the downward H+ flow. Consequently, the H+ density decreases on the dusk side and increases on the dawn side. It is also found that significant latitudinal variations in the ionospheric structures result from the L-dependency of these effects. In particular, the H+ density at 1000 km level becomes very low in the region of the plasmasphere bulge on the dusk side. In the second model, it is assumed that the outer portion of the plasmasphere is in the recovery state after depletions during geomagnetically disturbed periods. The result for this model shows that the upward H+ flux increases with latitude and consequently the H+ density decreases with latitude in the region of the outer plasmasphere. In summary, the present theoretical study provides a basis for comparison between the equatorial plasmapause and the trough features in the topside ionosphere. 相似文献
18.
Observations have consistently pointed out that the longitudinal and latitudinal motions of sunspots are correlated. The magnitude of the covariance was found to increase with latitude, and its sign was found to be positive in the N-hemisphere and negative in the S-hemisphere. This correlation was believed to be due to the underlying turbulence where the sunspot flux tubes are anchored, and the covariance had the right sign and magnitude needed to explain the transfer of angular momentum toward the equator through Reynolds stresses.Here we present an alternate explanation for these sunspot velocity correlations: It is believed that the dynamo operates in a thin overshoot layer beneath the base of the convection zone, and the flux tubes generated there produce sunspots at the photosphere. By studying the dynamics of flux tubes emerging from the base of the convection zone to the photosphere, we show that these velocity correlations of sunspots could be merely a consequence of the effect of Coriolis force on rising flux tubes. The effect of the Coriolis force, as demonstrated by even a back-of-the-envelope calculation, is to push the faster rotating spots equatorward and the slower rotating spots poleward, giving rise to a correlation in their longitudinal and latitudinal velocities, which is positive in the N-hemisphere and negative in the S-hemisphere. The increase in the correlation with latitude is due to the increase in magnitude of the Coriolis force. Hence we show that these velocity correlations might have nothing to do with the Reynolds stresses of the underlying turbulence.We present analyses of observations, and show that the covariances of plages are an order of magnitude higher than the sunspot covariances. If plages and sunspots share the same origin, and if their horizontal velocity correlations are wholly due to the effect of Coriolis force on rising flux tubes, then the study of their dynamics suggests that the flux tubes that form plages should have diameters of a couple of thousand km at the base of the convection zone and remain intact until they reach the photosphere, whereas sunspots should be formed by a collection of small flux tubes (each measuring about a hundred km in diameter), that rise through the convection zone as individual elements and coalesce when they emerge through the photosphere.Operated by the Association of Universities for Research in Astronomy, Inc., under Cooperative Agreement with the National Science Foundation. 相似文献
19.
A model of the interaction of the solar wind with Venus is proposed including magnetic barrier formation, ionopause structure, plasma dynamics in the magnetic barrier, and the formation of the Venusian tail (wake). It is shown that under stationary conditions the ionopause is practically an equipotential boundary and its current is determined by a diamagnetic drift. The source of the plasma mantle can be provided by photoions appearing in the magnetic barrier and convecting toward the wake as a result of both magnetic pressure gradient and magnetic tension. The formation of the magnetic tail is determined by convection of magnetic barrier flux tubes in which the solar-wind plasma is replaced by ions of planetary origin. Compared to observational data the proposed model gives somewhat overestimated values of ion convective velocity and magnetic barrier thickness near the terminator and underestimated values of number density and magnetic field strength in the tail. Accordingly this suggests the possible influence of the anomalous ionization effects in the solar wind—Venus interaction. 相似文献
20.
The source of coronal magnetic energy and helicity lies below the surface of the Sun, probably in the convective zone dynamo. Measurements of magnetic and velocity fields can capture the fluxes of both magnetic energy and helicity crossing the photosphere. We point out the ambiguities which can occur when observations are used to compute these fluxes. In particular, we show that these fluxes should be computed only from the horizontal motions deduced by tracking the photospheric cut of magnetic flux tubes. These horizontal motions include the effect of both the emergence and the shearing motions whatever the magnetic configuration complexity is. We finally analyze the observational difficulties involved in deriving such fluxes, in particular the limitations of the correlation tracking methods. 相似文献