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1.
D. V. Erofeev 《Solar physics》1996,167(1-2):25-45
Discrete rigidly rotating components (modes) of the large-scale solar magnetic field have been investigated. We have used a specially calculated basic set of functions to resolve the observed magnetic field into discrete components. This adaptive set of functions, as well as the expansion coefficients, have been found by processing a series of digitized synoptic maps of the background magnetic field over a 20-year period. As a result, dependences have been obtained which describe the spatial structure and the temporal evolution of the 27-day and 28-day rigidly rotating modes of the Sun's magnetic field.The spatial structure of the modes has been compared with simulations based on the known flux-transport equation. In the simulations, the rigidly rotating modes were regarded as stationary states of the magnetic field whose rigid rotation and stability were maintained by a balance between the emergence of magnetic flux from stationary sources located at low latitudes and the horizontal transport of flux by turbulent diffusion and poleward directed meridional flow. Under these assumptions, the structure of the modes is determined solely by the horizontal velocity field of the plasma, except for the low-latitude zone where sources of magnetic flux concentrate. We have found a detailed agreement between the simulations and the results of the data analysis, provided that the amplitude of the meridional flow velocity and the diffusion constant are equal to 9.5 m s–1 and 600 km2 s–1, respectively.The analysis of the expansion coefficients has shown that the rigidly rotating modes undergo rapid step-like variations which occur quasi-periodically with a period of about two years. These variations are caused by separate surges of magnetic flux in the photosphere, so that each new surge gives rise to a rapid replacement of old large-scale magnetic structures by newly arisen ones.  相似文献   

2.
We have analyzed the effects that differential rotation and a hypothetical meridional flow would have on the evolution of the Sun's mean line-of-sight magnetic field as seen from Earth. By winding the large-scale field into strips of alternating positive and negative polarity, differential rotation causes the mean-field amplitude to decay and the mean-field rotation period to acquire the value corresponding to the latitude of the surviving unwound magnetic flux. For a latitudinally broad two-sector initial field such as a horizontal dipole, the decay is rapid for about 5 rotations and slow with a t –1/2 dependence thereafter. If a poleward meridional flow is present, it will accelerate the decay by carrying the residual flux to high latitudes where the line-of-sight components are small. The resulting decay is exponential with an e-folding time of 0.75 yr (10 rotations) for an assumed 15 m s–1 peak meridional flow speed.E.O. Hulburt Center for Space Research.Laboratory for Computational Physics.  相似文献   

3.
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   

4.
From a set of stellar spectropolarimetric observations, we report the detection of surface magnetic fields in a sample of four solar-type stars, namely HD 73350, HD 76151, HD 146233 (18 Sco) and HD 190771. Assuming that the observed variability of polarimetric signal is controlled by stellar rotation, we establish the rotation periods of our targets, with values ranging from 8.8 d (for HD 190771) to 22.7 d (for HD 146233). Apart from rotation, fundamental parameters of the selected objects are very close to the Sun's, making this sample a practical basis to investigate the specific impact of rotation on magnetic properties of Sun-like stars.
We reconstruct the large-scale magnetic geometry of the targets as a low-order  (ℓ < 10)  spherical harmonic expansion of the surface magnetic field. From the set of magnetic maps, we draw two main conclusions. (i) The magnetic energy of the large-scale field increases with rotation rate. The increase in chromospheric emission with the mean magnetic field is flatter than observed in the Sun. Since the chromospheric flux is also sensitive to magnetic elements smaller than those contributing to the polarimetric signal, this observation suggests that a larger fraction of the surface magnetic energy is stored in large scales as rotation increases. (ii) Whereas the magnetic field is mostly poloidal for low rotation rates, more rapid rotators host a large-scale toroidal component in their surface field. From our observations, we infer that a rotation period lower than ≈12 d is necessary for the toroidal magnetic energy to dominate over the poloidal component.  相似文献   

5.
Three-dimensional non-linear magnetoconvection in a strongly stratified compressible layer exhibits different patterns as the strength of the imposed magnetic field is reduced. There is a transition from a magnetically dominated regime, with small-scale convection in slender hexagonal cells, to a convectively dominated regime, with clusters of broad rising plumes that confine the magnetic flux to narrow lanes where fields are locally intense. Both patterns can coexist for intermediate field strengths, giving rise to flux separation: clumps of vigorously convecting plumes, from which magnetic flux has been excluded, are segregated from regions with strong fields and small-scale convection. A systematic numerical investigation of these different states shows that flux separation can occur over a significant parameter range and that there is also hysteresis. The results are related to the fine structure of magnetic fields in sunspots and in the quiet Sun.  相似文献   

6.
Numerical simulations of the Sun's mean line-of-sight magnetic field suggest an origin for the 28-to 29-day recurrent patterns of the field and its associated interplanetary phenomena. The patterns are caused by longitudinal fluctuations in the eruption of new magnetic flux, the transport of this flux to mid latitudes by supergranular diffusion and meridional flow, and the slow rotation of the resulting flux distributions at the 28- to 29-day periods characteristic of those latitudes.E. O. Hulburt Center for Space Research.Laboratory for Computational Physics.  相似文献   

7.
We have followed disappearing elements of magnetic flux to determine the smallest elements detectable with the Big Bear videomagnetograph. All the elements followed were disappearing through interaction with elements of opposite polarity. The last remaining visible segment of magnectic field of such features can be used to infer the total magnetic flux of these and other small flux elements visible on the magnetograms.We used both photographic and digital videomagnetograms combining 4096 Zeeman frames made at Big Bear. Fifteen elements were measured near the vanishing point, in a 2–8 hr period. The minimum observable fluxes fall in the range of 1.0 × 1016 to 1.4 × 1017 Mx, and the apparent size of these elements is in the range of 1 to 9 square arc sec. The process of disappearance appears to be a smooth one. The smallest detectable elements of network field and ephemeral regions (ER) appear to be the same as the small intra-network (IN) field elements. The present limit is still instrumental; elements smaller than 1 × 1016 would not have been detected.Visiting Associates from Beijing Observatory, Academia Sinica, Beijing, China.  相似文献   

8.
We compare the shape and position of some plasma formations visible in the polar corona with the cyclic evolution of the global magnetic field. The first type of object is polar crown prominences. A two-fold decrease of the height of polar crown prominences was found during their poleward migration from the middle latitudes to the poles before a polar magnetic field reversal. The effect could be assigned to a decrease of the magnetic field scale. The second type of object is the polar plumes, ray like structures that follow magnetic field lines. Tangents to polar ray structures are usually crossed near some point, “a magnetic focus,” below the surface. The distance q between the focus and the center of the solar disk changes from the maximum value about 0.65 R ⊙ at solar minimum activity to the minimum value about 0.45 R ⊙ at solar maximum. At first glance this behaviour seems to be contrary to the dynamics of spherical harmonics of the global magnetic field throughout a cycle. We believe that the problem could be resolved if one takes into account not only scale changes in the global magnetic field but also the phase difference in the cyclic variations of large-scale and small-scale components of the global field.  相似文献   

9.
An autocorrelation of the direction of the large-scale photospheric magnetic field observed during 1959–1967 has yielded evidence that the field structure at some heliographic latitudes can display both differential rotation and rigid rotation properties.  相似文献   

10.
Durrant  C.J.  Mccloughan  J. 《Solar physics》2004,219(1):55-78
We describe the application of the synoptic transport equation to simulate the temporal evolution of the magnetic flux over the solar surface. This provides a means of predicting each day both the synoptic maps for the Carrington rotation starting the next day and the instantaneous map of the solar flux over the whole solar surface for the next day. The reliability of the predicted synoptic maps is tested by comparing the locations of the zero-flux contour with those of the observed maps produced by the National Solar Observatory, Kitt Peak and with the locations of Hα filaments measured on filtergrams obtained by the Big Bear Solar Observatory. We conclude that the best match at high latitudes is obtained by long-term simulations (over 20 rotations) with flux updates each rotation between latitudes ± 60°. We illustrate the use of the simulations to describe the evolution of the polar fields at the time of the polarity reversals in Cycle 23. The reconstruction of the instantaneous maps is tested by comparison with full-disk magnetograms. The method provides a simple means of estimating the large-scale flux distribution over the whole surface. It does not take account of flux emerging after the central meridian passage each rotation so it is only approximate in the activity belts but provides a reliable map beyond those latitudes.  相似文献   

11.
We present a straightforward comparison of model calculations for the α-effect, helicities, and magnetic field line twist in the solar convection zone with magnetic field observations at atmospheric levels. The model calculations are carried out in a mixing-length approximation for the turbulence with a profile of the solar internal rotation rate obtained from helioseismic inversions. The magnetic field data consist of photospheric vector magnetograms of 422 active regions for which spatially-averaged values of the force-free twist parameter and of the current helicity density are calculated, which are then used to determine latitudinal profiles of these quantities. The comparison of the model calculations with the observations suggests that the observed twist and helicity are generated in the bulk of the convection zone, rather than in a layer close to the bottom. This supports two-layer dynamo models where the large-scale toroidal field is generated by differential rotation in a thin layer at the bottom while the α-effect is operating in the bulk of the convection zone. Our previous observational finding was that the moduli of the twist factor and of the current helicity density increase rather steeply from zero at the equator towards higher latitudes and attain a certain saturation at about 12 – 15∘. In our dynamo model with algebraic nonlinearity, the increase continues, however, to higher latitudes and is more gradual. This could be due to the neglect of the coupling between small-scale and large-scale current and magnetic helicities and of the latitudinal drift of the activity belts in the model.  相似文献   

12.
This study aims to quantify characteristic features of the bipolar flux appearance of solar intranetwork (IN) magnetic elements. To attack this problem, we use the Narrowband Filter Imager (NFI) magnetograms from the Solar Optical Telescope (SOT) on board Hinode; these data are from quiet and enhanced network areas. Cluster emergence of mixed polarities and IN ephemeral regions (ERs) are the most conspicuous forms of bipolar flux appearance within the network. Each of the clusters is characterized by a few well-developed ERs that are partially or fully coaligned in magnetic axis orientation. On average, the sampled IN ERs have a total maximum unsigned flux of several 1017 Mx, a separation of 3 – 4 arcsec, and a lifetime of 10 – 15 minutes. The smallest IN ERs have a maximum unsigned flux of several 1016 Mx, separations of less than 1 arcsec, and lifetimes as short as 5 minutes. Most IN ERs exhibit a rotation of their magnetic axis of more than 10 degrees during flux emergence. Peculiar flux appearance, e.g., bipole shrinkage followed by growth or the reverse, is not unusual. A few examples show repeated shrinkage–growth or growth–shrinkage, like magnetic floats in the dynamic photosphere. The observed bipolar behavior seems to carry rich information on magnetoconvection in the subphotospheric layer.  相似文献   

13.
Choudhuri and Gilman (1987) considered certain implications of the hypothesis that the magnetic flux within the Sun is generated at the bottom of the convection zone and then rises through it. Taking flux rings symmetric around the rotation axis and using reasonable values of different parameters, they found that the Coriolis force deflects these flux rings into trajectories parallel to the rotation axis so that they emerge at rather high latitudes. This paper looks into the question of whether the action of the Coriolis force is subdued when the initial configuration of the flux ring has non-axisymmetries in the form of loop structures. The results depend dramatically on whether the flux ring with the loops lies completely within the convection zone or whether the lower parts of it are embedded in the stable layers underneath the convection zone. In the first case, the Coriolis force supresses the non-axisymmetric perturbations so that the flux ring tends to remain symmetric and the trajectories are very similar to those of Choudhuri and Gilman (1987). In the second case, however, the lower parts of the flux ring may remain anchored underneath the bottom of the convection zone, but the upper parts of the loops still tend to move parallel to the rotation axis and emerge at high latitudes. Thus the problem of the magnetic flux not being able to come out at the sunspot latitudes still persists after the non-axisymmetries in the flux rings are taken into account.National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

14.
We study the zonal flow in solar subsurface layers, analyzing about six years of GONG++ high-resolution Doppler data with ring-diagram analysis. We focus on the variation of zonal flow with magnetic activity over a range of depths from the surface to about 16 Mm. There is a positive correlation between unsigned magnetic flux and zonal flow at most depths. We calculate the average zonal flow for a quiet- and an active-region subset defined as dense-pack locations with an unsigned magnetic flux less than 3.4 G and locations with greater than 65.0 G, respectively. The average zonal flow of active regions is about 4 m s−1 larger than the average flow of quiet regions. This difference increases slightly with increasing depth, which might be explained by a nonradial inclination of the flux tubes or a different extent in depth of different magnetic features. The difference shows no apparent pattern in time and latitude, which makes it unlikely that it is simply a manifestation of the torsional-oscillation pattern. As a byproduct, we find that the size of the North – South asymmetry of the rotation rate decreases during the same epoch.  相似文献   

15.
Horizontal large-scale velocity field describes horizontal displacement of the photospheric magnetic flux in zonal and meridian directions. The flow systems of solar plasma, constructed according to the velocity field, create the large-scale cellular-like patterns with up-flow in the center and the down-flow on the boundaries. Distribution of the largescale horizontal eddies (with characteristic scale length from 350 to 490 Mm) was found in the broad equatorial zone, limited by 60&amp;#x2021; latitude circles on both hemispheres. The zonal averages of the zonal and meridian velocities, and the total horizontal velocity for each Carrington rotation during the activity cycles no. 21 and 22 varies during the 11-yr activity cycle. Plot of RMS values of total horizontal velocity is shifted about 1.6 years before the similarly shaped variation of the magnetic flux.  相似文献   

16.
Kitchatinov  L.L.  Mazur  M.V. 《Solar physics》2000,191(2):325-340
We analyse stability and equilibrium of a unipolar large-scale magnetic field pervading a plane horizontal subphotospheric layer with the possible implications for sunspots in mind. Eddy diffusivity is applied to account for the effects of the small-scale convective turbulence. Diffusivity quenching by magnetic field results in a secondary large-scale instability. A linear stability analysis is performed to define the marginal stability boundary in parametric space and the unstable mode structure. The nonlinear dynamics of the unstable modes are followed numerically. The original state of a uniform vertical magnetic field is transformed via the instability into the nonlinear dynamical equilibrium with a highly intermittant distribution of the magnetic field. Magnetic flux is concentrated in a relatively small area surrounded by an almost field-free region. The role of the fluid motion in the hydromagnetic equilibrium is emphasized. Although the relevance of the instability to the process of sunspot formation is rather questionable, the resulting equilibrium structures are similar to mature spots in their thermal and magnetic properties. Also, the simulated flow structure agrees with helioseismic tomography results.  相似文献   

17.
S. Latushko 《Solar physics》1996,166(2):261-266
A study is made of the rotation of large-scale magnetic fields using the synoptic maps from the Kitt Peak National Observatory for the time interval 1976–1985. The auto-correlation method and the mass-centers method of magnetic structures was applied to infer mean differential rotation profiles and rotation profiles separately for each magnetic field polarity. It has been found that in both hemispheres the leading polarity rotates faster than the following polarity at all latitudes by about 0.04° day–1. The maximum rotation rate of the leading polarity is reached at about 6° latitude. In the mean profile for both polarities, this brings about two angular velocity maxima at 6° latitudes in both hemispheres. Such a profile appears as to have a dimple on the equator.  相似文献   

18.
Erofeev  D.V. 《Solar physics》1999,186(1-2):431-447
Large-scale distribution of the sunspot activity of the Sun has been analyzed by using a technique worked out previously (Erofeev, 1997) to study long-lived, non-axisymmetric magnetic structures with different periods of rotation. Results of the analysis have been compared with those obtained by analyzing both the solar large-scale magnetic field and large-scale magnetic field simulated by means of the well-known flux transport equation using the sunspot groups as a sole source of new magnetic flux in the photosphere. A 21-year period (1964–1985) has been examined.The rotation spectra calculated for the total time interval of two 11-year cycles indicate that sunspot activity consists of a series of discrete components (modes) with different periods of rotation. The largest-scale component of the sunspot activity reveals modes with 27-day and 28-day periods of rotation situated, correspondingly, in the northern and southern hemispheres of the Sun, and two modes with rotation periods of about 29.7 days situated in both hemispheres. Such a modal structure of the sunspot activity agrees well with that of the large-scale solar magnetic field. Moreover, the magnetic field distribution simulated with the flux transport equation also reveals the same modal structure. However, such an agreement between the large-scale solar magnetic field and both the sunspot activity and simulated magnetic field is unstable in time; so, it is absent in the northern hemisphere of the Sun during solar cycle No. 20. Thus the sources of magnetic flux responsible for formation of the large-scale, rigidly rotating magnetic patterns appear to be closely connected, but are not identical with the discrete modes of the sunspot activity.  相似文献   

19.
In the absence of new bipolar sources of flux, the large-scale magnetic field at the solar photosphere decays due to differential rotation, meridional flow, and supergranular diffusion. The rotational shear quickly winds up the nonaxisymmetric components of the field, increasing their latitudinal gradients and thus the rates of diffusive mixing of their flux. This process is particularly effective at mid latitudes, where the rotational shear is largest, so that eventually low- and high-latitude remnants of the initial, nonaxisymmetric field pattern survive. In this paper I solve analytically the transport equation describing the evolution of the large-scale photospheric field, to study its time-asymptotic behavior. The solutions are rigidly rotating, uniformly decaying distributions of flux, wound up by differential rotation and localized near either the equator or the poles. A balance between azimuthal transport of flux by the rotational shear and meridional transport by the diffusion gives rise to the rigidly rotating field patterns. The time-scale on which this balance is achieved, and also on which the nonaxisymmetric flux decays away, is the geometric mean of the short time-scale for shearing by differential rotation and the long time-scale for dispersal by supergranular diffusion. A poleward meridional flow alters this balance on its own, intermediate time-scale, accelerating the decay of the nonaxisymmetric flux at low latitudes. Such a flow also hastens the relaxation of the axisymmetric field to a modified dipolar configuration.  相似文献   

20.
A numerical model of axisymmetric convection in the presence of a vertical magnetic flux bundle and rotation about the axis is presented. The model contains a compressible plasma described by the non-linear MHD equations, with density and temperature gradients simulating the upper layer of the Sun's convection zone. The solutions exhibit a central magnetic flux tube in a cylindrical numerical domain, with convection cells forming collar flows around the tube. When the numerical domain is rotated with a constant angular velocity, the plasma forms a Rankine vortex, with the plasma rotating as a rigid body where the magnetic field is strong, as in the flux tube, while experiencing sheared azimuthal flow in the surrounding convection cells, forming a free vortex. As a result, the azimuthal velocity component has its maximum value close to the outer edge of the flux tube. The azimuthal flow inside the magnetic flux tube and the vortex flow is prograde relative to the rotating cylindrical reference frame. A retrograde flow appears at the outer wall. The most significant convection cell outside the flux tube is the location for the maximum value of the azimuthal magnetic field component. The azimuthal flow and magnetic structure are not generated spontaneously, but decay exponentially in the absence of any imposed rotation of the cylindrical domain.  相似文献   

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