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1.
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
2.
We have studied the evolution of several high-latitude flux `plumes', i.e., unipolar regions, trailing from active regions which emerged near sunspot maximum in cycle 23. The observed patterns are compared with simulations using a simple flux transport equation based on the observed flux for an earlier Carrington rotation. In addition to the long recognized poleward migration and diffusion of flux from active regions, it is found that the evolution of the trailing plumes may be influenced by flux which emerges above latitude 35° over areas of all scales. We describe two cases in which the emerging flux appears in the form of bipolar flux patterns which are not obviously related to sunspots. Further, we find instances in which the observed surface flux decreases or spreads at rates which cannot be explained solely in terms of diffusion using the normally accepted rates. Thus in several cases the poleward migration of flux cannot be described in terms of passive transport by advection and diffusion as considered here, and further investigation of the processes that contribute to the evolution of the polar fields is required. 相似文献
3.
A. R. Yeates 《Solar physics》2014,289(2):631-648
Coupled flux transport and magneto-frictional simulations are extended to simulate the continuous magnetic-field evolution in the global solar corona for over 15 years, from the start of Solar Cycle 23 in 1996. By simplifying the dynamics, our model follows the build-up and transport of electric currents and free magnetic energy in the corona, offering an insight into the magnetic structure and topology that extrapolation-based models cannot. To enable these extended simulations, we have implemented a more efficient numerical grid, and have carefully calibrated the surface flux-transport model to reproduce the observed large-scale photospheric radial magnetic field, using emerging active regions determined from observed line-of-sight magnetograms. This calibration is described in some detail. In agreement with previous authors, we find that the standard flux-transport model is insufficient to simultaneously reproduce the observed polar fields and butterfly diagram during Cycle 23, and that additional effects must be added. For the best-fit model, we use automated techniques to detect the latitude–time profile of flux ropes and their ejections over the full solar cycle. Overall, flux ropes are more prevalent outside of active latitudes but those at active latitudes are more frequently ejected. Future possibilities for space-weather prediction with this approach are briefly assessed. 相似文献
4.
To explain the observed intermingling of polarities in the magnetic field distributions of rapidly rotating stars, surface magnetic flux transport models demand the presence of fast meridional flows.We combine simulations of the pre-eruptive and post-eruptive magnetic flux transport in cool stars to investigate the influence of a fast meridional circulation on the latitudinal eruption pattern of magnetic flux tubes and on the polar magnetic field properties. Magnetic flux tubes rising through the convection zone experience an enhanced latitude-dependent poleward deflection through meridional flows, which renders the wings of stellar butterfly diagrams convex. The larger amount of magnetic flux emerging at higher latitudes supports the intermingling of opposite polarities of polar magnetic fields and yields magnetic flux densities in the polar regions about 20% higher than in the case disregarding the pre-eruptive deflection. Taking the pre-eruptive evolution of magnetic flux into account therefore eases the need for the fast meridional flows predicted by previous investigations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
5.
We define for observational study two subsets of all polar zone filaments, which we call polemost filaments and polar filament bands. The behavior of the mean latitude of both the polemost filaments and the polar filament bands is examined and compared with the evolution of the polar magnetic field over an activity cycle as recently distilled by Howard and LaBonte (1981) from the past 13 years of Mt. Wilson full-disk magnetograms. The magnetic data reveal that the polar magnetic fields are built up and maintained by the episodic arrival of discrete f-polarity regions that originate in active region latitudes and subsequently drift to the poles. After leaving the active-region latitudes, these unipolar f-polarity regions do not spread equatorward even though there is less net flux equatorward; this indicates that the f-polarity regions are carried poleward by a meridional flow, rather than by diffusion. The polar zone filaments are an independent tracer which confirms both the episodic polar field formation and the meridional flow. We find:
- The mean latitude of the polemost filaments tracks the boundary of the polar field cap and undergoes an equatorward dip during each arrival of additional polar field.
- Polar filament bands track the boundary latitudes of the unipolar regions, drifting poleward with the regions at about 10 m s-1.
- The Mt. Wilson magnetic data, combined with a simple model calculation, show that the filament drift expected from diffusion alone would be slower than observed, and in some cases would be equatorward rather than poleward.
- The observation that filaments drift poleward along with the magnetic regions shows that fields of both polarities are carried by the meridional flow, as would be expected, rather than only the f-polarity flux which dominates the strength. This leads to the prediction that in the mid-latitudes during intervals between the passage of f-polarity regions, both polarities are present in nearly equal amounts. This prediction is confirmed by the magnetic data.
6.
Observations of the first large-scale patterns of magnetic fields near the sunspot minimum of 1986 (the start of cycle 22) are presented using synoptic magnetic data provided by the National Solar Observatory and contour maps constructed from data provided by the Mount Wilson Solar Observatory. The latter are compared with simulated contour maps derived from numerical solutions of the flux transport equation using data from particular Carrington rotations as initial conditions.The simulated evolutions of the large-scale magnetic fields are qualitatively consistent with observed evolutions, but differ in several significant respects. Some of the differences can be removed by varying the diffusivity and the parameters of the large-scale velocity fields. The remaining differences include: (i) the complexity of fine structure, (ii) the response to differential rotation, (iii) the evolution of decaying active regions, and (iv) the emergence of new elements in the weak, large-scale fields independent of the evolution of the observed active regions.It is concluded that the patterns of weak magnetic fields which comprise the large-scale features cannot be formed entirely by the diffusive decay of active regions. There must be a significant contribution to these patterns by non-random flux eruptions within the network structure, independent of active regions. 相似文献
7.
V. Holzwarth D. H. Mackay M. Jardine 《Monthly notices of the Royal Astronomical Society》2006,369(4):1703-1718
Observations of rapidly rotating solar-like stars show a significant mixture of opposite-polarity magnetic fields within their polar regions. To explain these observations, models describing the surface transport of magnetic flux demand the presence of fast meridional flows. Here, we link subsurface and surface magnetic flux transport simulations to investigate (i) the impact of meridional circulations with peak velocities of ≤125 m s−1 on the latitudinal eruption pattern of magnetic flux tubes and (ii) the influence of the resulting butterfly diagrams on polar magnetic field properties. Prior to their eruption, magnetic flux tubes with low field strengths and initial cross-sections below ∼300 km experience an enhanced poleward deflection through meridional flows (assumed to be polewards at the top of the convection zone and equatorwards at the bottom). In particular, flux tubes which originate between low and intermediate latitudes within the convective overshoot region are strongly affected. This latitude-dependent poleward deflection of erupting magnetic flux renders the wings of stellar butterfly diagrams distinctively convex. The subsequent evolution of the surface magnetic field shows that the increased number of newly emerging bipoles at higher latitudes promotes the intermingling of opposite polarities of polar magnetic fields. The associated magnetic flux densities are about 20 per cent higher than in the case disregarding the pre-eruptive deflection, which eases the necessity for fast meridional flows predicted by previous investigations. In order to reproduce the observed polar field properties, the rate of the meridional circulation has to be of the order of 100 m s−1 , and the latitudinal range from which magnetic flux tubes originate at the base of the convective zone (≲50°) must be larger than in the solar case (≲35°). 相似文献
8.
Observations of X-ray bright points (XBP) over a six-month interval in 1973 show significant variations in both the number density of XBP as a function of heliographic longitude and in the full Sun average number of XBP from one rotation to the next. The observed increases in XBP emergence are estimated to be quivalent to several large active regions emerging per day for several months. The number of XBP emerging at high latitudes also varies, in phase with the low latitude variation and reaches a maximum approximately simultaneous with a major outbreak of active regions. The quantity of magnetic flux emerging in the form of XBP at high latitudes alone is estimated to be as large as the contribution from all active regions.Harvard College Observatory/Smithsonian Astrophysical Observatory. 相似文献
9.
Surface magnetic fields during the solar activity cycle 总被引：1，自引：0，他引：1
We examine magnetic field measurements from Mount Wilson that cover the solar surface over a 13 1/2 year interval, from 1967 to mid-1980. Seen in long-term averages, the sunspot latitudes are characterized by fields of preceding polarity, while the polar fields are built up by a few discrete flows of following polarity fields. These drift speeds average about 10 m s-1 in latitude - slower early in the cycle and faster later in the cycle - and result from a large-scale poleward displacement of field lines, not diffusion. Weak field plots show essentially the same pattern as the stronger fields, and both data indicate that the large-scale field patterns result only from fields emerging at active region latitudes. The total magnetic flux over the solar surface varies only by a factor of about 3 from minimum to a very strong maximum (1979). Magnetic flux is highly concentrated toward the solar equator; only about 1% of the flux is at the poles. Magnetic flux appears at the solar surface at a rate which is sufficient to create all the flux that is seen at the solar surface within a period of only 10 days. Flux can spread relatively rapidly over the solar surface from outbreaks of activity. This is presumably caused by diffusion. In general, magnetic field lines at the photospheric level are nearly radial.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands. 相似文献
10.
C. Richard DeVore 《Solar physics》1987,112(1):17-35
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
11.
P. R. Wilson 《Solar physics》1992,138(1):11-21
Observations of the first major active regions and large-scale magnetic field patterns of Cycle 22 are presented. These show that, following the emergence of a trans-equatorial pattern, or cell, of positive flux related to old cycle activity, the first new cycle active regions of the longitude range emerged across the neutral lines of this cell, which continued to grow and expand across the equator for several rotations. The development of a parallel trans-equatorial band of flux of opposite (negative) polarity and the emergence of both new and old cycle active regions across a neutral line of this cell are also described.Simulations using the flux transport equation, and based on synoptic magnetic data provided by the Mount Wilson Observatory, show that, while the growth of the positive region could, in part, be explained by the decay of flux from these new regions, there were significant differences between synoptic contour charts based on the simulations and those constructed from the observed fields. They also show that the development of the negative region cannot reasonably be explained by the decay of the observed active regions.A further example of the counter rotation of decaying active region fields is reported. Here the initial tilt of the negative-positive magnetic axes of two adjacent regions is normal, and simulations based on these data show their combined follower flux moving preferentially polewards. However, the observations show that, after three rotations, the decaying leader flux is entirely poleward of the follower flux.On leave from the School of Mathematics, University of Sydney. 相似文献
12.
C. R. DeVore N. R. Sheeley Jr. J. P. Boris T. R. Young Jr. K. L. Harvey 《Solar physics》1985,102(1-2):41-49
We simulate the evolution of several observed solar active regions by solving a transport equation for magnetic flux at the photosphere. The rates of rotation, meridional flow, and diffusion of the flux are determined self-consistently in the calculations. Our findings are in good quantitative agreement with previous measures of the rotation rate and diffusion constant associated with photospheric magnetic fields. Although our meridional velocities are consistent in direction and magnitude with recently reported poleward flows, relatively large uncertainties in our velocity determinations make this result inconclusive.Laboratory for Computational Physics.E. O. Hulburt Center for Space Research. 相似文献
13.
The structure and variations of open field regions (OFRs) are analyzed against the solar cycle for the time interval of 1970–1996. The cycle of the large-scale magnetic field (LSMF) begins in the vicinity of maximum Wolf numbers, i.e. during the polar field reversal. At the beginning of the LSMF cycle, the polar and mid-latitude magnetic field systems are connected by a narrow bridge, but later they evolve independently. The polar field at the latitudes above 60° has a completely open configuration and fills the whole area of the polar caps near the cycle minimum of local fields. At this time, essentially all of the open solar flux is from the polar caps. The mid-latitude open field regions (OFRs) occur at a latitude of 30–40° away from solar minimum and drift slowly towards the equator to form a typical 'butterfly diagram' at the periphery of the local field zone. This supports the concept of a single complex – 'large-scale magnetic field – active region – coronal hole'. The rotation characteristics of OFRs have been analyzed to reveal a near solid-body rotation, much more rigid than in the case of sunspots. The rotation characteristics are shown to depend on the phase of the solar cycle. 相似文献
14.
Y.-M. Wang 《Solar physics》2004,224(1-2):21-35
The Sun’s large-scale external field is formed through the emergence of magnetic flux in active regions and its subsequent
dispersal over the solar surface by differential rotation, supergranular convection, and meridional flow. The observed evolution
of the polar fields and open flux (or interplanetary field) during recent solar cycles can be reproduced by assuming a supergranular
diffusion rate of 500 – 600 km2 s−1 and a poleward flow speed of 10 –20 m s−1. The nonaxisymmetric component of the large-scale field decays on the flow timescale of ∼1 yr and must be continually regenerated
by new sunspot activity. Stochastic fluctuations in the longitudinal distribution of active regions can produce large peaks
in the Sun’s equatorial dipole moment and in the interplanetary field strength during the declining phase of the cycle; by
the same token, they can lead to sudden weakenings of the large-scale field near sunspot maximum (Gnevyshev gaps). Flux transport
simulations over many solar cycles suggest that the meridional flow speed is correlated with cycle amplitude, with the flow
being slower during less active cycles. 相似文献
15.
In this paper the origin and evolution of the Sun's open magnetic flux is considered by conducting magnetic flux transport simulations over many solar cycles. The simulations include the effects of differential rotation, meridional flow and supergranular diffusion on the radial magnetic field at the surface of the Sun as new magnetic bipoles emerge and are transported poleward. In each cycle the emergence of roughly 2100 bipoles is considered. The net open flux produced by the surface distribution is calculated by constructing potential coronal fields with a source surface from the surface distribution at regular intervals. In the simulations the net open magnetic flux closely follows the total dipole component at the source surface and evolves independently from the surface flux. The behaviour of the open flux is highly dependent on meridional flow and many observed features are reproduced by the model. However, when meridional flow is present at observed values the maximum value of the open flux occurs at cycle minimum when the polar caps it helps produce are the strongest. This is inconsistent with observations by Lockwood, Stamper and Wild (1999) and Wang, Sheeley, and Lean (2000) who find the open flux peaking 1–2 years after cycle maximum. Only in unrealistic simulations where meridional flow is much smaller than diffusion does a maximum in open flux consistent with observations occur. It is therefore deduced that there is no realistic parameter range of the flux transport variables that can produce the correct magnitude variation in open flux under the present approximations. As a result the present standard model does not contain the correct physics to describe the evolution of the Sun's open magnetic flux over an entire solar cycle. Future possible improvements in modeling are suggested. 相似文献
16.
High-resolution magnetograph observations of the polar magnetic fields have been obtained at intervals of time since the end of 1986 at Big Bear Solar Observatory. The Big Bear data differ from the low-resolution, full-disk magnetograph observations in that the 2 arc sec resolution makes it possible to resolve concentrated field upward of 100 G. The purpose of this ongoing observation is to examine the evolution of polar fields during the expected polarity reversal as cycle 22 passes its maximum phase, and secondly, to study the polar magnetic field: its true field strength, distribution, and how it compares to other parts of the quiet Sun.We find that the >70° net polar flux of both poles has not reversed as of the end of 1989. However, in the lower latitudes of both poles, 50° to 70°, there are signs reminiscent of those preceding the reversals in cycles 19 and 20. These include: decreasing field intensity in the old polarity fluctuations in net flux between the old and new polarities.We find that the net average longitudinal polar fields (above 50°) are 1–2 G, in agreement with results found in cycles 19 and 20. For individual elements, however, the strongest observed field strength poleward of 70° is over 100 G.We compare the polar fields with the equatorial limb as a function of latitute and longitude, respectively, and find the polar fields are comparable to (or stronger than) the quiet equatorial limb. When the observed mean flux density of the polar field as a function of latitude is corrected for limb-darkening and projection effects (assuming the field is radial), the result is nearly constant. These results suggest that despite the high latitudes, the polar fields have field strength and distribution similar to other parts of the quiet Sun. 相似文献
17.
Models of the polarity reversals of the Sun's polar magnetic fields based on the surface transport of flux are discussed and are tested using observations of the polar fields during Cycle 23 obtained by the National Solar Observatory at Kitt Peak. We have extended earlier measurements of the net radial flux polewards of ±60° and confirm that, despite fluctuations of 20%, there is a steady decline in the old polarity polar flux which begins shortly after sunspot minimum (although not at the same time in each hemisphere), crosses the zero level near sunspot maximum, and increases, with reversed polarity during the remainder of the cycle. We have also measured the net transport of the radial field by both meridional flow and diffusion across several latitude zones at various phases of the Cycle. We can confirm that there was a net transport of leader flux across the solar equator during Cycle 23 and have used statistical tests to show that it began during the rising phase of this cycle rather than after sunspot maximum. This may explain the early decrease of the mean polar flux after sunspot minimum. We also found an outward flow of net flux across latitudes ±60° which is consistent with the onset of the decline of the old polarity flux. Thus the polar polarity reversals during Cycle 23 are not inconsistent with the surface flux-transport models but the large empirical values required for the magnetic diffusivity require further investigation. 相似文献
18.
We present recent 3-D MHD numerical simulations of the non-linear dynamical evolution of magnetic flux tubes in an adiabatically stratified convection zone in spherical geometry, using the anelastic spherical harmonic (ASH) code.We seek to understand the mechanism of emergence of strong toroidal fields from the base of the solar convection zone to the solar surface as active regions. We confirm the results obtained in cartesian geometry that flux tubes that are not twisted split into two counter vortices before reaching the top of the convection zone. Moreover, we find that twisted tubes undergo the poleward-slip instability due to an unbalanced magnetic curvature force which gives the tube a poleward motion both in the non-rotating and in the rotating case. This poleward drift is found to be more pronounced on tubes originally located at high latitudes. Finally, rotation is found to decrease the rise velocity of the flux tubes through the convection zone, especially when the tube is introduced at low latitudes. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
19.
J. O. Stenflo 《Solar physics》1974,36(2):495-515
The differential rotation and sector structure of solar magnetic fields has been studied using digitized data on photospheric magnetic fields recorded at the Mount Wilson Observatory during the period August 1959–May 1970. The power spectra show considerable power in high-frequency peaks, corresponding to harmonic components with wavelengths less than 1/10 solar rotation. Calculations for a series of shorter time intervals show how the distribution of power over the various harmonic components in the sector pattern varies strongly with the solar cycle. The equatorial rotation rate of solar magnetic fields is about 0.1 km s-1 faster than that of the photospheric plasma determined from Doppler shifts. It is shown that the Doppler measurements mainly refer to the non-network regions. The differential flow of 0.1 km s-1 forms streamlines around the magnetic fine structures. The different rotation rates of various solar features can be explained in terms of the rotation rates of magnetic and non-magnetic regions. The rotation rates of the magnetic fields in active and quiet regions agree at the equator. At higher latitudes, however, the background fields deviate less from solid-body rotation, indicating that their source is below the deepest layers to which the sunspot magnetic fields penetrate. This suggests that turbulent diffusion of the field in old active regions may not be the main source for the background magnetic field, but that the source is located close to a rigidly rotating solar core with a synodic rotation period of 26.87 days. 相似文献
20.
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