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1.
The revised Mount Wilson synoptic magnetic data for the period September 1987 through March 1996 are used as the basis of numerical simulations of the evolution of both the northern and southern polar magnetic fields during the reversal and declining phases of cycle 22. The simulations are based on numerical solutions of the flux-transport equation which involve, as parameters, the maximum meridional flow speed, v
0, and the supergranule diffusivity, . By matching characteristics of the observed and simulated fields, such as the observed reversal times, the evolution of the net flux above 60 °, and the migration of the polar crown, empirical values of these parameters, i.e., v
0=11 m s–1,=600 km2 s–1, may be determined. Further, the observed decrease in the mean net flux above 60 ° during the late declining phase of cycle 22 can be simulated only by increasing the diffusivity to 900 km2 s–1. However, direct observations of the supergranule velocities yield values of the diffusivity of order 200 km2 s–1, and we show that the inclusion of a pattern of emerging bipoles in the simulations can increase the diffusion of these fields and that, together with a more realistic value of the diffusivity, it is possible to reproduce qualitatively the features of the observed polar field reversals. 相似文献
2.
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
3.
We trace the photospheric motions of 170 concentrations of magnetic flux tubes in and around the decaying active region No. 19824 (CMP 23 October 1986), using a series of magnetograms obtained at the Big Bear Solar Observatory. The magnetograms span an interval of just over five days and cover an area of about 4 × 5 arc min centered on the active region. We find a persistent large-scale flow pattern that is superposed on the small-scale random motions of both polarities. Correction for differential rotation unveils the systematic, large-scale flow surrounding the core region of the magnetic plage. The flow (with a mean velocity of 30 m s–1) is faster and more pronounced around the southern side of the core region than around the northern side, and it accelerates towards the western side of the active region. The northern and southern branches of the large-scale flow converge westward of the core region, dragging along the westernmost sunspot and some of the magnetic flux near it. The overall pattern of the large-scale flow resembles the flow of a river around a sand bar. The long-term evolution of the active region suggests that the flow persists for several months. We discuss the possible association of the large-scale flow with the torsional oscillation.We correct the observed motions of concentrations of flux tubes for the large-scale flow in order to study their random motions. The small-scale random motions (with a mean speed of 150 m s–1) can be characterized by a diffusion coefficient of 250 km2 s–1 for the area surrounding the core region of the magnetic plage. The diffusion coefficient characterizing the small-scale motions within the core region (mostly observed near its periphery and in areas of relatively low flux density) is only 110 km2 s–1. The lower diffusion coefficient in the core region appears to be caused mainly by a smaller step length rather than by a distinct difference in velocities.Visitor at the Lockheed Palo Alto Research Laboratories. 相似文献
4.
Autocorrelation and cross-correlation techniques have been applied to obtain quantitative information about the dynamics of magnetic flux on the solar surface. The speed of network magnetic elements and the diffusion coefficient associated with their random motion is derived. The speed is found to be about 0.1 km s–1, independent of activity level. However, the diffusion coefficient shows a strong activity dependence: it is about 370–500 km2 s–1in the quiet network and 135–210 km2 s–1in the enhanced network. It is found that the lifetime of the enhanced network relative to the quiet network is compatible with that suggested by a comparison of their respective diffusion coefficients. This supports the proposition that a diffusion-like dispersion of magnetic flux is the dominant factor in the large-scale, long-term evolution of the network. 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.
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
7.
We calculate analytical and numerical solutions to the magnetic flux transport equation in the absence of new bipolar sources of flux, for several meridional flow profiles and a range of peak flow speeds. We find that a poleward flow with a broad profile and a nominal 10 m s–1 maximum speed concentrates the large-scale field into very small caps of less than 15° half-angle, with average field strengths of several tens of gauss, contrary to observations. A flow which reaches its peak speed at a relatively low latitude and then decreases rapidly to zero at higher latitudes leads to a large-scale field pattern which is consistent with observations. For such a flow, only lower latitude sunspot groups can contribute to interhemispheric flux annihilation and the resulting decay and reversal of the polar magnetic fields. 相似文献
8.
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
9.
The evolution of the velocity and magnetic fields associated with supergranulation has been investigated using the Sacramento Peak Observatory Diode Array Magnetograph. The observations consist of time sequences of simultaneous velocity, magnetic field, and chromospheric network measurements. From these data it appears that the supergranular velocity cells may have lifetimes in excess of the accepted value of 24 hours. Magnetic field motions associated with supergranulation were infrequent and seem to be accompanied by changes in the velocity field. More prevalent were the slow dissipation and diffusion of stationary flux points. Vertical velocity fields of 200 m s–1 appear to be confined to downflows in magnetic field regions at supergranular boundaries. These downflows are only observed using certain absorption lines. Corresponding upflows in the center of supergranules of less than 50 m s–1 may be present but cannot be confirmed. 相似文献
10.
An Evolving Synoptic Magnetic Flux map and Implications for the Distribution of Photospheric Magnetic Flux 总被引：1，自引：0，他引：1
We describe a procedure intended to produce accurate daily estimates of the magnetic flux distribution on the entire solar surface. Models of differential rotation, meridional flow, supergranulation, and the random emergence of background flux elements are used to regularly update unobserved or poorly observed portions of an initial traditional magnetic synoptic map that acts as a seed. Fresh observations replace model estimates when available. Application of these surface magnetic transport models gives us new insight into the distribution and evolution of magnetic flux on the Sun, especially at the poles where canopy effects, limited spatial resolution, and foreshortening result in poor measurements. We find that meridional circulation has a considerable effect on the distribution of polar magnetic fields. We present a modeled polar field distribution as well as time series of the difference between the northern and southern polar magnetic flux; this flux imbalance is related to the heliospheric current sheet tilt. We also estimate that the amount of new background magnetic flux needed to sustain the `quiet-Sun' magnetic field is about 1.1×1023 Mx d–1 (equivalent to several large active regions) at the spatial resolution and epoch of our maps. We comment on the diffusive properties of supergranules, ephemeral regions, and intranetwork flux. The maps are available on the NSO World Wide Web page. 相似文献
11.
Harold Zirin 《Solar physics》1987,110(1):101-107
We discuss the weak solar magnetic fields as studied with the BBSO videomagnetograph (VMG). By weak fields we mean those outside active and unipolar regions. These are found everywhere on the Sun, even where there never have been sunspots. These fields consist of the network and intranetwork (IN) elements. The former move slowly and live a day or more; the latter move rapidly (typically 300 m s–1) and live only hours. To all levels of sensitivity the flux is concentrated in discrete elements, and the background field has not been detected. The smallest detectable elements at present are 1016 Mx. The IN elements emerge in bipolar form but appear to flow in a random pattern rather than to the network edges; however, any expanding network element is constrained by geometry to move toward the edges.Because of the great number and short lifetime of the IN elements the total flux emerging in that form exceeds that emerging in the ER by two orders of magnitude and the flux in sunspots, by a factor 104. However, the flux separation is small and there is no contribution to the overall field. In contrast with our earlier results, merging of IN fields is more important than the ephemeral regions as a source of new network elements.The conjecture that all solar magnetic fields are intrinsically strong is discussed and evidence pro and con presented. For the IN fields the evidence suggests they cannot exceed 100 G. For the network fields there is evidence on either side.Reconnection and merging of magnetic fields takes place continually in the conditions studied.Because there is a steady state distribution, the amout of new elements created by merging or emergence must balance that destroyed by reconnection or fission and diffusion of the stronger elements.Solar Cycle Workshop Paper. 相似文献
12.
A. Khlystova 《Solar physics》2013,284(2):343-361
The dynamics of horizontal plasma flows during the first hours of the emergence of active region magnetic flux in the solar photosphere have been analyzed using SOHO/MDI data. Four active regions emerging near the solar limb have been considered. It has been found that extended regions of Doppler velocities with different signs are formed in the first hours of the magnetic flux emergence in the horizontal velocity field. The flows observed are directly connected with the emerging magnetic flux; they form at the beginning of the emergence of active regions and are present for a few hours. The Doppler velocities of flows observed increase gradually and reach their peak values 4?–?12 hours after the start of the magnetic flux emergence. The peak values of the mean (inside the ±?500 m?s?1 isolines) and maximum Doppler velocities are 800?–?970 m?s?1 and 1410?–?1700 m?s?1, respectively. The Doppler velocities observed substantially exceed the separation velocities of the photospheric magnetic flux outer boundaries. The asymmetry was detected between velocity structures of leading and following polarities. Doppler velocity structures located in a region of leading magnetic polarity are more powerful and exist longer than those in regions of following polarity. The Doppler velocity asymmetry between the velocity structures of opposite sign reaches its peak values soon after the emergence begins and then gradually drops within 7?–?12 hours. The peak values of asymmetry for the mean and maximal Doppler velocities reach 240?–?460 m?s?1 and 710?–?940 m?s?1, respectively. An interpretation of the observable flow of photospheric plasma is given. 相似文献
13.
Results of a detailed study on supergranule lifetime and velocity fields are presented. We show the correlation between the observed downdraft velocity and the network magnetic flux elements on the quiet sun. After excluding areas with magnetic flux density 25 G, we find that the upper limit of the supergranule vertical speed is 0.1 km s–1 for both downdraft and updraft, and the r.m.s. speed is 0.03 km s–1. By observing the evolution of individual supergranules, we find that the average lifetime of supergranules might be 50 hours. We describe different ways of formation and decay of supergranular cells. New cells usually form in an area containing no pre-existing supergranule velocity fields. Cells may disappear in two ways: fragmentation and fading away. 相似文献
14.
Haimin Wang 《Solar physics》1988,116(1):1-16
To obtain quantitative temporal and spatial information on the network magnetic fields, we applied auto- and cross-correlation techniques to the Big Bear videomagnetogram (VMG) data. The average size of the network magnetic elements derived from the auto-correlation curve is about 5700 km. The distance between the primary and secondary peak in the auto-correlation curve is about 17000 km, which is half of the size of the supergranule as determined from the velocity map. The correlation time is about 10 to 20 hours. The diffusion constant derived from the cross-correlation curve is 150 km2 s-1. We also found that in the quiet regions the total magnetic flux in a window 3 × 4 changes very little in nearly 10 hours. That means the emergence and the disappearance of magnetic flux are in balance. The cancelling features and the emergence of ephemeral regions are the major sources for the loss and replenishment of magnetic flux on the quiet Sun. 相似文献
15.
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
16.
Robert Howard 《Solar physics》1983,82(1-2):437-437
A series of digitized synoptic observations of solar magnetic and velocity fields has been carried out at the Mount Wilson Observatory since 1967. In recent studies (Howard and LaBonte, 1980; LaBonte and Howard, 1981), the existence of slow, large-scale torsional (toroidal) oscillations of the Sun has been demonstrated. Two modes have been identified. The first is a travelling wave, symmetric about the equator, with wave number 2 per hemisphere. The pattern-alternately slower and faster than the average rotation-starts at the poles and drifts to the equator in an interval of 22 years. At any one latitude on the Sun, the period of the oscillation is 11 years, and the amplitude is 3 m s-1. The magnetic flux emergence that is seen as the solar cycle occurs on average at the latitude of one shear zone of this oscillation. The amplitude of the shear is quite constant from the polar latitudes to the equator. The other mode of torsional oscillation, superposed on the first mode, is a wave number 1 per hemisphere pattern consisting of faster than average rotation at high latitudes around solar maximum and faster than average rotation at low latitudes near solar minimum. The amplitude of the effect is about 5 m s-1. For the first mode, the close relationship in latitude between the activity-related magnetic flux eruption and the torsional shear zone suggests strongly that there is a close connection between these motions and the cycle mechanism. It has been suggested (Yoshimura, 1981; Schüssler, 1981) that the effect is caused by a subsurface Lorentz force wave resulting from the dynamo action of magnetic flux ropes. But, this seems unlikely because of the high latitudes at which the shear wave is seen to originate and the constancy of the magnitude of the shear throughout the life time of the wave. 相似文献
17.
It is a basic feature of the Babcock-Leighton model of the solar cycle that the polar field reversal is due to the diffusive decay and poleward drift of the active region fields. The flux from follower regions moves preferentially polewards in each hemisphere, where it cancels with, and then replaces, the previously existing polar fields. A number of workers have attempted to model this process by numerical solutions of the flux transport equation, which include the surface effects of supergranule diffusion, differential rotation and meridional flow, with conflicting results.Here we describe recent changes in the polar fields using synoptic magnetic data provided by the Mount Wilson Observatory, and compare them with simulations using the flux transport equation and based on the observed fields for Carrington rotation 1815. These changes include a part-reversal of the north polar field. It is shown that the evolution of the polar fields cannot be reproduced accurately by simulations of the diffusion and poleward drift of the emerging active regions at sunspot latitudes.Histograms of the distribution of the field intensities derived from the daily magnetograms obtained at the Kitt Peak Station of the National Solar Observatory provide independent evidence that flux is emerging at high latitudes and that this flux makes a contribution to the evolution of these patterns. This implies the presence of some form of sub-surface dynamo action at high latitudes.On leave from the School of Mathematics, University of Sydney. 相似文献
18.
In this paper we study the evolution of magnetic fields of a 1F/2.4C solar flare and following magnetic flux cancellation. The data are Big Bear Solar Observatory and SOHO/MDI observations of active region NOAA 8375. The active region produced a multitude of subflares, many of them being clustered along the moat boundary in the area with mixed polarity magnetic fields. The study indicates a possible connection between the flare and the flux cancellation. The cancellation rate, defined from the data, was found to be 3×1019 Mx h–1. We observed strong upward directed plasma flows at the cancellation site. Suggesting that the cancellation is a result of reconnection process, we also found a reconnection rate of 0.5 km s–1, which is a significant fraction of Alfvén speed. The reconnection rate indicates a regime of fast photospheric reconnection happening during the cancellation. 相似文献
19.
Properties of a latitude zonal component of the large-scale solar magnetic field are analyzed on the basis of H charts for 1905–1982. Poleward migration of prominences is used to determine the time of reversal of the polar magnetic field for 1870–1905. It is shown that in each hemisphere the polar, middle latitude and equatorial zones of the predominant polarity of large-scale magnetic field can be detected by calculating the average latitude of prominence samples referred to one boundary of the large-scale magnetic field. The cases of a single and three-fold polar magnetic field reversal are investigated. It is shown that prominence samples referred to one boundary of the large-scale magnetic field do not have any regular equatorward drift. They manifest a poleward migration with a variable velocity up to 30 m s-1 depending on the phase of the cycle. The direction of migration is the same for both low-latitude and high-latitude zones. Two different time intervals of poleward migration are found. One lasts from the beginning of the cycle to the time of polar magnetic field reversal and the other lasts from the time of reversal to the time of minimum activity. The velocity of poleward migration of prominences during the first period is from 5 m s-1 to 30 m s-1 and the second period is devoid of regular latitude drift. 相似文献
20.
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