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1.
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
2.
Observations of the reappearance of polar coronal holes and the reversal of the polar magnetic field
We examine observations relating to the evolution of the polar magnetic field around sunspot maximum, when the net polar flux reverses polarity and coronal holes redevelop around the poles. Coronal hole observations during the last two solar maxima are examined in detail. Long-term averages of the latitudinal dependence of the photospheric magnetic field and the evolutionary pattern of the polar crown filaments are used to trace the poleward motion of the reversal of the large-scale surface field, and are compared to the redevelopment of the polar holes. The polar holes evolve from small, mid-latitude holes of new-cycle polarity which expand poleward until they join and cover the pole. We find that the appearance of these mid-latitude holes, the peak of flux emergence at low latitudes, and the polar polarity reversal all occur within a few solar rotations. Lagging 6 months to 1 1/2 yr after this time, the polar crown disappears and the polar holes redevelop.These results are examined in the context of phenomenological models of the solar cycle. We believe the following results in particular must be accounted for in successful models of the solar cycle: (1) The process of polarity reversal and redevelopment of the polar holes is discontinuous, occurring in 2 or 3 longitude bands, with surges of flux of old-cycle polarity interrupting the poleward migration of new-cycle flux. There is a persistent asymmetry in these processes between the two hemispheres; the polarity reversal in the two hemispheres is offset by 6 months to 1 1/2 yr. (2) Contrary to the Babcock hypothesis, the polar crown disappears months after the magnetic polar reversal. We suggest one possible scenario to explain this effect. (3) Our observations support suggestions of a poleward meridional flow around solar maximum that cannot be accounted for by Leighton-type diffusion. 相似文献
3.
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
4.
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
5.
Regarding new bipolar magnetic regions as sources of flux, we have computed the evolution of the photospheric magnetic field during 1976–1984 and derived the corresponding evolution of the mean line-of-sight field as seen from Earth. We obtained a good, but imperfect, agreement between the observed mean field and the field computed for a nominal choice of flux transport parameters. Also, we determined the response of the computed mean field to variations in the transport parameters and the source properties. The results lead us to regard the mean-field evolution as a random-walk process with dissipation. New eruptions of flux produce the random walk, and together differential rotation, meridional flow (if present), and diffusion provide the dissipation. The net effect of each new source depends on its strength and orientation (relative to the strength and orientation of the mean field) and on the time elapsed before the next eruption (relative to the decay time of the field). Thus the mean field evolves principally due to the contributions of the larger sources, which produce a strong, gradually evolving field near sunspot maximum but a weak, sporadically evolving field near sunspot minimum.E. O. Hulburt Center for Space Research.Laboratory for Computational Physics. 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.
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
8.
Valery P. Mikhailutsa 《Solar physics》1995,159(1):29-44
The evolution of the background magnetic field with the solar cycle has been studied using the dipole-quadrupole magnetic energy behaviour in a cycle. The combined energy of the axisymmetric dipole, non-axisymmetric quadrupole, and equatorial dipole is relatively lowly variable over the solar cycle. The dipole field changed sign when the quadrupole field was near a maximum, andvice versa. A conceptual picture involving four meridional magnetic polarity sectors proposed to explain these features may be in agreement with equatorial coronal hole observations. The rate of sector rotation is estimated to be 8 heliographic degrees per year faster than the Carrington rotation (P = 27.23d synodic). Polarity boundaries of sectors located 180° apart show meridional migrations in one direction, while the boundaries of the other two sectors move in the opposite direction. A simple model of how the magnetic field energy varies, subject to specifying reasonable initial photospheric magnetic and velocity field patterns, follows the observed evolution of the dipole and quadrupole field energies quite nicely. 相似文献
9.
In order to understand whether major flares or coronal mass ejections (CMEs) can be related to changes in the longitudinal photospheric magnetic field, we study 4 young active regions during seven days of their disk passage. This time period precludes any biases which may be introduced in studies that look at the field evolution during the short-term flare or CME period only. Data from the Michelson Doppler Imager (MDI) with a time cadence of 96 min are used. Corrections are made to the data to account for area foreshortening and angle between line of sight and field direction, and also the underestimation of the flux densities. We make a systematic study of the evolution of the longitudinal magnetic field, and analyze flare and CME occurrence in the magnetic evolution. We find that the majority of CMEs and flares occur during or after new flux emergence. The flux in all four active regions is observed to have deviations from polarity balance both on the long term (solar rotation) and on the short term (few hours). The long-term imbalance is not due to linkage outside the active region; it is primarily related to the east–west distance from central meridian, with the sign of polarity closer to the limb dominating. The sequence of short-term imbalances are not closely linked to CMEs and flares and no permanent imbalance remains after them. We propose that both kinds of imbalance are due to the presence of a horizontal field component (parallel to the photospheric surface) in the emerging flux. 相似文献
10.
Using Fe ix/x 17.1 nm observations from the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO), we have identified many coronal plumes inside low-latitude coronal holes as they transited the solar limb during the late
declining phase of cycle 23. These diffuse, linear features appear to be completely analogous to the familiar polar plumes.
By tracking them as they rotate from the limb onto the disk (or vice versa), we confirm that EUV plumes seen against the disk appear as faint, diffuse blobs of emission surrounding a brighter core.
When the EIT images are compared with near-simultaneous magnetograms from the SOHO Michelson Doppler Imager (MDI), the low-latitude, on-disk plumes are found to overlie regions of mixed polarity, where small
bipoles are in contact with unipolar flux concentrations inside the coronal hole. The birth and decay of the plumes are shown
to be closely related to the emergence of ephemeral regions, their dispersal in the supergranular flow field, and the cancellation
of the minority-polarity flux against the dominant-polarity network elements. In addition to the faint polar and nonpolar
plumes associated with ephemeral regions, we note the existence of two topologically similar coronal structures: the giant
plume-like features that occur above active regions inside coronal holes, and the even larger scale “pseudostreamers” that
separate coronal holes of the same polarity. In all three cases, the basic structure consists of open field lines of a given
polarity overlying a photospheric region of the opposite polarity; ongoing interchange reconnection at the X-point separating
the open field domains from the underlying double-arcade system appears to result in the steady evaporation of material from
the closed into the open region. 相似文献
11.
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
12.
Regarding new bipolar magnetic regions as sources of flux, we have simulated the evolution of the radial component of the solar photospheric magnetic field during 1976–1984 and derived the corresponding evolution of the line-of-sight polar fields as seen from Earth. The observed timing and strength of the polar-field reversal during cycle 21 can be accounted for by supergranular diffusion alone, for a diffusion coefficient of 800 km2 s-1. For an assumed 300 km2 s-1 rate of diffusion, on the other hand, a poleward meridional flow with a moderately broad profile and a peak speed of 10 m s-1 reached at about 5° latitude is required to obtain agreement between the simulated and observed fields. Such a flow accelerates the transport of following-polarity flux to the polar caps, but also inhibits the diffusion of leading-polarity flux across the equator. For flows faster than about 10 m s-1 the latter effect dominates, and the simulated polar fields reverse increasingly later and more weakly than the observed fields.Laboratory for Computational Physics and Fluid Dynamics.E. O. Hulburt Center for Space Research. 相似文献
13.
The customary notion that high-latitude filaments arise from magnetic flux originating in the active-region belts finds its modern expression in numerical models that generate filament channels from flux patterns migrating from active latitudes to the polar caps. Polarity inversions underlying high-latitude filament channels are swept into distinct patterns called `switchbacks' under the joint influence of differential rotation, supergranular diffusion, and meridional flow. The numerical model of Mackay and van Ballegooijen (2001) predicts a heretofore unsuspected solar-cycle dependence to the hemispheric pattern of filament magnetic fields. Observations presented here of a switchback formed early in cycle 21 confirm some key aspects of their model. In this remarkable example the flux diffusing out of the source region migrates to the opposite side of the Sun before it encounters another active region with which to create the quadrupolar field configuration wherein a return arm forms to complete the switchback. 相似文献
14.
Using Stanford large-scale magnetic field synoptic charts of rotation 1676 to 1739 and by delineating LLUMR, i.e., long-lived unipolar magnetic regions of both polarities surviving at least for four solar rotations, the semi-regular nature of their photospheric magnetic field pattern and their rotational properties have been examined. The investigation demonstrates the existence of regularities in the background field patterns as shown from the regular patterns of LLUMR rows and streams. This confirms the results of Bumba and Howard concerning regularities in large-scale photospheric magnetic field patterns. LLUMR streams seem to be arranged in a wave pattern of alternating polarities. Coronal holes and associated sections of photospheric field patterns suffer differential rotation. The rotation rates of the background field patterns which are not associated with the coronal holes are different from those which are. 相似文献
15.
Bipolar active regions (ARs) are thought to be formed by twisted flux tubes, as the presence of such twist is theoretically
required for a cohesive rise through the whole convective zone. We use longitudinal magnetograms to demonstrate that a clear
signature of a global magnetic twist is present, particularly, during the emergence phase when the AR is forming in a much
weaker pre-existing magnetic field environment. The twist is characterised by the presence of elongated polarities, called
“magnetic tongues”, which originate from the azimuthal magnetic field component. The tongues first extend in size before retracting
when the maximum magnetic flux is reached. This implies an apparent rotation of the magnetic bipole. Using a simple half-torus
model of an emerging twisted flux tube having a uniform twist profile, we derive how the direction of the polarity inversion
line and the elongation of the tongues depend on the global twist in the flux rope. Using a sample of 40 ARs, we verify that
the helicity sign, determined from the magnetic polarity distribution pattern, is consistent with the sign derived from the
photospheric helicity flux computed from magnetogram time series, as well as from other proxies such as sheared coronal loops,
sigmoids, flare ribbons and/or the associated magnetic cloud observed in situ at 1 AU. The evolution of the tongues observed in emerging ARs is also closely similar to the evolution found in recent MHD
numerical simulations. We also found that the elongation of the tongue formed by the leading magnetic polarity is significantly
larger than that of the following polarity. This newly discovered asymmetry is consistent with an asymmetric Ω-loop emergence,
trailing the solar rotation, which was proposed earlier to explain other asymmetries in bipolar ARs. 相似文献
16.
The most pertinent effect of the currents in the coronal-interplanetary space is their alteration of the magnetic topology to form configurations of open field lines. The important currents seem to be those in the neighborhoods of the interfaces between closed and open field lines or between oppositely directed open field lines in the coronal helmet-streamer structures. Thus, the coronal-interplanetary space may be regarded as being partitioned by current-sheets into several piecewise current-free regions. These current sheets overlie the photospheric neutral lines, where the vertical component of the magnetic field reverses its polarity on the solar surface. But, their locations and strengths are determined by force balance between the magnetic field and the gas pressure in the coronal-interplanetary space. Since the pressure depends on the flow velocity of the solar wind and the solar wind channels along magnetic flux tubes, there is a strong magnetohydrodynamic coupling between the magnetic field and the solar wind. The sheetcurrent approach presented in this paper seems to be a reasonable way to account for this complicated interaction.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
17.
Photospheric magnetic fluxes and average field strengths have been measured beneath 33 coronal holes observed on 63 occasions during 1975–1980. The principal result is that low-latitude holes contained 3 times more flux near sunspot maximum than near minimum despite the fact that their sizes were essentially the same. Average magnetic field strengths ranged from 3–36 G near sunspot maximum compared to 1–7 G near minimum. Evidently the low-latitude coronal holes received a proportion of the extra flux that was available at low latitudes near sunspot maximum.Visiting Astronomer, KPNO.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 相似文献
18.
Vector magnetogram and dopplergram observation of magnetic flux emergence and its explanation 总被引：1，自引：0，他引：1
During 23–28 August 1988, at the Huairou Solar Observation Station of Beijing Observatory, the full development process of the region HR 88059 was observed. It emerged near the center of the solar disk and formed a medium active region. A complete series of vector magnetograms and photospheric and chromospheric Dopplergrams was obtained. From an analysis of these data, combined with some numerical simulations, the following conclusions can be drawn. (1) The emergence of new magnetic flux from enhanced networks followed by sunspot formation is an interesting physical process which can be simply described by MHD numerical simulation. The phenomena accompanying it occur according to a definite law summarized by Zwaan (1985). The condition for gas cooling and sunspot formation seems to be transverse field strength > 50 G together with longitudinal field strength > 700 G. For a period of 4 to 5 hours, the orientation of the transverse field shows little change. The configuration of field lines may be derived from vector magnetograms. The arch filament system can be recognized as an MHD shock. (2) New opposite bipolar features emerge within the former bipolar field with an identical strength which will develop a sunspot group complex. Also, arch filament systems appear there located in the position of flux emergence. The neutral line is often pushed aside and curved, leading to faculae heating and the formation of a current sheet. In spite of complicated Dopplergrams, the same phenomena occur at the site of flux emergence as usual: upward flow appears at the location of the emerging and rapidly varying flux near the magnetic neutral line, and downdraft occurs over large parts of the legs of the emerging flux tubes. The age of magnetic emerging flux (or a sunspot) can be estimated in terms of transverse field strengths: when 50 G < transverse field < 200 G, the longitudinal magnetogram and Dopplergram change rapidly, which indicates a rigourously emerging magnetic flux. When the transverse field is between 200 and 400 G, the area concerned is in middle age, and some of the new flux is still emerging there. When the transverse field > 400 G, the variation of the longitudinal magnetogram slows down and the emerging arch becomes relatively stable and a photospheric Evershed flow forms at the penumbra of the sunspot. 相似文献
19.
《天文和天体物理学研究(英文版)》2015,(9)
EUV cyclones are rotating structures in the solar corona, and they are usually rooted in the underlying rotating network magnetic fields in the photosphere.However, their connection with the surrounding magnetic fields remains unknown.Here we report an observational study of four typical cyclones which are rooted in different kinds of magnetic fields. We use Solar Dynamics Observatory/Atmospheric Imaging Assembly data to investigate the rotation of EUV features in cyclones and Helioseismic and Magnetic Imager data to study the associated magnetic fields. The results show that,(1) an EUV cyclone rooted in a sunspot rotates with the photospheric magnetic field;(2) two EUV cyclones in two faculae of an active region are connected to the same sunspot of the active region but rotate oppositely;(3) an EUV cyclone is rooted in a coronal hole with weak open magnetic fields;(4) a pair of conjugated cyclones is rooted in magnetic fields that have opposite polarity with opposite directions of rotation. The differences in the spatial extent of a cyclone, characteristics of its rotation and underlying fields indicate that cyclones are ubiquitous over the solar atmosphere and that the magnetic structures relevant to the cyclones are more complicated than expected. 相似文献
20.
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