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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.
The structure of electric current and magnetic helicity in the solar corona is closely linked to solar activity over the 11-year cycle, yet is poorly understood. As an alternative to traditional current-free “potential-field” extrapolations, we investigate a model for the global coronal magnetic field which is non-potential and time-dependent, following the build-up and transport of magnetic helicity due to flux emergence and large-scale photospheric motions. This helicity concentrates into twisted magnetic flux ropes, which may lose equilibrium and be ejected. Here, we consider how the magnetic structure predicted by this model – in particular the flux ropes – varies over the solar activity cycle, based on photospheric input data from six periods of cycle 23. The number of flux ropes doubles from minimum to maximum, following the total length of photospheric polarity inversion lines. However, the number of flux rope ejections increases by a factor of eight, following the emergence rate of active regions. This is broadly consistent with the observed cycle modulation of coronal mass ejections, although the actual rate of ejections in the simulation is about a fifth of the rate of observed events. The model predicts that, even at minimum, differential rotation will produce sheared, non-potential, magnetic structure at all latitudes. 相似文献
3.
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
4.
It is shown that leading and following magnetic field lines are inclined toward each other by a few degrees at nearly all latitudes in both the north and south hemispheres. The amplitudes of these inclinations are lower by about a factor 3 for weak fields than for strong fields. There are significant differences between the hemispheres and from one activity cycle to the next in the leading and following polarity field-line inclinations at latitudes poleward of the activity latitudes. In a narrow latitude zone just south of the solar equator the inclinations of both the leading and following fields reduce to zero (or perhaps slightly negative values). Although one would expect such a zone at the equator, where diffusion will mix field lines with opposite inclinations from the two hemispheres, it is not clear why this zone should be on one side of the equator only. The results discussed here were obtained with Mount Wilson magnetograph data (1967–1992), and are confirmed in many respects with National Solar Observatory/Kitt Peak (NSO/KP) data (1976–1986).Operated by the Association of Universities for Research in Astronomy, Inc., under Cooperative Agreement with the National Science Foundation. 相似文献
5.
J. H. Piddington 《Astrophysics and Space Science》1983,90(1):217-230
Newly formed stars have magnetic fields provided by the compression of the interstellar field, and contrary to a widely accepted idea these fields are not destroyed by convective motions. For the same reason, the fallacy of ‘turbulent diffusion’, turbulent dynamo action is not possible in any star. Thus all stellar magnetic fields have a common origin, and persist throughout the lifetime of each star, including degenerate phases. This common origin, and a general similarity in stellar evolutionary processes, suggest that the fields may develop similar structural characteristics and MHD effects. This would open new possibilities of coordinating the studies of different types of stars and relating them to solar physics which has tended to become isolated from general stellar physics. As an initial step we consider three features of solar magnetic fields and their MHD effects. First, the solar magnetic field comprises two separate components: a poloidal field and a toroidal field. The former is a dipole field, permeating the entire Sun and closely aligned with the rotational axis; at the surface it is always concealed by much stronger elements of the toroidal field. The latter is probably wound from the former by differential rotation at latitudes below about 35°, where sections emerge through the solar surface and are then carried polewards. The second feature of solar magnetic fields is that all flux is concentrated into flux tubes of strength some kG, isolated within a much larger volume of non-magnetic plasma. The third feature is that the flux tubes are helically twisted into flux ropes (up to ?1022Mx) and smaller elements ranging down to flux fibres (? 1018Mx). Some implications of similar features in other stars are discussed. 相似文献
6.
Sixteen years of WSO magnetogram data have been studied to determine the solar cycle variation and latitude dependence of the east-west inclination of photospheric magnetic field lines. East-west inclination is here defined as the angle between a field line and its local radial vector, as projected onto the plane of the latitude and line of sight. Inclination is determined by a least-squares fit of observed magnetic fields to a simple projection model, and is found to depend on polarity and to change with the solar cycle. Leading and following polarities are tipped towards each by about 9° and have an overall net tilt in the direction of rotation (to the west) of 0.6°. New cycles are seen to begin at high latitudes and to grow through the lower latitudes over approximately 5 years, providing evidence for an extended cycle length of 16–18 years. 相似文献
7.
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
8.
《天文和天体物理学研究(英文版)》2016,(6)
It has been found that photospheric magnetic fields can change in accordance with restructuring of the three-dimensional magnetic field following solar eruptions.Previous studies mainly use vector magnetic field data taken for events near the disk center.In this paper,we analyze the magnetic field evolution associated with the 2012 October 23 X1.8 flare in NOAA AR 11598 that is close to the solar limb,using both the 45 s cadence line-of-sight and 12 min cadence vector magnetograms from the Helioseismic and Magnetic Imager on board Solar Dynamics Observatory.This flare is classified as a circular-ribbon flare with spine-fan type magnetic topology containing a null point.In the line-of-sight magnetograms,there are two apparent polarity inversion lines(PILs).The PIL closer to the limb is affected more by the projection effect.Between these two PILs there lie positive polarity magnetic fields,which are surrounded by negative polarity fields outside the PILs.We find that after the flare,both the apparent limb-ward and disk-ward negative fluxes decrease,while the positive flux in-between increases.We also find that the horizontal magnetic fields have a significant increase along the disk-ward PIL,but in the surrounding area,they decrease.Synthesizing the observed field changes,we conclude that the magnetic fields collapse toward the surface above the disk-ward PIL as depicted in the coronal implosion scenario,while the peripheral field turns to a more vertical configuration after the flare.We also suggest that this event is an asymmetric circular-ribbon flare:a flux rope is likely present above the disk-ward PIL.Its eruption causes instability of the entire fan-spine structure and the implosion near that PIL. 相似文献
9.
The coronal magnetic field (CMF) during Carrington rotation 1870 is inferred by using the so-called 'potential model' with the photospheric magnetic field observed at Kitt Peak. Magnetic field lines starting at areas restricted (90deg; ± 30deg; in longitude and 0deg; ± 20deg; in latitude) in both the photosphere and the source surface of 2.5 solar radii are traced to examine fine geometrical structures of the CMF. We found a well-ordered planar magnetic structure (PMS) near 90° Carrington longitude in the corona. The PMS consists of magnetic flux of negative polarity emanating from several small areas in the photosphere. The magnetic flux expands into a wide longitudinal angle in the source surface making a planar magnetic structure. 相似文献
10.
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
11.
It is known for over two decades now that the rotation of the photospheric magnetic fields determined by two different methods
of correlation analysis leads to two vastly differing rotation laws - one the differential and the other rigid rotation. Snodgrass
and Smith (2001) reexamining this puzzle show that the averaging of the correlation amplitudes can tilt the final profile
in favour of rigid rotation whenever the contribution of the rigidly rotating large-scale magnetic structures (the plumes)
to the correlation dominates over that of the differentially rotating small-scale and mesoscale features. We present arguments
to show that the large-scale unipolar structures in latitudes >40 deg, which also show rigid rotation (Stenflo, 1989), are
formed mainly from the intranetwork magnetic elements (abbreviated as IN elements). We then estimate the anchor depths of
the various surface magnetic elements as locations of the Sun's internal plasma layers that rotate at the same rate as the
flux elements, using the rotation rates of the internal plasma layers given by helioseismology. We infer that the anchor depths
of the flux broken off from the decay of sunspot active regions (the small-scale and mesoscale features that constitute the
plumes) are located in the shallow layers close to the solar surface. From a similar comparison with helioseismic rotation
rates we infer that the rigid rotation of the large-scale unipolar regions in high latitudes could only be coming from plasma
layers at a radial distance of about 0.66–0.68 R
⊙ from the Sun's centre. Using Stenflo's (1991) ‘balloon man’ analogy, we interpret these layers as the source of the magnetic
flux of the IN elements. If so, the IN flux elements seem to constitute a fundamental component of solar magnetism. 相似文献
12.
Because of the progressive decrease in rotation rate of the solar plasma at increasing latitudes, the photospheric foot-points of large-scale closed magnetic structures in the corona, which are originally widely separated in longitude, may ultimately be brought into proximity. Magnetic mergers and reconnections between magnetic fields of opposite polarity are presumed to occur, producing major structural changes in the corona and in the locations of underlying filaments. Thus we believe that the differential rotation phenomenon is essential to understanding both gradual (evolutionary) and sudden (transient) changes in the corona, and that they can occur without any observable change in the photospheric magnetic flux. A process is suggested for the splitting or bifurcation of a high-latitude magnetic structure, producing two separate components at the same latitude, whose rotation rates are influenced by their respective magnetic linkages to other regions on the Sun.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
13.
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
14.
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
15.
We present high-resolution studies of the solar polar magnetic fields near sunspot maximum in 1989 and towards sunspot minimum in 1995. We show that, in 1989, the polar latitudes were covered by several unipolar regions of both polarities. In 1995, however, after the polar field reversal was complete, each pole exhibited only one dominant polarity region.Each unipolar region contains magnetic knots of both polarities but the number count of the knots of the dominant polarity exceeds that of the opposite polarity by a ratio of order 4:1, and it is rare to find opposite polarity pairs, i.e., magnetic bipoles.These knots have lifetimes greater than 7 hours but less than 24 hours. We interpret the longitudinal displacement of the knots over a 7-hour period as a measure of the local rotation rate. This rotation rate is found to be generally consistent with Snodgrass' (1983) magnetic rotation law.In an attempt to obtain some insight into the operation of the solar dynamo, sketches of postulated subsurface field configurations corresponding to the observed surface fields at these two epochs of the solar cycle are presented. 相似文献
16.
A clearly evident large-scale pattern in the interplanetary magnetic field during 1964 is used to search for a similar large-scale pattern in the solar magnetic field. It is found that such a pattern did exist in the photospheric field observations on both sides of the equator over a range of at least 40°N to 35°S. The pattern is basically similar at all these latitudes, and differs from that to be expected from solar differential rotation in three important respects. It is found that the solar magnetic pattern changed at all latitudes investigated within an interval of a few solar rotations. 相似文献
17.
It is well known that magnetic activity on the Sun modulates from one cycle to the next. The most striking occurrence of this is called a grand minimum where magnetic activity all but disappears. The latest grand minimum occurred between the years 1645 and 1715 and is called the Maunder minimum. In this paper magnetic flux transport simulations are used to consider what type of surface magnetic field configurations may be produced both during and after a grand minimum depending on how the grand minimum occurs. It is shown that the surface configurations during and after a grand minimum strongly depend on the phase of the cycle in which the grand minimum starts and whether it lasts for an odd or even number of cycles. If the grand minimum starts around cycle minimum then a significant amount of large-scale magnetic flux may persist on the Sun at high latitudes during the grand minimum. In contrast, if it starts at cycle maximum during the grand minimum it is possible for there to be essentially zero large-scale magnetic flux over the entire surface of the Sun. It is shown that for a single grand minimum event the reversal of the polar fields at the presently observed time in the solar cycle is only reproduced if the event starts at cycle minimum and extends over an even number of cycles. In contrast, if the grand minimum runs for an odd number of cycles it is possible for there to be no reversal of the polar fields or for the reversals to occur at times inconsistent with our present understanding of the solar cycle. Consequences of the assumptions made in the modelling are discussed and the significance of the simulations for direct modelling of events such as the Maunder minimum are considered. 相似文献
18.
High-resolution magnetograms of the solar polar region were used for the study of the polar magnetic field. In contrast to low-resolution magnetograph observations which measure the polar magnetic field averaged over a large area, we focused our efforts on the properties of the small magnetic elements in the polar region. Evolution of the filling factor - the ratio of the area occupied by the magnetic elements to the total area - of these magnetic elements, as well as the average magnetic field strength, were studied during the maximum and declining phase of solar cycle 22, from early 1991 to mid-1993.We found that during the sunspot maximum period, the polar regions were occupied by about equal numbers of positive and negative magnetic elements, with equal average field strength. As the solar cycle progresses toward sunspot minimum, the magnetic field elements in the polar region become predominantly of one polarity. The average magnetic field of the dominant polarity elements also increases with the filling factor. In the meanwhile, both the filling factor and the average field strength of the non-dominant polarity elements decrease. The combined effects of the changing filling factors and average field strength produce the observed evolution of the integrated polar flux over the solar cycle.We compared the evolutionary histories of both filling factor and average field strength, for regions of high (70°–80°) and low (60°–70°) latitudes. For the south pole, we found no significant evidence of difference in the time of reversal. However, the low-latitude region of the north pole did reverse polarity much earlier than the high-latitude region. It later showed an oscillatory behavior. We suggest this may be caused by the poleward migration of flux from a large active region in 1989 with highly imbalanced flux. 相似文献
19.
The structure of the large-scale background magnetic field evolves in time and space. The large-scale horizontal transport
velocity field of the magnetic flux patterns was inferred over the whole solar photosphere in the course of two solar activity
cycles from year 1976 to 1999. The method of velocity determination and the testing procedures of the velocity accuracy are
presented. The non-axially symmetric component of the horizontal velocity was found and both zonal and meridional velocity
regions were described. The horizontal large-scale transport velocity regions vary in shape and the intensity during different
phases of the 11-year solar activity cycle. The total horizontal transport velocity is characterized by the presence of variable
amounts of the vector field vortices with symmetric orientation relative to the solar equator. The zonal velocity regions,
distributed inside of the zonal belt limited by latitudes ± 35°, are persistent for about 4 Carrington rotations. Recurrent
structures of similar velocity distributions are not coherent over the whole solar photosphere. 相似文献
20.
Makarov V.I. Tlatov A.G. CALLEBaUT D.K. Obridko V.N. Shelting B.D. 《Solar physics》2001,198(2):409-421
Hα magnetic synoptic charts of the Sun are processed for 1915–1999 and the spherical harmonics are calculated. It is shown
that the polarity distribution of the magnetic field on Hα charts is similar to the polarity distribution of the Stanford
magnetic field observations during 1975–1999. The index of activity of the large-scale magnetic field A(t), representing the sum of the intensities of dipole and octupole components, is introduced. It is shown that the cycle of
the large-scale magnetic field of the Sun precedes on the average by 5.5 years the sunspot activity cycle, W(t). This means that the weak large-scale magnetic fields of the Sun do not result from decay and diffusion of strong fields
from active regions as it is supposed in all modern theories of the solar cycle. On the basis of the new data the intensity
of the current solar cycle 23 is predicted and some aspects of the theory of the solar cycle are discussed. 相似文献