Polar magnetic field reversals of the Sun in Maunder Minimum

A possible scenario of polar magnetic field reversal of the Sun during the Maunder Minimum (1645–1715) is discussed using data of magnetic field reversals of the Sun for 1880–1991 and the¹⁴ C content variations in the bi-annual rings of the pine-trees in 1600–1730 yrs.     

Short-period interplanetary and polar magnetic field variations

It is shown that the dependence of the variations of vertical component of the polar cap magnetic field on the sector structure (actually, the azimuthal or Y component) of the interplanetary magnetic field as first discovered by Svalgaard (1968) and Mansurov (1969) extends to variations as brief as 1 hr or even less. The relation between sector structure dependent variations and substorm fields as indicated by the southward-directed component of the interplanetary magnetic field is investigated by comparing brief variations over selected intervals of time. The independence of the variations of the polar cap vertical and horizontal components suggests that there are at least two different current systems which produce brief variations in the polar cap. One of the current systems is related to the substonn field; the other is strongly seasonally dependent and is confined to the dayside sector of the Earth.     

The magnetic field in the prominences of the polar crown

The Hanle effect method has been applied to the determination of the magnetic field in 120 prominences of the polar crown observed during the 1974–1980 period, which is the ascending phase of cycle XXI. The average field strength which was about 6 G at the beginning of the cycle reached twice this value just before the maximum. There is also a clear trend for a increase of the prominence field with the altitude. We confirm the fact that the magnetic vector makes a small angle (25 °) with the long axis of the prominence. As to the field orientation, we show that the most striking feature lies in the regular pattern of the component which is parallel to the axis of the filament; its direction seems to depend closely on the polarities of the high latitude photospheric field.     

Implications of a strongly peaked polar magnetic field

Using the flux-transport equation in the absence of sources, we study the relation between a highly peaked polar magnetic field and the poleward meridional flow that concentrates it. If the maximum flow speed _m greatly exceeds the effective diffusion speed /R, then the field has a quasi-equilibrium configuration in which the poleward convection of flux via meridional flow approximately balances the equatorward spreading via supergranular diffusion. In this case, the flow speed () and the magnetic field B() are related by the steady-state approximation () (/R)B()/B() over a wide range of colatitudes from the poles to midlatitudes. In particular, a general flow profile of the form sin ^p cos ^q which peaks near the equator (q p) will correspond to a cos ⁿ magnetic field at high latitudes only if p = 1 and _m = n /R. Recent measurements of n 8 and 600 km² s^–1 would then give _m 7 m s^–1.     

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.     

On the relationship between polar faculae and large-scale magnetic field

We investigate a latitude–time distribution of polar faculae observed at Ussuriysk Observatory in years 1966–1986. The distribution is compared with the longitude-averaged (zonal) magnetic field of the Sun calculated from the data obtained at Mount Wilson Observatory in the years 1966–1976, and at Kitt Peak National Observatory during the period from 1976 to 1985. We found that slow, poleward-directed migration of the polar faculae zones occurring during the course of the solar cycle is not a continuous process, but it contains several episodes of appearance and fast poleward drift of new zones of polar faculae. At the rising phase of the solar cycle, new zones of polar faculae appear at latitudes as low as 40°, but the ones observed during the declining phase of the solar cycle originate at higher latitudes of 50–55°. Such episodes of appearance and fast migration of the polar faculae zones are associated with the poleward-directed streams of magnetic field originated at low latitudes. Moreover, we found some evidence for existence of an additional component of the polar faculae activity that reveals an equatorward migration during the course of the solar cycle. We also investigated a relationship between the number of polar faculae, n, and absolute magnetic flux _z of the zonal mode of the solar magnetic field. We found that within the polar zones of the Sun, substantial correlation between temporal variations of n and _z takes place both on the time scale of the solar cycle and on a shorter time scale of 2–4 years. The relationship between the number of polar faculae and magnetic flux may be approximated by a linear dependence n=0.12_z (where _z is expressed in 10²¹ Mx), except for time interval 1977 through 1980 for which the factor of proportionality is found to have a systematically larger value of 0.20.     

Convection in the polar cap after the interplanetary magnetic field becomes northward

As shown by Iwasaki (1971); Maezawa (1976); Kuznetsov and Troshichev (1977) and other investigators, the electric field and the plasma convection in the polar cap change their direction after an appearance of a significant northward component of the interplanetary magnetic field. Two possible mechanisms of this phenomenon may be suggested: (i) the direct penetration of the dusk-to-dawn electric field from the solar wind into the magnetosphere, and (ii) the generation of the observed electric field and convection in a process of the decay of the three-dimensional current system which existed before the appearance of the northward interplanetary field. The latter mechanism implies that the value of the electric field generated in the polar cap will decrease with time after the appearance of the northward interplanetary magnetic field. The results of the experimental investigation show such a decrease which favours the second mechanism.     

North-south asymmetry of the polar solar wind and some characteristics of solar magnetic field

We have found correlated variations of the yearly averaged north-south asymmetry in the polar solar wind speed (_sol) and the ratio of the zonal quadrupolar to the zonal dipolar contribution in the inferred coronal magnetic field during the declining phase of sunspot cycle 21. A physically meaningful association between _sol and some polar solar magnetic field proxies is also observed during the low sunspot activity periods of the above cycle.     

Time-sequence of polar magnetic substorms

The time-sequence of polar magnetic substorms is discussed to clarify some controversies on the magnetospheric substorm model including the growth phase. The main purpose of the analyses is to examine magnetic variations in the polar cap and in low latitudes. The onset of the expansion phase is confirmed to be reasonably defined by a vector change of polar-cap magnetic disturbance, a sharp intensification of the auroral electrojet disturbance and the beginning of positive ΔH disturbance in midlatitudes near midnight. It is shown that the growth phase signatures so far proposed are consistent when the onset of the expansion phase is identified from the above mentioned features.     

Twenty-two year modulation of cosmic rays associated with polarity reversal of polar magnetic field of the sun

The existence of the 22-year modulation of cosmic ray intensity is pointed out, using data of the ion chamber at Huancayo and the neutron monitors at Ottawa and Deep River for about four solar cycles. The modulation consists of two discrete states (high and low intensities), corresponding respectively to those of the polarity of the polar magnetic field of the Sun. This can be interpreted on the basis of the following hypothesis; when the polar magnetic field of the Sun is nearly parallel to the galactic magnetic field, they could easily connect with each other, so that galactic cosmic rays could intrude more easily into the heliomagnetosphere along the magnetic line of force, as compared with those in the anti-parallel state of the magnetic fields. The observed intensity difference between two states is about 4.3 ± 0.2% for neutron monitor (P_c = 1.5GV). The abnormal increase in proton (0.28–0.42 GV) and electron (0.41-3.24 GV) fluxes in the 20th solar cycle and the sudden appearance of anomalous components (He⁺, etc.) since 1972 can be also explained on the basis of the present hypothesis. The transition between the two states has a time lag behind the polarity reversal, depending on the cosmic ray rigidity, such as about 1 year for the neutron monitor (P_c = 1.5 GV) and about 3.5 years for low rigidity components (P < 1 GV). These time lags could be explained on the basis of the generalized Simpson's coasting solar wind model and the general diffusion-convection theory on some assumptions.     

Origin and characteristics of the Mars north polar basal unit and implications for polar geologic history

Building upon previous studies, we have used Mars Orbiter Camera and Mars Orbiter Laser Altimeter data to characterize in detail the newly discovered north polar basal unit. Lying stratigraphically between the polar layered deposits, from which it is likely separated by an unconformity, and the Vastitas Borealis Formation, this unit has introduced new complexity into north polar stratigraphy and has important implications for polar history. Exposures of the basal unit in Olympia Planitia and Chasma Boreale reveal relatively dark layers which exhibit differential erosion. Eroded primarily by wind, the basal unit may be the major if not sole source for the north polar dunes and ergs and has contributed material to the lower polar cap layers. We investigate four possible origins for the basal unit (outflow channel/oceanic deposits, basal ice, paleopolar deposits, and eolian deposits). The patchy layering within the unit, its likely sandy grain size, and presence only in the north polar basin suggest that it is primarily an eolian deposit, supporting Byrne and Murray's 2002 earlier conclusion. This implies that at some time during the Early to Late Amazonian, migrating sand was mixed with water ice, forming a relatively dark, sandy deposit. During this time, either no classic polar layered deposits were forming or smaller caps were growing and shrinking, possibly adding material to the basal unit.     

The reversal of the solar polar magnetic fields

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.     

The reversal of the solar polar magnetic fields

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.     

The reversal of the solar polar magnetic fields

Some theoretical difficulties confronting the current model of the polar magnetic reversal by cancellation with the flux remnants of decaying active regions are discussed. It is shown that the flux transport equation does not adequately describe the essential physical consequences of the transport of large-scale fields, linked to deep subsurface toroids, over distances comparable with the solar radius. The possibility that subsurface reconnections may release these fields to form U-loops is discussed but it is shown that, in this event, the loops will quickly rise to the surface. Mechanisms whereby the flux may escape through the surface are considered.     

The reversal of the solar polar magnetic fields

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.     

The reversal of the solar polar magnetic fields

The Mount Wilson synoptic magnetic data from CRs 1815 to 1866 are used to describe the reversal of the solar polar magnetic fields during the period May 1989–March 1993. These are compared with simulations based on the observed fields for CR 1815 using the flux transport equation. Simulations including the emergence of small bipoles with preferred poleward orientations are also described. It is shown that, while the former can provide a qualitative account of the evolution of the southern fields between CRs 1815 and 1860, only the latter can describe the evolution of the northern fields between CRs 1815 and 1865.     

Comparison of the mean photospheric magnetic field and the interplanetary magnetic field

The mean photospheric magnetic field of the sun seen as a star has been compared with the interplanetary magnetic field observed with spacecraft near the earth. Each change in polarity of the mean solar field is followed about 4 1/2 days later by a change in polarity of the interplanetary field (sector boundary). The scaling of the field magnitude from sun to near earth is within a factor of two of the theoretical value, indicating that large areas on the sun have the same predominant polarity as that of the interplanetary sector pattern. An independent determination of the zero level of the solar magnetograph has yielded a value of 0.1±0.05 G. An effect attributed to a delay of approximately one solar rotation between the appearance of a new photospheric magnetic feature and the resulting change in the interplanetary field is observed.     

High-resolution observations of the polar magnetic fields of the sun

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.     

Origin of stellar magnetic fields

It is pointed out that the magnetic field of a star is originated from dynamo action associated with the stellar evolution. The magnetic field of a star is related with how much nuclear energy is generated in its phase of evolution. From this we can explain why some stars possess a magnetic field high than that of the Sun. In our case the magnetic field of the star is a by-product of the stars evolution and it has no influence on the internal structure of the star but it does have influence on the flare, chromosphere and coronal activities of the star. Again it is stressed that to confirm the activities of the star, the details of evolution of stars should be calculated according to the photon-neutrino coupling theory.     

Burial of the polar magnetic field of an accreting neutron star – II. Hydromagnetic stability of axisymmetric equilibria

The theory of polar magnetic burial in accreting neutron stars predicts that a mountain of accreted material accumulates at the magnetic poles of the star, and that, as the mountain spreads equatorward, it is confined by, and compresses, the equatorial magnetic field. Here, we extend previous, axisymmetric, Grad–Shafranov calculations of the hydromagnetic structure of a magnetic mountain up to accreted masses as high as   M _a= 6 × 10⁻⁴ M_⊙  , by importing the output from previous calculations (which were limited by numerical problems and the formation of closed bubbles to   M _a < 10⁻⁴ M_⊙  ) into the time-dependent, ideal-magnetohydrodynamic code zeus-3d and loading additional mass on to the star dynamically. The rise of buoyant magnetic bubbles through the accreted layer is observed in these experiments. We also investigate the stability of the resulting hydromagnetic equilibria by perturbing them in zeus-3d . Surprisingly, it is observed that the equilibria are marginally stable for all   M _a≤ 6 × 10⁻⁴ M_⊙  ; the mountain oscillates persistently when perturbed, in a combination of Alfvén and acoustic modes, without appreciable damping or growth, and is therefore not disrupted (apart from a transient Parker instability initially, which expels <1 per cent of the mass and magnetic flux).