Evidence for non-radial fields in the Sun's photosphere and a possible explanation of the polar magnetic signal

The appearance of the Hα fibrils suggests the presence of magnetic fields inclined at noticeably non-radial angles in the Sun's chromosphere. We present evidence to suggest that these angles continue into the photosphere. The presence even of small non-radial inclinations can significantly affect the appearance of regions observed by a longitudinal magnetograph. In particular, a simple bipolar loop can appear unbalanced when viewed near the limb. We suggest that the observed polar signal may be nothing more than a geometric effect arising when a balanced but systematically aligned array of bipolar pairs is viewed at an angle.     

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.     

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

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

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

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 polar magnetic fields of July and August 1968

Observations of the polar magnetic fields were made during the period July 3–August 23, 1968, with the Mt. Wilson magnetograph. The scanning aperture was 5 × 5. The magnetic field was found to be ofS polarity near the heliographic north pole and ofN polarity near the south pole. At lower latitudes the polarity was the opposite. The polarity reversal occurred at a latitude of about +70° in the north and -55° in the south hemisphere. This coincides with the position of the polar prominence zones at that time. The observations indicate that the average field strength at the south pole was well above 5 G.Synoptic charts of the magnetic fields have been plotted in a polar coordinate system for two consecutive solar rotations.     

The Sun's Distance from the Galactic Plane

Use is made of 93,106 parallaxes from the Hipparcos catalog, with a mixture of spectrum-luminosity classes, to derive the position of the Galactic plane. The reduction technique, mixed total least squares-least squares, takes into account the errors in the parallaxes, and the condition that the direction cosines of the Galactic pole have unit Euclidean norm is rigorously enforced. To obtain an acceptable solution it is necessary to eliminate the stars of classes O and B that belong to the Gould belt. The Sun is found to lie 34.56±0.56 pc above the plane. The coordinates of the Galactic pole, l _g , b _g, are found to be: l _g =0.°004±0.°039; b _g =89.°427±0.°035.This agrees well with what radio observations find and demonstrates that the IAU's recommendation in 1960 to use only radio observations to determine the Galactic pole, although correct at the time because of the paucity of optical observations, can no longer be justified given the plethora of observations contained in the Hipparcos catalog and an adequate reduction technique, unavailable in 1960. The reduction technique is also demonstrably superior to others because it involves fewer assumptions and calculates smaller mean errors. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

The Sun's immutable basal quiet atmosphere

We employ limb darkening, spectral energy distribution (color), and center-disk spectrum line strength to investigate photospheric temporal variability. Current limb-darkening curves agree to 1% with past observations taken at different epochs extending back to 1975. Concerning color, from the data of Labs and Neckel (Cox, 1999) we deduce that the solar limb is 1000 Å more red than disk center. But when integrated over the entire disk to represent the Sun-as-a-star, the color shift is only 30 Å. Color is therefore not a very sensitive indicator of full-disk photospheric change. We examine the center-disk time series for C 5380 Å and Fe 5379 Å equivalent width and the Ca K index. The ratio C 5380/Fe 5379 in equivalent width is 0.4221+0.00011 (±0.00003) y ^–1, indicating secular change but with no cycle modulation. Converted to temperature this variance amounts to ±0.028 K. This is in contrast to the full-disk cycle modulation of these lines reported by Gray and Livingston (1997b). Ca K index also exhibits no cycle variation at disk center. Taking into account these findings, plus the small fraction of the photosphere occupied by magnetic elements as revealed in high-resolution G-band pictures, we suggest that cycle magnetic fields thread through the basal atmosphere without physical effect; that the basal quiet atmosphere is observationally immutable to the magnetic cycle within the limits given above.     

The roots of coronal structure in the Sun's surface

We have compared the structures seen on X-ray images obtained by a flight of the NIXT sounding rocket payload on July 11, 1991 with near-simultaneous photospheric and chromospheric structures and magnetic fields observed at Big Bear. The X-ray images reflect emission of both Mgx and Fexvi, formed at 1 × 10⁶ K and 3 × 10⁶ K, respectively. The brightest H sources correspond to a dying sub-flare and other active region components, all of which reveal coronal enhancements situated spatially well above the H emission. The largest set of X-ray arches connected plages of opposite polarity in a large bipolar active region. The arches appear to lie in a small range of angle in the meridian plane connecting their footpoints. Sunspots are dark on the surface and in the corona. For the first time we see an emerging flux region in X-rays and find the emission extends twice as high as the H arches. Many features which we believe to correspond to X-ray bright points (XBPs) were observed. Whether by resolution or spectral band, the number detected greatly exceeds that from previous work. All of the brighter XBPs correspond to bipolar H features, while unipolar H bright points are the base of more diffuse comet-like coronal arches, generally vertical. These diverge from individual features by less than 30°, and give a good measure of what the canopies must do. The H data shows that all the H features were present the entire day, so they are not clearly disappearing or reappearing. We find a new class of XBPs which we call satellite points, elements of opposite polarity linked to nearby umbrae by invisible field lines. The satellite points change rapidly in X-ray brightness during the flight. An M1.9 flare occurred four hours after the flight; examination of the pre-flare structures reveals nothing unusual.     

Simulations of the sun's polar magnetic fields during sunspot cycle 21

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.     

Evolution of the Sun's magnetic polarities

The global magnetic-field resonances previously found in a modal analysis of a 25 yr Mt Wilson-Kitt Peak data set of synoptic magnetic maps are also revealed when only the magnetic polarities are used, disregarding the magnitude of the flux. Thus the topological organization of the magnetic polarities alone suffices to bring out the correct modal structure, although the results are noisier as compared with the case when the magnetic fluxes are included.This result suggests that magnetic polarities indirectly inferred from H data may be used to study the resonances. Whereas synoptic magnetograph observations are not available before 1959, H observations date back to the last century, and could in principle be used to enhance the frequency resolution in the power spectra.Using zonal distributions of magnetic polarities inferred from H data by Makarov for the period 1941–1983, we have performed a power spectrum analysis of the rotationally symmetric spherical harmonic modes (m = 0), and compared the results with the corresponding analysis for the magnetic fields and polarities determined from real magnetograph data. The same parity selection rule that governs the 22 yr magnetic cycle in the magnetograph data is also revealed by the H data. The H results are however much noisier and do not show the pattern of resonant frequencies discovered in the magnetograph data for the modes of even parity. It is concluded that only real magnetograph data should be used to investigate the global resonances in the magnetic-field pattern.     

A model combining the polar and the sector structured solar magnetic fields

A phenomenological model of the interplay between the polar magnetic fields of the Sun and the solar sector structure is discussed. Current sheets separate regions of opposite polarity and mark the sector boundaries in the corona. The sheets are visible as helmet streamers. The solar sector boundary is tilted with respect to central meridian, and boundaries with opposite polarity change are oppositely tilted. The tilt of a given type of boundary [(+, ?) or (?, +)] changes systematically during the sunspot cycle as the polarity of the polar fields reverses. Similar reversals of the position of the streamers at the limbs takes place. If we consider (a) a sunspot cycle where the northern polar field is inward (?) during the early part of the cycle and (b) a (+, ?) sector boundary at central meridian then the model predicts the following pattern; a streamer at high northern latitudes should be observed over the west limb together with a corresponding southern streamer over the east limb. The current sheet runs now NW-SE. At sunspot maximum the boundary is more in the N-S direction; later when the polar fields have completed their reversal the boundary runs NE-SW and the northern streamer should be observed over the east limb and the southern streamer over the west limb. Observational evidence in support of the model is presented, especially the findings of Hansen, Sawyer and Hansen and Koomen and Howard that the K-corona is highly structured and related to the solar sector structure.     

The Mechanism involved in the Reversals of the Sun's Polar Magnetic Fields

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.     

The interaction of the spiral density wave and the Sun's galactic orbit

The galactocentric radial motion of the Sun introduces another periodicity to the encounters between the Sun and the spiral density wave. We describe a model simulation of the effect and present the associated periods.     

Electric fields and currents in the earth's polar caps

A model for solar wind flow around the magnetopause incorporating a stagnation line at the frontside magnetopause is used to derive a formula for the electric field intensity and polar cap potential drop. These relationships are compared to experimental data from polar orbiting satellites. The relation between solar wind parameters and auroral arc velocity is also studied.     

On the Sun's limb brightening in the visible

It is shown that in practice the method by Julius (1906) is incapable to determine the limb intensity drop. The brief intensity reversal near the extreme limb as derived from cinematography of flash spectra can be explained by diffraction.