CORONAL HOLES AND THE POLAR FIELD REVERSALS. 1. Introduction

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1 CORONAL HOLES AND THE POLAR FIELD REVERSALS P. FOX 1, P. McINTOSH 2 and P. R. WILSON 3 1 High Altitude Observatory, Boulder, CO , U.S.A. 2 HelioSynoptics Inc., Boulder, CO , U.S.A. 3 School of Mathematics, University of Sydney, Australia (Received 22 April 1996; accepted 22 May 1997) Abstract. A description of the reversal of the solar north polar magnetic field during cycle 22 is provided using polar projections which combine the large-scale magnetic fields as inferred from Hæ synoptic charts and coronal holes mapped from He I ç1083 nm spectroheliograms. These plots are supported by polar plots of the magnetic fields derived from synoptic magnetic field data from the Mount Wilson Observatory. The coronal holes showed some unexpected evolutionary patterns in relation to the polarity reversals, and these patterns appear to be coordinated with changes in the global patterns of coronal holes and the heliospheric current sheet, suggesting that the polar reversal originates from global processes rather than from local magnetic flux annihilation. Similar patterns have been observed in the reversal of the southern polar magnetic field in cycle 22 and in both hemispheres in cycle 21. The consequences of these findings for the solar dynamo process are discussed. 1. Introduction Because two eleven-year sunspot cycles must be completed before the magnetic field configurations associated with the sunspot groups return to their original form, the solar activity cycle has been identified as a 22-year magnetic cycle. Further, measurements of the polar field strengths at the Mount Wilson and the Wilcox Solar Observatories over the past four solar cycles indicate the presence of a 22- year cyclic variation in these fields, associated with sunspots and involving polarity reversals (Babcock and Livingston, 1958). The time of maximum of the polar fields is 90 æ out of phase with the peak of the smoothed sunspot number for that sunspot cycle (Webb, Davis, and McIntosh, 1984; Layden et al., 1991). Although the polar magnetic fields are weak compared to the fields of active regions, their 22- year cyclic variation suggests that they are an important component of the solar magnetic cycle. Thus an understanding of the physical processes which control the reversal of the polar fields should provide some insight into the solar dynamo process. The standard model for this reversal (e.g., Leighton, 1964; DeVore, Sheeley, and Boris, 1984; DeVore and Sheeley, 1987; Sheeley, Nash, and Wang, 1987; Wang, Nash, and Sheeley, 1989) assumes that the diffusive effects of supergranule motions, differential rotation, and meridional flows disperse the active-region magnetic fields to form the large-scale field patterns. These fields drift polewards, under the influence of meridional flow, and it is claimed that the polar reversals are effected by flux cancellation and that the observed features of the large-scale fields can be simulated by suitably varying the parameters of the flux transport equation. Solar Physics 177: , cæ 1998 Kluwer Academic Publishers. Printed in Belgium.

2 376 P. FOX, P. McINTOSH, AND P. R. WILSON However, this model faces difficulties, both theoretical and observational. In the flux transport equation, the radial component of the surface magnetic fields is regarded as a scalar function of surface position, and the three-dimensional nature of these fields, in particular the sub-surface connections of these field lines and the transverse magnetic tensions which would be created by their apparent surface transport, are ignored in the surface flux transport model. From an observational perspective, Murray and Wilson (1992) and Wilson and Giovannis (1994) described recent changes to the polar fields using synoptic magnetic data provided by the Mount Wilson Observatory. Wilson and Giovannis produced polar projections of the large-scale fields through the period of the polar field reversal. By comparing the observed polar fields with simulations using the flux transport equation, based on the observed fields for Carrington rotation (CR) 1815, they showed that many of the high-latitude patterns could not be reproduced, even qualitatively, by the simulations of the surface transport of low-latitude flux. Although it was not provided with the Mount Wilson data, Ulrich (1992) indicated that a center-to-limb calibration is required for their magnetograms, and this might change some of the details of the flux distribution. Further discussion of this point is beyond the scope of this paper. When theory and observations are not coincident, some progress may be achieved by studying observations of different, but related, phenomena. An important property of the large-scale fields is their relation to coronal holes, which are defined as regions of open field lines (Zirker, 1977) and these have been identified with the darkest features in X-ray images of the Sun and are particularly obvious in the images provided by Yohkoh. In this paper we will not discuss the magnetic nature of coronal holes in any detail (for two recent viewpoints see Mikic and McClymont (1994) and Wang, Hawley, and Sheeley (1996)). Coronal holes are, however, generally centered in large unipolar magnetic cells, and their low coronal density is probably a consequence of the magnetic field lines being open to interplanetary space. Coronal holes are also identified with bright features in ground-based images taken in the infrared line He I 1083 nm (the so-called coronal holes) resulting, it is believed, from the conduction of heat from the corona along the open field lines (Avrett, Fontenla, and Loeser, 1992). In coronal holes, the chromospheric network has a diffuse, low-contrast structure. The distribution of coronal holes over the solar surface at the various phases of the solar cycle has been discussed by McIntosh (1993). During sunspot minimum the dominant coronal holes on the Sun are those associated with the polar fields, and the reversal of these fields (after solar maximum) requires that each polar coronal hole must be replaced by one of opposite polarity. In this paper, we describe the evolution of the polar coronal holes during the north-polar field reversal of cycle 22 and their relation to changes in the coronal hole patterns at lower latitudes. In Section 2 we discuss the preparation of the polar projection from the synoptic charts. In Section 3 we describe the polar field reversal process for the north pole

3 CORONAL HOLES AND THE POLAR FIELD REVERSALS 377 Figure 1. Polar crown filament chains overlay lines of polarity reversal near the north solar pole on 23 July 1990 (Carrington rotation 1831), six months before the reversal of polarity at this pole. Neutral lines from this image were used to construct the polar projection for CR 1831 shown in Figure 3 and the partial Hæ synoptic chart (last panel) in Figure 5. Hæ filtergram obtained at the Boulder Solar Observatory of the Space Environment Lab., N.O.A.A. of cycle 22 as an example. In Section 4 we test our inferences of the magnetic field patterns against magnetogram data from Mount Wilson. In Section 5 we discuss the role of the equatorial coronal holes in the evolution of the polar coronal holes, and in Section 6 we summarize and discuss our findings. Finally, in Section 7 we present our conclusions. 2. Polar Projections of the Polar Magnetic Fields from Hæ Charts 2.1. THE Hæ SYNOPTIC CHARTS The reversals of the polar magnetic fields were originally observed using magnetograph data (Babcock and Babcock, 1955; Babcock, 1959), but the weakness of these fields and their proximity to the solar limb combine to make such measurements particularly difficult. An alternative method of charting the large-scale magnetic field patterns and their evolution is afforded by careful mapping of Hæ filaments, fibril patterns, and plage corridors, and this is of particular advantage for fields near the solar poles (Figure 1). Because they tend to be stable and attached to the solar surface at arch footpoints (where the fields are likely to be potential), these Hæ features are reliable markers of magnetic neutral lines (i.e., lines of polarity reversal of the large-scale solar magnetic fields) in areas of the quiet Sun and have been used to compile global maps of the large-scale solar polarity patterns for the past 30 years (McIntosh,

4 378 P. FOX, P. McINTOSH, AND P. R. WILSON 1972a, b, 1979, 1981; McIntosh, Willock, and Thompson, 1991). Portions of these Hæ synoptic charts were used in a study of the solar polar reversals for cycles 20 and 21 (Webb, Davis, and McIntosh, 1984). The construction of the synoptic charts for the most recent cycle, cycle 22, included a search for filaments near the polar limbs so that the polar reversal could be studied in considerable detail, particularly during the six months that the B-angle was favorable (the B-angle measures the tilt of the solar rotation axis toward the Earth). A particular feature of these maps is the polar crown, a ring of high-latitude filaments which almost encircles the polar regions (McIntosh, 1992). During most of the cycle, this crown is broken by a long-lived, well-defined gap, the polar crown gap (McIntosh, 1980), through which the polar fields connect with largescale fields of like polarity at lower latitudes. During the ascending phase of the cycle, the polar crown drifts to higher latitudes and, just prior to the polar field reversal, the gap closes. The neutral line defined by the polar crown is part of the longest neutral line on the solar surface, having a physical connection with filaments and active regions near the solar equator, and with the polar crown of the opposite solar hemisphere (McIntosh, 1992). Late in the cycle, it forms a pattern that is somewhat like the seam which separates the leather covers of a baseball. The neutral line appears to underlie nearly all of the heliospheric current sheet observed or inferred in the corona (McIntosh, 1993) CORONAL HOLES MAPPED FROM HE I1083 NM SPECTROHELIOGRAMS The coronal holes have been included on the Hæ synoptic charts from 1975 through May 1997, although they have been included in maps published in Solar Geophysical Data (SGD) only since In He I 1083 nm spectroheliograms, they are seen as regions brighter than the rest of the solar disk, and containing a diffuse network fine structure (see Figure 2). The boundaries of the coronal holes are, however, often less distinct than those shown in X-ray images, and the quality of the boundary of a particular coronal hole often varies around its perimeter. Early comparisons between coronal hole outlines and coronal holes recorded in X-rays indicated an imperfect fit between the two types of data (Webb et al., 1984, andreferencestherein), butmore recentcomparisonswith the Yohkoh X-ray images (Uchida et al., 1994) show that most of the differences are due to masking of the coronal holes by the 3-dimensional bright streamers in the X-ray images. When comparing data only from disk center, the discrepancies are greatly reduced, and it should be remembered that both the soft X-ray dark regions and the bright regions are simply indicators of the open field-line regions which are the coronal holes. Further, since neither provide information concerning the polarity of these fields, they should be interpreted in conjunction with magnetogram data and neutral lines mapped from Hæ images.

5 CORONAL HOLES AND THE POLAR FIELD REVERSALS 379 Figure 2. Coronal holes appear (right of center, and in most of the north polar portion of this image) as the brightest portions of this He I 1083 nm spectroheliogram from the National Solar Observatory (Kitt Peak). This was one of many images used to construct the Hæ synoptic chart for Carrington rotation 1844, appearing in Figure 3 in polar projection. Seen in He I 1083 nm, the most distinct coronal hole boundaries consist of darkened network structure contiguous with especially bright and featureless coronal hole interiors. Well-defined coronal holes occur more often during the active portion of the solar cycle when magnetic field strengths near and within the coronal holes are relatively strong. In general, the field strength within a coronal hole

6 380 P. FOX, P. McINTOSH, AND P. R. WILSON is weak, but it is usually stronger than that in the surrounding quiet regions of the Sun (Zirker, 1977; Harvey et al., 1982). The less distinct coronal hole boundaries are mapped either by taking an average of several daily solar images, or by choosing the most distinct boundary. In some cases it seems there are systematic motions over the period of the disk passage, and these are noted in the original SGD maps by two boundaries with arrows between them indicating the direction of motion. In the polar regions, the coronal holes are less precisely defined, as are the polarity boundaries. Further, the weaker fields and coronal holes here show relatively large day-to-day variations. The He I 1083 nm images sometimes give additional difficulties by virtue of a slight limb darkening associated with the He I structure, often filling areas that were seen as distinctly bright coronal-holelike patterns further onto the solar disk. Despite these difficulties, however, time sequences of coronal hole charts show systematic motions and evolutions similar to those constructed from X-ray images obtained by Yohkoh (Uchida et al., 1994), and we believe that coronal hole observations based on the He I 1083 nm images provide a valuable source of information concerning these features of the Sun THE CONSTRUCTION OF THE POLAR PROJECTIONS North polar projection maps for 36 solar rotations in are presented in Figure 3. These were constructed by shading the negative polarity regions (the positive regions are left white) and using a darker shading for the coronal holes. A complete Carrington rotation is represented on a mercator projection, to which a coordinate transformation is applied so that the polar fields are displayed in polar projection down to latitude 45 æ.theb-angle is indicated in each case because its variation may affect the accuracy of the polar plots. The final charts were transported into a Mcintosh graphics program which allowed accurate overlays of shading patterns to emphasize the distinction between the large-scale field patterns and the coronal holes. Identical shading has been applied to the corresponding partial synoptic charts shown in Figure 5. From Figure 3, we note first some aspects of the polar fields that are not specific to our discussion of the polarity reversals: (1) The polar coronal hole is asymmetric in all but a very few solar rotations and exhibits a tendency to touch the pole at one boundary rather than cover the location of the rotational axis. (2) The apparent rate of solar rotation inferred from the systematic motion of the major axis of the polar coronal hole is variable, but generally appears slower than the day Carrington rotation period. A more detailed analysis reveals that this rotation rate is faster than would be expected for the polar regions.

7 CORONAL HOLES AND THE POLAR FIELD REVERSALS Description of the Polar Field Reversal The complete sequence of northern-hemisphere Hæ polar plots from Carrington rotations (CR) 1815 to 1850 is shown in Figure 3 and this may be compared with polar plots derived from the Mount Wilson magnetograph data for the same rotations presented by Wilson and Giovannis (1994), a selection of which is shown in Figure 4. In CR 1815, the old-polarity (negative) field, occupied the polar region. Within this field is found a negative coronal hole, although it is not symmetrical about the pole. A coronal hole of opposite polarity can also be seen at latitude æ in the Carrington longitude range L = æ within the region of positive largescale field below the polar crown. This configuration is typical of the polar field distribution during the rising phase of the cycle and we define it as Phase A of the reversal process. As shown in the plot for CR 1816, the polar coronal hole extends to at least latitude 45 æ through the polar crown gap near L = 300 æ, i.e., diametrically opposite the positive coronal hole, now located in the longitude range L = 60 æ 150 æ (Phase B). While similar extensions (or equatorward lobes) of the polar coronal hole can be observed from time to time prior to the reversal, this observation is significant because, during the next few rotations, the poleward component of this coronal hole weakened and; by CR 1820, had disappeared from the polar region altogether. The equatorward component can be seen near latitude ç 45 æ until CR 1822, at an ç 180 æ longitudinal displacement from the positive coronal hole. The polar field remained negative, connected to the lower-latitude region of negative flux through the polar crown gap (Phase C). This general configuration persisted until CR 1830, when the region of negative field at the pole became isolated from the lower-latitude field of the same sign by the closure of the polar crown gap (Phase D). In the plot for CR 1831 a second positive (new-cycle polarity) coronal hole appears in the longitude range L = æ within the lower latitude region of positive flux, which, by now, completely encircles the old-polarity polar field. After CR 1831, the first new-cycle hole can no longer be seen in the polar plots and, during the next few rotations, (CRs ), the second new-cycle hole extends to higher latitudes (70 80 æ ) (Phase E). In the plot for CR 1838, the last remnant of the old polar field disappears; and the new-polarity coronal hole extends into the polar region, just touching the pole in CRs 1838 and 1839 (Phase F). Here it consolidates over the next several rotations; and, by CR 1846, the polar region is dominated by the positive field with a large but non-symmetrically distributed coronal hole occupying all regions above latitude 70 æ (Phase G). We have also studied the Hæ synoptic charts for the reversal of the southern magnetic polarity in cycle 22 and for both reversals in cycle 21. The same steps or phases noted above may also be found during these reversals, with some interesting differences which will be discussed in more detail in a later paper.

8 382 P. FOX, P. McINTOSH, AND P. R. WILSON Figure 3a.

9 CORONAL HOLES AND THE POLAR FIELD REVERSALS 383 Figure 3b.

10 384 P. FOX, P. McINTOSH, AND P. R. WILSON Figure 3c. Figure 3a c. A complete sequence of northern-hemisphere polar plots from CRs 1815 to 1861 (cycle 22) is shown. Here the large-scale magnetic field distributions are inferred from synoptic observations of the Hæ filaments and a fibril pattern (negative polarity regions are shaded with light grey and positive polarity is white) and the coronal holes (indicated by darker shading within each polarity) are derived from He I 1083 nm spectroheliograms.

11 Figure 4. Polar plots of the large-scale magnetic fields derived from the Mount Wilson synoptic magnetic data are shown for selected Carrington rotations. CORONAL HOLES AND THE POLAR FIELD REVERSALS 385

12 386 P. FOX, P. McINTOSH, AND P. R. WILSON The phases of the reversals may be identified as follows: (A) The old-polarity coronal hole occupies the polar region and is surrounded by old-polarity field. A new-polarity coronal hole can be seen near latitude 45 æ. (B) The old-polarity coronal hole extends through the polar crown gap to the old-polarity region near L = 45 æ about 180 æ displaced in longitude from the new-polarity coronal hole. (C) The old-polarity coronal hole vacates the polar region but the polar field retains the old-polarity. The equatorward component of the old-polarity coronal hole can be seen near latitude 45 æ for about another year. (D) The polar crown gap closes and the old-polarity coronal hole disappears from within the mid-latitude old-polarity field. (E) A new-polarity coronal hole extends to higher latitudes. (F) The new-polarity coronal hole moves into the polar region and the newpolarity field replaces the old-polarity field. (G) The new-polarity coronal hole occupies the polar region and is surrounded by the field with new cycle polarity. 4. Polar Plots Derived from Mount Wilson Magnetic Data Although Mount Wilson magnetograms do not show coronal holes, they do provide direct information regarding the magnetic fields and their polarities. A selection of the magnetograms in polar projection (covering a period similar to that described in Figure 3, and showing the large-scale fields above latitude 35 æ ) is provided in Figure 4. This sequence supports the general pattern of evolution of the large-scale magnetic fields described above but with some discrepancies, such as in CR 1922, in which the polar crown gap appears at a slightly different longitude, compared to the Hæ projections. The Mount Wilson data extend only to latitude 75 æ ; and in these plots, the fields above this latitude are estimated by setting the fields at latitude 85 æ equal to the mean of the fields at all longitudes at latitude 75 æ and linearly interpolating between the two values at each longitude. The rotations shown in Figure 4 were chosen so that the B-angle is near zero in each case. In the plot for CR 1816 (which corresponds to Phase B defined above), the polar region is occupied by negative field surrounded by a well-defined positive-negative (from lower latitudes to higher) neutral line which corresponds to the polar crown as displayed in the Hæ plots. This neutral line does not completely encircle the pole in this plot, since the polar field connects through the polar crown gap to likepolarity fields at lower latitudes. By CR 1822 (Phase C), the neutral line almost completely encircles the pole except for a connection through the narrower gap to a second (negative-positive) neutral line at lower latitudes. The magnetic field distribution shown in this plot corresponds closely to that shown in Figure 3 for the same Carrington rotation but the gap occurs at about 10 æ in Figure 4, but at 60 æ in Figure 3. This discrepancy may be due to the weakness of the fields adjacent

13 CORONAL HOLES AND THE POLAR FIELD REVERSALS 387 to the neutral line which, in the plots of the Mount Wilson data, is derived from a contouring algorithm. In the plots for CRs 1829 and 1836 (Phases D and E), the polarity of the polar region is confused; the well-defined positive-negative neutral line has broken up, following the closure of the polar crown gap in CR 1830 (the plotting routine generated several smaller closed positive-negative neutral lines which probably reflect only the weakness of the fields in the polar region and this highlights an inherent difficulty with using the zero level contour lines for a quantity whose statistical error may be several Gauss). By this rotation the lower latitude negativepositive neutral line had become the locally dominant neutral line in the region, although in the plots for CRs 1843 and 1849 it is displaced to lower latitudes. However, in the plots from CR 1843 onwards (Phases F and G), the polar field is seen to be generally positive, surrounded by the, by then, well-defined but incomplete negative-positive neutral line. While it is less straightforward to identify the phases of the cycle from the plots shown in Figure 4, they confirm that the large-scale field distributions inferred from the Hæ data in Figure 3 are essentially correct. 5. Equatorial Coronal Holes and their Relation to the Polar Holes It was noted from the study of the polar plots in Figure 3 that coronal holes of both polarities appear and disappear at mid-latitudes (45 50 æ ); but it was not clear from this figure whether this involved a genuine creation and destruction process, or whether these holes arise from and descend to lower latitudes. This question can be addressed by reference to selected synoptic charts for the northern hemisphere for CRs shown in Figure 5 (a complete set may be found in the Solar-Geophysical Data Reports for the period). In these charts we have labeled the coronal holes by considering the complete sequence of charts and noting the continuity and sequence of origin of new components, not all of which are obvious from the partial sequence we present here. The old-cycle (negative polarity) coronal hole, which can be seen in the polar plot for CR 1815, is identified here (by the letter A) in the longitude range L = æ and, in both the polar and equatorial plots for CR 1816, a lobe from this hole can be seen extending through the polar crown gap (again identified by A). It was noted that this hole disappeared from the polar region in CR 1820, and can no longer be seen in the polar plots after CR Here it can be seen at mid-latitudes in the longitude range L = æ in CR 1820 (A), and its migration to lower latitudes can be followed in Figure 5 through CR 1828, sometimes as a single feature and sometimes subdivided into several components (when subdivided, the componentsare identified by A1, A2, etc.). After CR 1828, the trailing component (A1) is no longer evident but the leading component (A3) can be seen near the equator, particularly in the plot for CR 1831 in the longitude range L = 0 70 æ.

14 388 P. FOX, P. McINTOSH, AND P. R. WILSON Figure 5a. The new-polarity (positive) coronal hole, which was seen in the polar plot for CR 1815 in the longitude range L = æ, can be seen here within a large region of positive flux in the map for CR 1815, and is identified by the letter B. In the discussion of Figure 3 it was noted that this hole could be followed through CR 1831 when a second positive polarity coronal hole appeared in the longitude range L = æ, and this development can be followed in more detail in the equatorial plots.

15 CORONAL HOLES AND THE POLAR FIELD REVERSALS 389 Figure 5b. Figure 5a b. A selection of Hæ synoptic charts for the northern hemisphere for CRs is shown. Coronal holes are labeled to distinguish those of opposite polarity (A versus B) and numbered to identify the sequential development of related components. The charts are shaded in the same way as Figure 3. In CR 1815 this hole appears in Figure 5 as a single extended feature, B, but, in CR 1816, it appears to have subdivided into three coronal holes within the same large-scale region of positive flux, and these are identified as B 1, B 2,andB 3.This system can be followed through CR 1830, sometimes appearing as a single hole and

16 390 P. FOX, P. McINTOSH, AND P. R. WILSON sometimes subdivided into two or three holes. In the plot for CR 1830, three positive polarity holes can be identified, extending from longitude L = æ,andare labeled B2, B 5,andB4. It is not clear whether they share a common origin, but we note that they appear within the same large-scale field region of positive polarity. In CR 1830, the leading component of this system (B4) can be seen equatorwards of a large region of negative flux, within which a new cell of positive flux has formed to the north-west of B4. In CR 1831, the positive unipolar region expands and connects with the positive regions above and below the negative region, and what appears to be a new (positive) coronal hole, B6, appears within and above this region, separated by ç 180 æ from the old-cycle negative polarity coronal hole, A7. The polar plots of Figure 3 show that B6 is the hole which migrates to higher latitudes and becomes the new-cycle polar coronal hole. A second hole, B4, can be seen at lower latitudes, but B 5 is no longer observed. The above account is essentially descriptive and, although we have identified the individual components of each system by the letters A i and B i, we do not assert unequivocally that they are physically connected but they may be so. We simply report the synoptic evolution of these coronal holes and note that, since by CR 1831, the polar fields had not reversed, the Sun exhibited a weak quadrupolar field structure at this stage, since two prominent coronal holes, of opposite polarity existed at high latitudes and there was no dominant axial component of the largescale field. After CR 1831, this new-cycle polarity hole (B6) advanced poleward, and this has been described as phase E of the polar field reversal. It would seem that the evolution of the polar coronal holes described in Section 3 above is part of a general evolutionary pattern of coronal holes which is related to the Sun s global magnetic field. 6. Discussion It would appear that the large-scale magnetic fields in the polar regions, which dominate the poloidal component of the large-scale solar magnetic field, are not solely the product of decaying active regions. Some local effects related to subsurface processes would seem to be involved. Active region fields, on the other hand, are presumed to be generated by the subsurface toroidal fields built up over the course of the solar cycle. Previously, a coronal hole has been regarded simply as a surface and upper atmospheric phenomenon, a property of the corona as the name implies, and it has been argued (Wang, Hawley, and Sheeley, 1996) that coronal holes form and decay as a result of reconnections between the global bipole field and the fields of active regions. The longevity of the coronal holes described above, and their relationship to the reversals of the polar magnetic fields suggest that they may have a more direct involvement with cyclic processes.

17 CORONAL HOLES AND THE POLAR FIELD REVERSALS 391 The particular phases of the polar field reversal that we have outlined in this paper are striking in the fact that the evolution of the polar coronal hole seems to be part of a more fundamental process; the polar coronal hole does not simply disappear in situ, prior to the reversal, but appears to move out of the polar regions. Correspondingly, a coronal hole does not just appear at the pole after the polar field has reversed, but migrates into the polar regions from mid-latitudes. Another feature of the patterns formed by the global distribution of coronal holes is the apparent 180 æ longitude separation of the holes that are part of the reversal process. In particular, it was noted above that the component of the positive coronal hole, B 6, which became the new-cycle polar coronal hole, began its poleward progress in CR 1831 at a longitude separation of 180 æ from the old-cycle polarity coronal hole, A 3. The strong connections between the polar and equatorial coronal holes indicated above, their latitudinal displacements of ç 180 æ, and their almost synchronized evolution (one away from the pole, the other toward) suggest these holes may be connected by subsurface fields which provide a quadrupolar component to the global field. If so, the locations of the axes of the quadrupole change during the reversal, and each hemisphere seems to evolve independently of the other, only to resynchronize once both reversals are complete. It was noted in Section 2.1 that, during the rising phase of the cycle, the polar crowns migrate poleward and the polar field reversal follows shortly after the closure of the polar crown gap. Here we have described the departure of the old polar coronal hole through this gap, just prior to its closure, and we suggest that this is a component of the reversal process. Note that the actual reversal, after the gap closes, occurs on a very short time scale compared to the evolution leading up to or following the reversal, and distinctly shorter than the solar cycle time scale. Further, this characteristic behavior is seen in all four reversals we have studied. The importance of the polar crown gap was also noted by Webb, Davis, and McIntosh (1984). These observations provide important constraints for dynamo models, since (i) the evolution of the quadrupolar-like configuration of the poloidal field during the reversal occurs over a short period of time compared to the cycle itself, yet (ii) it extends over large (global) scales. These features should be predicted by a successful dynamo (or other) model of the solar cycle. 7. Conclusion This paper has described the evolution of coronal holes and associated features in relation to the polar field reversal which is common for the four polar field reversals that we have studied (cycles 21 and 22, north and south). As one might expect, there are aspects of each reversal that differ from each other but the remarkable relation between the equatorial and the polar coronal hole patterns during the

18 392 P. FOX, P. McINTOSH, AND P. R. WILSON reversal process leads us to conclude that the global evolution of coronal holes is a fundamental component of the reversal process. The relation between the equatorial and the polar coronal holes will be further explored in a later paper. Acknowledgements We wish to acknowledge the stimulating participation of Herschel Snodgrass in this project which was initiated during a mini-workshop held at Sydney University. We also acknowledge the contribution of Dr Li Yan, of the University of Sydney who prepared the polar plots of the magnetic fields derived from the Mount Wilson data. We thank Dick White, Peter Gilman, Paul Charbonneau, and the anonymous referee for constructive comments on the manuscript. A number of institutions and funding agencies have made this work possible. We thank the University of Sydney, School of Mathematics, the High Altitude Observatory, and NOAA Space Environment Laboratory. PRW acknowledges funding from ARC Grant No. A References Avrett, E. H., Fontenla, J., and Loeser, R.: 1992, in D. M. Rabin and J. T. Jefferies (eds), Infrared Solar Physics, IAU Symp Babcock, H. D.: 1959, Astrophys. J. 130, 364. Babcock, H. D. and Livingston, W. C.: 1958, Science 127, Babcock, H. W. and Babcock, H. D.: 1955, Astrophys. J. 121, 349. DeVore, C. R., and Sheeley, N. R., Jr.: 1987, Solar Phys. 108, 47. DeVore, C. R., Sheeley, N. R., Jr., and Boris, J. P.: 1984, Solar Phys. 92, 1. Harvey, K. H., Sheeley, N. R., Jr, and Harvey, J.: 1982, Solar Phys. 79, 149. Layden, A. C., Fox, P. A., Howard, J. M., Sarajedini, A., Schatten, K. H., and Sofia, S.: 1991, Solar Phys. 132, 1. Leighton, R. B.: 1964, Astrophys. J. 140, McIntosh, P. S.: 1972a, in P. S. McIntosh and M. Dryer (eds), Solar Activity Observations and Predictions, The MIT Press, Boston, Massachusetts, p. 65. McIntosh, P. S.: 1972b, Rev. Geophys. Space Sci. 10, 837. McIntosh, P. S.: 1979, UAG Rep. 70, World Data Center A for Solar-Terrestrial Physics, Boulder, Colorado. McIntosh, P. S.: 1980, in M. Dryer and E. Tanberg-Hanssen (eds), Solar and Interplanetary Dynamics, IAU Symp. 91, 25. McIntosh, P. S.: 1981, in L. E. Cram and J. H. Thomas (eds), The Physics of Sunspots, Sacramento Peak Observatory, Sunspot, New Mexico, p. 7. McIntosh, P. S.: 1992, in K. L. Harvey (ed.), Proceedings of the National Solar Observatory / Sacramento Peak 12th Summer Workshop, San Francisco, California, p. 14. McIntosh, P. S.: 1993, in J. Hruška, M. A. Shea, D. F. Smart, and G. Heckman (eds), Solar-Terrestrial Predictions IV, Proceedings of a Workshop at Ottawa, Canada, May, 1992, Vol.2,U.S.Deptof Commerce, Boulder, CO, p. 20. McIntosh, P. S., Willock, E. C., and Thompson, R. J.: 1991, UAG Rep. 101, World Data Center A for Solar-Terrestrial Physics, Boulder, Colorado. Mikic, Z. and McClymont, A. N.: 1994, Astron. Soc. Pacific Conf. Ser. 68, 225. Murray, N. and Wilson, P. R.: 1992, Solar Phys. 142, 221. Sheeley, N. R., Jr., Nash, A. G., and Wang, Y.-M.: 1987, Astrophys. J. 319, 481.

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