CORONAL HOLES AND THE POLAR FIELD REVERSALS

P. FOX1, P. McINTOSH2 and P. R. WILSON31High Altitude Observatory, Boulder, CO 80307–3000, U.S.A.

2HelioSynoptics Inc., Boulder, CO 80307–3000, U.S.A.3School 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 isprovided using polar projections which combine the large-scale magnetic fields as inferred fromH� synoptic charts and coronal holes mapped from HeI �1083 nm spectroheliograms. These plotsare supported by polar plots of the magnetic fields derived from synoptic magnetic field data fromthe Mount Wilson Observatory. The coronal holes showed some unexpected evolutionary patternsin relation to the polarity reversals, and these patterns appear to be coordinated with changes inthe global patterns of coronal holes and the heliospheric current sheet, suggesting that the polarreversal originates from global processes rather than from local magnetic flux annihilation. Similarpatterns have been observed in the reversal of the southern polar magnetic field in cycle 22 and inboth hemispheres in cycle 21. The consequences of these findings for the solar dynamo process arediscussed.

1. Introduction

Because two eleven-year sunspot cycles must be completed before the magneticfield 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 WilcoxSolar Observatories over the past four solar cycles indicate the presence of a 22-year cyclic variation in these fields, associated with sunspots and involving polarityreversals (Babcock and Livingston, 1958). The time of maximum of the polar fieldsis 90� out of phase with the peak of the smoothed sunspot number for that sunspotcycle (Webb, Davis, and McIntosh, 1984; Laydenet al., 1991). Although thepolar 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 solarmagnetic cycle. Thus an understanding of the physical processes which controlthe reversal of the polar fields should provide some insight into the solar dynamoprocess.

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 supergran-ule motions, differential rotation, and meridional flows disperse the active-regionmagnetic 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 areeffected by flux cancellation and that the observed features of the large-scale fieldscan be simulated by suitably varying the parameters of the flux transport equation.

Solar Physics177: 375–393, 1998.c 1998Kluwer Academic Publishers. Printed in Belgium.

376 P. FOX, P. McINTOSH, AND P. R. WILSON

However, this model faces difficulties, both theoretical and observational. Inthe flux transport equation, the radial component of the surface magnetic fields isregarded as a scalar function of surface position, and the three-dimensional natureof these fields, in particular the sub-surface connections of these field lines andthe transverse magnetic tensions which would be created by their apparent surfacetransport, are ignored in the surface flux transport model.

From an observational perspective, Murray and Wilson (1992) and Wilsonand Giovannis (1994) described recent changes to the polar fields using synopticmagnetic data provided by the Mount Wilson Observatory. Wilson and Giovannisproduced polar projections of the large-scale fields through the period of the polarfield reversal. By comparing the observed polar fields with simulations using theflux 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) indicatedthat a center-to-limb calibration is required for their magnetograms, and this mightchange some of the details of the flux distribution. Further discussion of this pointis beyond the scope of this paper.

When theory and observations are not coincident, some progress may beachieved by studying observations of different, but related, phenomena. An import-ant property of the large-scale fields is their relation to coronal holes, which aredefined as regions of open field lines (Zirker, 1977) and these have been identifiedwith the darkest features in X-ray images of the Sun and are particularly obviousin the images provided byYohkoh. In this paper we will not discuss the magneticnature of coronal holes in any detail (for two recent viewpoints see Mikic andMcClymont (1994) and Wang, Hawley, and Sheeley (1996)). Coronal holes are,however, generally centered in large unipolar magnetic cells, and their low coron-al density is probably a consequence of the magnetic field lines being open tointerplanetary space.

Coronal holes are also identified with bright features in ground-based imagestaken in the infrared line HeI 1083 nm (the so-called 10830 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 networkhas a diffuse, low-contrast structure.

The distribution of coronal holes over the solar surface at the various phases ofthe solar cycle has been discussed by McIntosh (1993). During sunspot minimumthe dominant coronal holes on the Sun are those associated with the polar fields, andthe reversal of these fields (after solar maximum) requires that each polar coronalhole must be replaced by one of opposite polarity. In this paper, we describe theevolution of the polar coronal holes during the north-polar field reversal of cycle 22and 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 synopticcharts. In Section 3 we describe the polar field reversal process for the north pole

CORONAL HOLES AND THE POLAR FIELD REVERSALS 377

Figure 1. Polar crown filament chains overlay lines of polarity reversal near the north solar poleon 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 inFigure 3 and the partial H� synoptic chart (last panel) in Figure 5. H� filtergram obtained at theBoulder 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 fieldpatterns against magnetogram data from Mount Wilson. In Section 5 we discussthe 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 wepresent our conclusions.

2. Polar Projections of the Polar Magnetic Fields from H� Charts

2.1. THE H� SYNOPTIC CHARTS

The reversals of the polar magnetic fields were originally observed using mag-netograph data (Babcock and Babcock, 1955; Babcock, 1959), but the weaknessof these fields and their proximity to the solar limb combine to make such meas-urements particularly difficult. An alternative method of charting the large-scalemagnetic field patterns and their evolution is afforded by careful mapping of H�

filaments, fibril patterns, and plage corridors, and this is of particular advantage forfields 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 H� features are reliable markersof magnetic neutral lines (i.e., lines of polarity reversal of the large-scale solarmagnetic fields) in areas of the quiet Sun and have been used to compile globalmaps of the large-scale solar polarity patterns for the past 30 years (McIntosh,

378 P. FOX, P. McINTOSH, AND P. R. WILSON

1972a, b, 1979, 1981; McIntosh, Willock, and Thompson, 1991). Portions of theseH� synoptic charts were used in a study of the solar polar reversals for cycles 20and 21 (Webb, Davis, and McIntosh, 1984). The construction of the synoptic chartsfor the most recent cycle, cycle 22, included a search for filaments near the polarlimbs so that the polar reversal could be studied in considerable detail, particularlyduring the six months that theB-angle was favorable (theB-angle measures thetilt of the solar rotation axis toward the Earth).

A particular feature of these maps is thepolar crown, a ring of high-latitudefilaments which almost encircles the polar regions (McIntosh, 1992). During mostof the cycle, this crown is broken by a long-lived, well-defined gap, thepolarcrown gap(McIntosh, 1980), through which the polar fields connect with large-scale fields of like polarity at lower latitudes. During the ascending phase of thecycle, the polar crown drifts to higher latitudes and, just prior to the polar fieldreversal, the gap closes.

The neutral line defined by the polar crown is part of the longest neutral line onthe solar surface, having a physical connection with filaments and active regionsnear 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 theseam which separates the leather covers of a baseball. The neutral line appearsto underlie nearly all of the heliospheric current sheet observed or inferred in thecorona (McIntosh, 1993).

2.2. CORONAL HOLES MAPPED FROMHE I 1083NM SPECTROHELIOGRAMS

The 10830 coronal holes have been included on the H� synoptic charts from 1975through May 1997, although they have been included in maps published inSolarGeophysical Data(SGD) only since 1981. In HeI 1083 nm spectroheliograms, theyare seen as regions brighter than the rest of the solar disk, and containing a diffusenetwork fine structure (see Figure 2). The boundaries of the 10830 coronal holesare, however, often less distinct than those shown in X-ray images, and the qualityof the boundary of a particular coronal hole often varies around its perimeter. Earlycomparisons between 10830 coronal hole outlines and coronal holes recorded inX-rays indicated an imperfect fit between the two types of data (Webbet al., 1984,and references therein), but more recent comparisons with theYohkohX-ray images(Uchidaet al., 1994) show that most of the differences are due to masking of thecoronal holes by the 3-dimensional bright streamers in the X-ray images. Whencomparing data only from disk center, the discrepancies are greatly reduced, and itshould be remembered that both the soft X-ray dark regions and the 10830 brightregions are simply indicators of the open field-line regions which are the coronalholes. Further, since neither provide information concerning the polarity of thesefields, they should be interpreted in conjunction with magnetogram data and neutrallines mapped from H� images.

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 HeI 1083 nm spectroheliogram from the National Solar Observatory(Kitt Peak). This was one of many images used to construct the H� synoptic chart for Carringtonrotation 1844, appearing in Figure 3 in polar projection.

Seen in HeI 1083 nm, the most distinct coronal hole boundaries consist ofdarkened network structure contiguous with especially bright and featureless coron-al hole interiors. Well-defined coronal holes occur more often during the activeportion of the solar cycle when magnetic field strengths near and within the coron-al holes are relatively strong. In general, the field strength within a coronal hole

380 P. FOX, P. McINTOSH, AND P. R. WILSON

is weak, but it is usually stronger than that in the surrounding quiet regions of theSun (Zirker, 1977; Harveyet al., 1982).

The less distinct coronal hole boundaries are mapped either by taking an averageof several daily solar images, or by choosing the most distinct boundary. In somecases it seems there are systematic motions over the period of the disk passage, andthese are noted in the originalSGDmaps by two boundaries with arrows betweenthem indicating the direction of motion.

In the polar regions, the 10830 coronal holes are less precisely defined, as arethe polarity boundaries. Further, the weaker fields and coronal holes here showrelatively large day-to-day variations. The HeI 1083 nm images sometimes giveadditional difficulties by virtue of a slight limb darkening associated with theHeI structure, often filling areas that were seen as distinctly bright coronal-hole-like patterns further onto the solar disk. Despite these difficulties, however, timesequences of coronal hole charts show systematic motions and evolutions similarto those constructed from X-ray images obtained byYohkoh(Uchidaet al., 1994),and we believe that coronal hole observations based on the HeI 1083 nm imagesprovide a valuable source of information concerning these features of the Sun.

2.3. THE CONSTRUCTION OF THE POLAR PROJECTIONS

North polar projection maps for 36 solar rotations in 1989–1991 are presented inFigure 3. These were constructed by shading the negative polarity regions (thepositive regions are left white) and using a darker shading for the coronal holes. Acomplete Carrington rotation is represented on a mercator projection, to which acoordinate transformation is applied so that the polar fields are displayed in polarprojection down to latitude 45�. TheB-angle is indicated in each case because itsvariation may affect the accuracy of the polar plots.

The final charts were transported into a Mcintosh graphics program whichallowed accurate overlays of shading patterns to emphasize the distinction betweenthe large-scale field patterns and the coronal holes. Identical shading has beenapplied 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 specificto our discussion of the polarity reversals:

(1) The polar coronal hole is asymmetric in all but a very few solar rotationsand exhibits a tendency to touch the pole at one boundary rather than cover thelocation of the rotational axis.

(2) The apparent rate of solar rotation inferred from the systematic motion ofthe major axis of the polar coronal hole is variable, but generally appears slowerthan the 27.27-day Carrington rotation period. A more detailed analysis revealsthat this rotation rate is faster than would be expected for the polar regions.

CORONAL HOLES AND THE POLAR FIELD REVERSALS 381

3. Description of the Polar Field Reversal

The complete sequence of northern-hemisphere H� polar plots from Carringtonrotations (CR) 1815 to 1850 is shown in Figure 3 and this may be comparedwith polar plots derived from the Mount Wilson magnetograph data for the samerotations presented by Wilson and Giovannis (1994), a selection of which is shownin Figure 4.

In CR 1815, the old-polarity (negative) field, occupied the polar region. Withinthis field is found a negative coronal hole, although it is not symmetrical about thepole. A coronal hole of opposite polarity can also be seen at latitude 45–50� inthe Carrington longitude rangeL = 90–180� within the region of positive large-scale field below the polar crown. This configuration is typical of the polar fielddistribution during the rising phase of the cycle and we define it as Phase A of thereversal process.

As shown in the plot for CR 1816, the polar coronal hole extends to at leastlatitude 45� through the polar crown gap nearL = 300�, i.e., diametrically oppositethe positive coronal hole, now located in the longitude rangeL = 60�–150�

(Phase B). While similar extensions (or equatorward lobes) of the polar coronalhole can be observed from time to time prior to the reversal, this observationis significant because, during the next few rotations, the poleward component ofthis coronal hole weakened and; by CR 1820, had disappeared from the polarregion altogether. The equatorward component can be seen near latitude� 45�

until CR 1822, at an� 180� longitudinal displacement from the positive coronalhole. The polar field remained negative, connected to the lower-latitude region ofnegative flux through the polar crown gap (Phase C). This general configurationpersisted until CR 1830, when the region of negative field at the pole becameisolated from the lower-latitude field of the same sign by the closure of the polarcrown gap (Phase D).

In the plot for CR 1831 a second positive (new-cycle polarity) coronal holeappears in the longitude rangeL = 240–270� within the lower latitude region ofpositive flux, which, by now, completely encircles the old-polarity polar field. AfterCR 1831, the first new-cycle hole can no longer be seen in the polar plots and,during the next few rotations, (CRs 1831–35), the second new-cycle hole extendsto higher latitudes (70–80�) (Phase E). In the plot for CR 1838, the last remnantof the old polar field disappears; and the new-polarity coronal hole extends intothe polar region, just touching the pole in CRs 1838 and 1839 (Phase F). Hereit consolidates over the next several rotations; and, by CR 1846, the polar regionis dominated by the positive field with a large but non-symmetrically distributedcoronal hole occupying all regions above latitude 70� (Phase G).

We have also studied the H� synoptic charts for the reversal of the southernmagnetic polarity in cycle 22 and for both reversals in cycle 21. The same steps orphases noted above may also be found during these reversals, with some interestingdifferences which will be discussed in more detail in a later paper.

382 P. FOX, P. McINTOSH, AND P. R. WILSON

Figure 3a.

CORONAL HOLES AND THE POLAR FIELD REVERSALS 383

Figure 3b.

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 synopticobservations of the H� filaments and a fibril pattern (negative polarity regions are shaded with lightgrey and positive polarity is white) and the coronal holes (indicated by darker shading within eachpolarity) are derived from HeI 1083 nm spectroheliograms.

CO

RO

NA

LH

OL

ES

AN

DT

HE

PO

LA

RF

IEL

DR

EV

ER

SA

LS

38

5Figure 4. Polar plots of the large-scale magnetic fields derived from the Mount Wilson synoptic magnetic data are shown for selected Carrington rotations.

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�.(B) The old-polarity coronal hole extends through the polar crown gap to

the old-polarity region nearL = 45� about 180� displaced in longitude from thenew-polarity coronal hole.

(C) The old-polarity coronal hole vacates the polar region but the polar fieldretains the old-polarity. The equatorward component of the old-polarity coronalhole can be seen near latitude 45� for about another year.

(D) The polar crown gap closes and the old-polarity coronal hole disappearsfrom 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 new-

polarity 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 providedirect information regarding the magnetic fields and their polarities. A selection ofthe magnetograms in polar projection (covering a period similar to that describedin Figure 3, and showing the large-scale fields above latitude 35�) is provided inFigure 4. This sequence supports the general pattern of evolution of the large-scalemagnetic 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 tothe H� projections. The Mount Wilson data extend only to latitude 75�; and in theseplots, the fields above this latitude are estimated by setting the fields at latitude85� equal to the mean of the fields at all longitudes at latitude 75� and linearlyinterpolating between the two values at each longitude. The rotations shown inFigure 4 were chosen so that theB-angle is near zero in each case.

In the plot for CR 1816 (which corresponds to Phase B defined above), the polarregion is occupied by negative field surrounded by a well-defined positive-negative(from lower latitudes to higher) neutral line which corresponds to the polar crownas displayed in the H� plots. This neutral line does not completely encircle thepole in this plot, since the polar field connects through the polar crown gap to like-polarity fields at lower latitudes. By CR 1822 (Phase C), the neutral line almostcompletely encircles the pole except for a connection through the narrower gapto a second (negative-positive) neutral line at lower latitudes. The magnetic fielddistribution shown in this plot corresponds closely to that shown in Figure 3 forthe same Carrington rotation but the gap occurs at about 10� in Figure 4, but at 60�

in Figure 3. This discrepancy may be due to the weakness of the fields adjacent

CORONAL HOLES AND THE POLAR FIELD REVERSALS 387

to the neutral line which, in the plots of the Mount Wilson data, is derived from acontouring algorithm.

In the plots for CRs 1829 and 1836 (Phases D and E), the polarity of the polarregion 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 routinegenerated several smaller closed positive-negative neutral lines which probablyreflect only the weakness of the fields in the polar region and this highlights aninherent difficulty with using the zero level contour lines for a quantity whosestatistical error may be several Gauss). By this rotation the lower latitude negative-positive 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 seento be generally positive, surrounded by the, by then, well-defined but incompletenegative-positive neutral line. While it is less straightforward to identify the phasesof the cycle from the plots shown in Figure 4, they confirm that the large-scale fielddistributions inferred from the H� 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 ofboth polarities appear and disappear at mid-latitudes (45–50�); but it was not clearfrom this figure whether this involved a genuine creation and destruction process,or whether these holes arise from and descend to lower latitudes. This question canbe addressed by reference to selected synoptic charts for the northern hemispherefor CRs 1815–1831 shown in Figure 5 (a complete set may be found in theSolar-Geophysical Data Reportsfor the period). In these charts we have labeledthe coronal holes by considering the complete sequence of charts and noting thecontinuity and sequence of origin of new components, not all of which are obviousfrom the partial sequence we present here.

The old-cycle (negative polarity) coronal hole, which can be seen in the polarplot for CR 1815, is identified here (by the letterA) in the longitude rangeL = 250–340� and, in both the polar and equatorial plots for CR 1816, a lobe from thishole can be seen extending through the polar crown gap (again identified byA). Itwas noted that this hole disappeared from the polar region in CR 1820, and can nolonger be seen in the polar plots after CR 1821. Here it can be seen at mid-latitudesin the longitude rangeL = 100–180� in CR 1820 (A), and its migration to lowerlatitudes can be followed in Figure 5 through CR 1828, sometimes as a singlefeature and sometimes subdivided into several components (when subdivided, thecomponents are identified byA1,A2, etc.). After CR 1828, the trailing component(A1) is no longer evident but the leading component (A3) can be seen near theequator, particularly in the plot for CR 1831 in the longitude rangeL = 0–70�.

388 P. FOX, P. McINTOSH, AND P. R. WILSON

Figure 5a.

The new-polarity (positive) coronal hole, which was seen in the polar plot forCR 1815 in the longitude rangeL = 90–180�, can be seen here within a largeregion of positive flux in the map for CR 1815, and is identified by the letterB.In the discussion of Figure 3 it was noted that this hole could be followed throughCR 1831 when a second positive polarity coronal hole appeared in the longituderangeL = 240–270�, and this development can be followed in more detail in theequatorial plots.

CORONAL HOLES AND THE POLAR FIELD REVERSALS 389

Figure 5b.

Figure 5a–b. A selection of H� synoptic charts for the northern hemisphere for CRs 1815–1831 isshown. Coronal holes are labeled to distinguish those of opposite polarity (A versusB) and numberedto identify the sequential development of related components. The charts are shaded in the same wayas 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 samelarge-scale region of positive flux, and these are identified asB1,B2, andB3. Thissystem can be followed through CR 1830, sometimes appearing as a single hole and

390 P. FOX, P. McINTOSH, AND P. R. WILSON

sometimes subdivided into two or three holes. In the plot for CR 1830, three positivepolarity holes can be identified, extending from longitudeL = 70–270�, and arelabeledB2,B5, andB4. It is not clear whether they share a common origin, but wenote 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 equatorwardsof a large region of negative flux, within which a new cell of positive flux hasformed to the north-west ofB4. In CR 1831, the positive unipolar region expandsand connects with the positive regions above and below the negative region, andwhat appears to be a new (positive) coronal hole,B6, appears within and abovethis region, separated by� 180� from the old-cycle negative polarity coronal hole,A7. The polar plots of Figure 3 show thatB6 is the hole which migrates to higherlatitudes and becomes the new-cycle polar coronal hole. A second hole,B4, can beseen at lower latitudes, butB5 is no longer observed.

The above account is essentially descriptive and, although we have identifiedthe individual components of each system by the lettersAi andBi, we do notassert unequivocally that they are physically connected but they may be so. Wesimply report the synoptic evolution of these coronal holes and note that, since byCR 1831, the polar fields had not reversed, the Sun exhibited a weak quadrupolarfield structure at this stage, since two prominent coronal holes, of opposite polarityexisted at high latitudes and there was no dominant axial component of the large-scale 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 3above is part of a general evolutionary pattern of coronal holes which is related tothe Sun’s global magnetic field.

6. Discussion

It would appear that the large-scale magnetic fields in the polar regions, whichdominate the poloidal component of the large-scale solar magnetic field, are notsolely the product of decaying active regions. Some local effects related to sub-surface processes would seem to be involved. Active region fields, on the otherhand, are presumed to be generated by the subsurface toroidal fields built up overthe course of the solar cycle. Previously, a coronal hole has been regarded simply asa surface and upper atmospheric phenomenon, a property of the corona as the nameimplies, and it has been argued (Wang, Hawley, and Sheeley, 1996) that coronalholes form and decay as a result of reconnections between the global bipole fieldand the fields of active regions.

The longevity of the coronal holes described above, and their relationship tothe reversals of the polar magnetic fields suggest that they may have a more directinvolvement with cyclic processes.

CORONAL HOLES AND THE POLAR FIELD REVERSALS 391

The particular phases of the polar field reversal that we have outlined in thispaper are striking in the fact that the evolution of the polar coronal hole seemsto be part of a more fundamental process; the polar coronal hole does not simplydisappearin 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 fieldhas reversed, but migrates into the polar regions from mid-latitudes.

Another feature of the patterns formed by the global distribution of coronal holesis the apparent 180� longitude separation of the holes that are part of the reversalprocess. In particular, it was noted above that the component of the positive coronalhole,B6, which became the new-cycle polar coronal hole, began its polewardprogress in CR 1831 at a longitude separation of 180� from the old-cycle polaritycoronal hole,A3.

The strong connections between the polar and equatorial coronal holes indicatedabove, their latitudinal displacements of� 180�, and their almost synchronizedevolution (one away from the pole, the other toward) suggest these holes maybe connected by subsurface fields which provide a quadrupolar component to theglobal field. If so, the locations of the axes of the quadrupole change during thereversal, and each hemisphere seems to evolve independently of the other, only toresynchronize once both reversals are complete.

It was noted in Section 2.1 that, during the rising phase of the cycle, the polarcrowns migrate poleward and the polar field reversal follows shortly after theclosure of the polar crown gap. Here we have described the departure of the oldpolar coronal hole through this gap, just prior to its closure, and we suggest thatthis is a component of the reversal process. Note that the actual reversal, after thegap closes, occurs on a very short time scale compared to the evolution leadingup to or following the reversal, and distinctly shorter than the solar cycle timescale. Further, this characteristic behavior is seen in all four reversals we havestudied. 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 duringthe 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 asuccessful dynamo (or other) model of the solar cycle.

7. Conclusion

This paper has described the evolution of coronal holes and associated features inrelation to the polar field reversal which is common for the four polar field reversalsthat 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 remarkablerelation between the equatorial and the polar coronal hole patterns during the

392 P. FOX, P. McINTOSH, AND P. R. WILSON

reversal process leads us to conclude that the global evolution of coronal holes is afundamental componentof the reversal process. The relation between the equatorialand the polar coronal holes will be further explored in a later paper.

Acknowledgements

We wish to acknowledge the stimulating participation of Herschel Snodgrass in thisproject which was initiated during a mini-workshop held at Sydney University. Wealso acknowledge the contribution of Dr Li Yan, of the University of Sydney whoprepared the polar plots of the magnetic fields derived from the Mount Wilson data.We thank Dick White, Peter Gilman, Paul Charbonneau,and the anonymous refereefor constructive comments on the manuscript. A number of institutions and fundingagencies have made this work possible. We thank the University of Sydney, Schoolof Mathematics, the High Altitude Observatory, and NOAA Space EnvironmentLaboratory. PRW acknowledges funding from ARC Grant No. A69131266.

References

Avrett, E. H., Fontenla, J., and Loeser, R.: 1992, in D. M. Rabin and J. T. Jefferies (eds), ‘InfraredSolar Physics’,IAU Symp.154.

Babcock, H. D.: 1959,Astrophys. J.130, 364.Babcock, H. D. and Livingston, W. C.: 1958,Science127, 1058.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, 1547.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 Dynam-

ics’, 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 Observat-

ory / Sacramento Peak 12th Summer Workshop, San Francisco, California, p. 14.McIntosh, P. S.: 1993, in J. Hruska, 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. Dept ofCommerce, Boulder, CO, p. 20.

McIntosh, P. S., Willock, E. C., and Thompson, R. J.: 1991,UAG Rep. 101, World Data Center A forSolar-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.

CORONAL HOLES AND THE POLAR FIELD REVERSALS 393

Uchida, Y., Wanatabe, T., Shibata, K., and Hudson, H. S. (eds.): 1994,X-Ray Solar Physics fromYohkoh, Universal Academic Press Inc., Tokyo.

Ulrich, R. K.: 1992, in M. S. Giampapa and J. A. Bookbinder (eds),Cool Stars, Stellar Systems, andthe Sun, Astronomical Society of the Pacific, San Francisco, p. 265.

Wang, Y.-M., Nash, A. G., and Sheeley, N. R., Jr.: 1989,Science245, 681.Wang, Y.-M., Hawley, S. H., and Sheeley, N. R., Jr.: 1996,Science271, 464.Webb, D. F., Davis, J. M., and McIntosh, P. S: 1984,Solar Phys.92, 109.Wilson, P. R. and Giovannis, J.: 1994,Solar Phys.155, 29.Zirker, J. B.: 1977, in J. B. Zirker (ed.),Coronal Holes and High Speed Wind Streams, Colorado

University Press, Boulder, Colorado.