High-Resolution Studies of the Solar Polar Magneti

High-ResolutionStudiesoftheSolarPolarMagneticFieldsJ.R.VarsikBigBearSolarObservatory/NewJerseyInstituteofTechnology40386NorthShoreLane,BigBearCity,CA92314P.R.WilsonandY.LiUniversityofSydneySchoolofMathematicsandStatistics,Sydney,NSW2006(Received.....;Acceptedinnalform.....)Abstract.Wepresenthigh-resolutionstudiesofthesolarpolarmagneticeldsnearsunspotmaximumin1989andtowardssunspotminimumin1995.Weshowthat,in1989,thepolarlatitudeswerecoveredbyseveralunipolarregionsofbothpolarities.In1995,however,afterthepolareldreversalwascomplete,eachpoleexhibitedonlyonedominantpolarityregion.Eachunipolarregioncontainsmagneticknotsofbothpolaritiesbutthenumbercountoftheknotsofthedominantpolarityexceedsthatoftheoppositepolaritybyaratiooforder4:1,anditisraretondoppositepolaritypairs,i.e.magneticbipoles.Theseknotshavelifetimesgreaterthan7hoursbutlessthan24hours.Weinterpretthelongitudinaldisplacementoftheknotsovera7hourperiodasameasureofthelocalrotationrate.ThisrotationrateisfoundtobegenerallyconsistentwithSnodgrass’(1983)magneticrotationlaw.Inanattempttoobtainsomeinsightintotheoperationofthesolardynamo,sketchesofpostulatedsubsurfaceeldcongurationscorrespondingtotheobservedsurfaceeldsatthesetwoepochsofthesolarcyclearepresented.1.IntroductionThedirectmeasurementofthesplittingofaZeeman-sensitivespectrallinebythemagneticeldofasunspot,asmeasuredbyG.E.Halein1908(HaleandNicolson1938)openedanewerainsolarphysics,the‘Mt.Wilsonera’.ThedevelopmentofthephotoelectricmagnetographbyH.D.andH.W.Babcock(Babcock,H.D.andBabcock,H.W.1952)andtheimprovementswhichhavebeenmadetothisandotherinstru-mentsduringthepastthreedecades(e.g.BeckersandSchroter1968,Harvey1977)havepermittedhigh-resolutionstudiesofthenestruc-tureofthesolarmagneticelds.ReviewsofthisearlyworkhavebeenprovidedbySeverny(1972),Howard(1977,andZwaan(1987).InthesestudiesitwasshownthatthegeneralmagneticeldoftheSunconsistsofsmall(1-2arcsec),intense(1200G)uxknotsofmixedpolarities.Inhigh-resolutionmagnetogramsinwhichpositive2J.R.Varsiketal.eldsareindicatedbywhiteandnegativebyblack,theseregionsaresometimesdescribedas‘pepperandsalt’regionsbecausetheuxknotsofbothpolaritiesarefairlyevenlydistributed.Inotherregions,how-ever,theknotsofonepolarityareseentodominateovertheother,andSevernyshowsthat,atlowresolution,theknotsofthedominantpolaritytendtomaskthoseoftheoppositepolarity,theregionbeingoftendescribedasa‘unipolar’region.Aregiondenedbytheneutrallinesofthelarge-scaleeldseenatlowresolution,asgiveneitherbythezerolevelofacontourplotofthemagneticeldorasinferredfromlamentsseeninHspectroheliograms,issometimescalleda‘magneticcell’andisgenerallycospatialwithaunipolarregion.Duringmostofthecyclethepolarregionsareoccupiedbyunipolarmagneticcellsofoppositepolaritiesand,followingBabcock’sobserva-tionofthereversalofthepolareldsin1954,Severnyreportedthechangeofthemagneticpolaritiesatbothpolesin1964-65,notingthatthechangeswerenotsimultaneous.Recenthigh-resolutionstudiesofthepolarelds(Lin,Varsik,andZirin,1993)showthat,liketheeldsatlowerlatitudes,thepolarmagneticeldsareconcentratedintosmallmagneticelementsofbothpolarities.Althoughthepolarregionsmayappearasunipolarinlow-resolutionmagnetograms,theseobservationsshowthat,withinthoseregions,magneticelementsofbothpolaritiesarepresentandthatonepolaritytendtodominatetheother.Asthecycleprogresses,thepredominantpolarityineachhemispherechangesandFox,McIntosh,andWilson(1997)havedescribedtheevo-lutionofthelarge-scaleeldsandtheirassociationwithcoronalholesduringthepolareldreversals.Thisstudyshowsthatthedominantprocessestakeplaceonaglobalratherthanalocalscale.Thepresentstudiesareintendedtocomplementthatworkbyinvestigatingtheevo-lutionofthesmallscalestructuresofwhichthelarge-scaleunipolarregionsarecomposed.ItisgenerallyagreedthattheimperativesofMaxwell’sequations,inparticularrB=0;(1)requirethatmagneticeldsobservedatthesolarsurfacemustbecon-nectedeitheraboveorbelowthesurface.Whilemanywritershavedescribedthecoronalconnections,basedonHandX-Rayobserva-tionsoronpotentialeldcalculations(seee.g.Wilson1994,p57),fewhavebeensucientlybrave(orfoolhardy)toattempttodescribethesubsurfaceconnections.Nevertheless,ifwearetoobtainsomeguide-linesfromobservationastothenatureofthesolardynamo,somedescriptionsmustbeattempted.Inthispaper,furtherstudiesofthesmall-scaleandlarge-scalepolarmagneticeldsattheBigBearSolarpaper980909.tex;9/09/1998;11:08;nov.;p.2High-ResolutionStudiesoftheSolarPolarMagneticFields3Observatory(BBSO)in1989and1995(i.e.nearsunspotmaximumandminimum)arereportedandanattemptismadetosketchthesubsur-faceeldcongurationscorrespondingtotheobservedsurfaceeldsatthesetwoepochsofthesolarcycle.2.TheDataHigh-resolutionmagnetogramsofthesolarpolarregionshavebeenobtainedatBBSOduringtheperiodsJune19-July10,1989,i.e.Car-ringtonrotation(CR)1817andJune14-July10,1995,(Carringtonrotation1897).Thepixelscaleisabout0.6arcsec/pixelandtheseeingwasoftheorderof2arcseconthebestimages.Eachmagnetogramimageisapproximately300x225arcsecondsinsize.Individualmagnetogramshavebeencombinedtoyieldmosa