The origin and dynamics of solar magnetism PDF
Preview The origin and dynamics of solar magnetism
M.J. Thompson (cid:2) A. Balogh (cid:2) J.L. Culhane (cid:2) Å. Nordlund (cid:2) S.K. Solanki (cid:2) J.-P. Zahn Editors The Origin and Dynamics of Solar Magnetism Previously published in Space Science Reviews Volume 144, Issues 1–4, 2009 M.J.Thompson Å.Nordlund SchoolofMathematics&Statistics NielsBohrInstitute UniversityofSheffield UniversityofCopenhagen Sheffield,UK Copenhagen,Denmark A.Balogh S.K.Solanki InternationalSpaceScienceInstitute Max-Planck-Institut Bern,Switzerland fürSonnensystemforschungKatlenburg-Lindau Germany J.L.Culhane UniversityCollegeLondon J.-P.Zahn MullardSpaceScienceLaboratory LUTH,ObservatoiredeParis Dorking,UK Meudon,France Coverillustration:Continuumimage(blue)andline-of-sightcomponentsofthevelocity(yellow)and magneticfield(redandenhancedasgreen)obtainedfromMilne-EddingtoninversionsofStokesdata observed with the CRisp Imaging SPectropolarimeter (CRISP) on the Swedish 1-m Solar Telescope (SST).Shownisashortlivedactiveregionobservedon22April2008intheFeI6302line.Thespatial resolutionisclosetothediffractionlimitof0.16". CourtesyofTomasHillberg,GautamNarayanandGöranScharmer. Allrightsreserved. LibraryofCongressControlNumber:2009926695 DOI:10.1007/978-1-4419-0239-9 ISBN-978-1-4419-0238-2 e-ISBN-978-1-4419-0239-9 Printedonacid-freepaper. ©2009SpringerScience+BusinessMedia,BV Nopartofthisworkmaybereproduced,storedinaretrievalsystem,ortransmittedinanyformorbyany means,electronic,mechanical,photocopying,microfilming,recordingorotherwise,withoutthewritten permissionfromthePublisher,withtheexceptionofanymaterialsuppliedspecificallyforthepurpose ofbeingenteredandexecutedonacomputersystem,fortheexclusiveusebythepurchaserofthework. 1 springer.com Contents IntroductiontoSolarMagnetism:TheEarlyYears (cid:3) A.Balogh M.J.Thompson 1 SolarMagnetism:TheStateofOurKnowledgeandIgnorance E.N.Parker 15 ChaosandIntermittencyintheSolarCycle E.A.Spiegel 25 TheSolarDynamo (cid:3) N.O.Weiss M.J.Thompson 53 Flux-TransportSolarDynamos (cid:3) M.Dikpati P.A.Gilman 67 TheSolarDynamo:TheRoleofPenetration,RotationandShearonConvective Dynamos S.M.Tobias 77 AdvancesinTheoryandSimulationsofLarge-ScaleDynamos A.Brandenburg 87 PlanetaryDynamosfromaSolarPerspective (cid:3) (cid:3) U.R.Christensen D.Schmitt M.Rempel 105 ObservationsofPhotosphericDynamicsandMagneticFields:FromLarge-Scale toSmall-ScaleFlows (cid:3) N.Meunier J.Zhao 127 LargeScaleFlowsintheSolarConvectionZone (cid:3) A.S.Brun M.Rempel 151 PhotosphericandSubphotosphericDynamicsofEmergingMagneticFlux A.G.Kosovichev 175 TheTopologyandBehaviorofMagneticFieldsEmergingattheSolarPhotosphere B.W.Lites 197 Sunspots:FromSmall-ScaleInhomogeneitiesTowardsaGlobalTheory R.Schlichenmaier 213 RecentEvidenceforConvectioninSunspotPenumbrae G.B.Scharmer 229 HelioseismologyofSunspots:ACaseStudyofNOAARegion9787 (cid:3) (cid:3) (cid:3) (cid:3) (cid:3) (cid:3) L.Gizon H.Schunker C.S.Baldner S.Basu A.C.Birch R.S.Bogart (cid:3) (cid:3) (cid:3) (cid:3) (cid:3) (cid:3) D.C.Braun R.Cameron T.L.DuvallJr. S.M.Hanasoge J.Jackiewicz M.Roth (cid:3) (cid:3) T.Stahn M.J.Thompson S.Zharkov 249 Small-ScaleSolarMagneticFields (cid:3) (cid:3) (cid:3) A.G.deWijn J.O.Stenflo S.K.Solanki S.Tsuneta 275 CouplingfromthePhotospheretotheChromosphereandtheCorona (cid:3) (cid:3) S.Wedemeyer-Böhm A.Lagg Å.Nordlund 317 MagneticFluxEmergence,Activity,EruptionsandMagneticClouds:Following MagneticFieldfromtheSuntotheHeliosphere (cid:3) L.vanDriel-Gesztelyi J.L.Culhane 351 CoronalHolesandOpenMagneticFlux Y.-M.Wang 383 SolarCycleForecasting D.H.Hathaway 401 CoronalMagnetism:DifficultiesandProspects P.J.Cargill 413 ISSIWorkshoponSolarMagnetism:ConcludingRemarks J.-P.Zahn 423 Introduction to Solar Magnetism: The Early Years A.Balogh·M.J.Thompson OriginallypublishedinthejournalSpaceScienceReviews,Volume144,Nos1–4,1–14. DOI:10.1007/s11214-009-9493-x©SpringerScience+BusinessMediaB.V.2009 Abstract Theyear2008markedtheonehundredthanniversaryoftheobservationaldiscov- erybyGeorgeElleryHaleofmagneticfieldinsunspots(HaleinAstrophys.J.28:315–343, 1908).Thisobservation,thefirsttosuggestadirectlinkbetweenthebest-knownvariable featuresontheSunandmagnetism,startedalineofresearchthathaswidenedconsiderably over the last 100 years and is continuing today. Knowledge about all aspects of the Sun hasincreasedinaremarkablewayoverthepastfewdecades.Variationsintheappearance of the Sun and its corona, as well as deeper sources of quasi-regular and chaotic changes thatmakeupsolarvariabilityhavebeenextensivelydocumentedbybothground-basedand space-basedsolarobservatories.Ithasbeenrecognizedthatsolarmagnetismisthekeyphe- nomenonthatdrivessolarvariability.Theworkshopdevotedtotheoriginanddynamicsof solarmagnetismheldintheInternationalSpaceScienceInstituteinBern,Switzerland,from 21to25January2008reviewedthestatusofthefieldandhasledtothisvolumethatbrings togetherthebestavailableknowledgeandunderstandingofsolarmagnetism100yearsafter Hale’spioneeringpaper.Thisintroductorypapergivesanoutlineofthehistoryofresearch intosolarvariabilityuptotheworkofHaleandhiscolleagues.Theachievementsofthepast decadesarediscussedextensivelyintheothercontributionstothisvolume. Keywords Sun·Solarmagnetism·Sunspots·Solarcycle 1 SunspotsasIndicatorsofSolarVariability Sunspotsareprobablythemostobviousandlongestrecognizedmanifestationsofthevari- able Sun, and now of solar magnetic activity. These regions of relatively cool gas/plasma (cid:2) A.Balogh( ) InternationalSpaceScienceInstitute,Bern,Switzerland e-mail:[email protected] A.Balogh TheBlackettLaboratory,ImperialCollege,London,UK M.J.Thompson SchoolofMathematicsandStatistics,UniversityofSheffield,Sheffield,UK M.J.Thompsonetal.(eds.),TheOriginandDynamicsofSolarMagnetism. 1 DOI:10.1007/978-1-4419-0239-9_1 2 A.Balogh,M.J.Thompson at the Sun’s surface are caused by the suppression of convective heat transport by intense magnetic field generated in the solar convection zone. The number of sunspots visible on Sunandtheareatheycoveronthephotosphereexhibitanapproximately11-yearcycle,and overthatsametimethesolarlatitudeatwhichnewspotsappearmigratesfrommid-latitudes towardsthesolarequator.Thesearethetwomostrecognizablefeaturesofsolarvariability, buttherangeofmeasuresthatareusedtocharacterizetemporalchangesintheSunisvery large. Sunspotshavenowbeenobservedandcountedsincetheearly17th century,andstudied in ever increasing detail ever since then. (For a comprehensive account of the history of observationsofsunspotandsolaractivity,seeChapt.2.1inHoytandSchatten1997).They were first recognized as dark features, spots, on an otherwise idealized, unblemished Sun almost exactly 400 years ago, in and around 1610 or 1611, by Thomas Harriott, Johann Fabricius,ChristopheScheinerandGalileo,verysoonaftertheintroductionofthetelescope as a tool into astronomy. (There is some evidence for earlier observations, but those who madethemdidnotknowwhattheysawand,forculturalreasons,certainlydidnotinterpret theirobservationsasblemishesorspotsontheSun.)Anumberofotherobserversalsojoined indocumentingsunspots,althoughthecontroversyabouttheirnature,whetherreallyspots ontheSunor“clouds”oreven“transitingplanets”continuedforuptotwoorthreedecades. Thediscoveryandsustainedobservationofsunspotsinthefirsthalfofthe17th century immediately led to the discovery of solar rotation and its period, the determination of the rotationaxisoftheSun,andthelatitudedependenceoftherotationperiod.BothGalileoand ScheinerclaimedthemeritfordiscoveriesconcerningtheSun,basedontheirobservations of sunspots. These observations from this early phase of scientific solar research are very useful in providing a record of normality in solar activity prior to the start of a long, 70 yearinterval(fromabout1645to1715)whentheSunhadveryfewspots,nowcalledthe Maunderminimum. We can now be grateful for the nearly systematic observing and counting of sunspots since Galileo’s time because these records provide evidence of the longevity of what we nowknowtobethesolarmagneticactivitycycle,evenif,historically,itcanbeseentobe chaoticratherthanregular. Thecloseto11yearperiodicityinthenumberofsunspotswasonlynotedinthe1840s by Heinrich Schwabe, originally an apothecary, but then turned full-time solar observer. The observations and data used in the Schwabe’s conclusion that there was a ∼10 year periodicity in sunspots are shown in Fig. 1 (Schwabe 1843). In fact, the original data set wascomplementedwithsevenextrayearsofobservationsbySchwabeincludedinAlexan- dervonHumboldt’swork,Cosmos,publishedin1851.Theseextrayearsprovidedfurther evidenceoftheperiodicity,byincludingonemoremaximuminthenumberofsunspots. However, it was Rudolf Wolf (his portrait is shown in Fig. 2) who, following his own extensive observations in Switzerland, first in Bern, then in Zurich, established the stan- dardizedwaytocountsunspots,firstcalledtheZurichorWolfsunspotnumber.Wolfhad been drawn to the study of sunspots following the work of Schwabe already in 1848. He then went on to put the quasi-cyclic variations in sunspot numbers on a firm, long-term basis by collecting, examining and standardizing past observations, as well as adding his own. The history of Wolf’s work has been described by Izenman et al. (1983) who also hasprovidedacriticalstatisticalassessmentofthe“average”durationofthesunspotcycle derivedbyWolf(11.11years).Wolf’sdefinitionofrelativesunspotnumbersthathespent more than ten years to refine remained the accepted standard for measuring solar activity forover100years.Wolfcombinedcountsofsunspotgroupswiththoseofindividualspots. The historical data collected and refined by Wolf from the beginning of the 17th century IntroductiontoSolarMagnetism:TheEarlyYears 3 Fig.1 Thediscoveryofthe ∼11yearsolaractivitycycle throughsunspotobservations. Upperpanel:Schwabe’syearly sunspotgroupobservations(from 1826to1844)andcomplemented to1850byAlexandervon Humboldt.Blacksymbols: observednumbers,redsymbols: correctedforthenumberof observingdaysintheyear.Lower panel:Currentlyused(originally Wolf-orZurich)monthlysunspot numbersfor1800to1900 Fig.2 RudolfWolf(1816–1893) who,astheDirectorfirstofthe ObservatoryoftheUniversityof BernthenoftheObservatoryof theEidgenössischeTechnische Hochschule(ETH)inZurich madenotonlysystematicsunspot observationsoverseveral decades,butalsocollected historicsunspotdataanddevised thestillusedmeasureofstandard sunspotnumbers have become very important indicators to show that solar variability is very complex and notsimplyperiodic.InFig.1,Schwabe’sobservationsareputinthecontextoftherecord ofthecompletesunspotdatainthe19thcentury,usingtheacceptedZurichmonthlysunspot numbers. During the Maunder minimum in the second half of the 17th century, the very small numberofsunspotsdidnotexhibitthecyclicbehaviourthatisnowassociatedwithwhatwe knowtobethe11-yearperiodicity(Eddy1976).ThelackofsunspotsduringtheMaunder 4 A.Balogh,M.J.Thompson minimum is well documented, observations covered about 95% of the total interval. The existenceofother“grandminima”oflongdurationabsenceofsunspotsinearlierepochsis inferredfromproxyrecords,bysuchtechniquesascarbondatingandmeasuringisotopesin icecores(see,forrecentreviews,Usoskinetal.2007;Usoskin2008).Wehavenoadequate theory to predict when the next grand minimum will occur. On these longer scales, solar activityandsunspotsasitsmanifestationappeartobehaveinachaotic/stochasticmanner, andshownoevidenceofacyclicbehaviour(Spiegel2009).Thequasi-regularityofthe11- yearcycle,whenitpersistsforcenturies,isthereforeallthemoreremarkable.Someproxy recordsalsosupportthecontinuationofthe11-yearcyclethroughtheMaunderminimum (Beeretal.1998). The linking of geomagnetic variations with the sunspot cycle is associated primarily with Edward Sabine in 1952 who was also leading an initiative for the establishment of geomagneticobservatoriesworldwide(seeCliver1994andreferencestherein).Infactwhat Sabine noted was the coincidence of the 1843 minimum and 1848 maximum in sunspot numbers(seeFig.1)withaminimumandmaximumingeomagneticstormsattwowidely separated geomagnetic observatories in Hobart (Tasmania) and Toronto (Canada). Others, includingRudolfWolf,alsonotedtherelationshipbetweensolarandgeomagneticvariations (andotherindicatorssuchasthefrequencyofaurorae)thatremainedanunsolvedpuzzlefor manydecadesafteritsdiscovery. Another aspect of solar activity, introducing a concept of much shorter time-scale, ex- plosive variability was discovered by Richard Carrington during his routine observations ofsunspots(Carrington1860).Thisobservation,on1September1859,wasthefirstwhite- light,obviouslyveryintensesolarflarethatwasseennotonlyasaremarkablesolarphenom- enon,butalsonotedasbeingfollowedbyalargegeomagneticstorm(Cliver2006).Flaring ontheSunisrelatedtoactiveregionsandsunspotcomplexes,butwhiletheconnectionis well established, the occurrence of very large flares, the kind that Carrington observed, is related to sunspots much less predictably. The terrestrial effects of large solar flares were noted,manydecadesafterCarrington,byHale(1931). AnobservableeffectoftheSunonterrestrialphenomenaisrelatedtotheSun’srotation. Whilethiscorrelation,resultinginanapparent27-dayperiodicity,wasnotedbefore,thekey work on the details of the association were first published by Walter Maunder (Maunder 1904a), following an earlier compilation of observations by William Ellis. In particular, Maunder noted the association of the largest geomagnetic storms, with the shortest delay after the flares occurred when the sunspot group was within a privileged range of solar longitudes, between 19◦ East and 47◦ West of the central meridian, with a mean of 14◦ West.Thiswasanotherassociationwhichremainedapuzzleuntilthesecondhalfofthelast century. AnotherimportantindicatorofsolarvariabilitythatisassociatedwithEdwardMaunder istheevolution,inheliolatitude,ofthelocationofsunspotsasafunctionofheliolatitudeas the solar cycle progresses (Maunder 1904b). At the time of minimum activity, there are a veryfewsunspotsfromthepreviouscycleclosetotheequator,andalsoafewspotsassoci- atedwiththenewcycleathigherlatitudes,atorpolewardof30◦.Asthenumberofsunspots increasestowardssolarmaximum,theyareseentoemergeprogressivelyclosertothesolar equator.Followingmaximum,thenumberofsunspotsdiminishesbutthetrendofapproach- ingtheequatorcontinues.Whenthelocationofsunspotsisplotted,thewell-knownbutterfly diagramemerges.ThisisshowninFig.3inwhichMaunder’soriginalbutterflydiagramis given, illustrating the location of sunspots from 1874 to 1902, as well as the equivalent 28yearintervalincludingthesunspots(codedwiththeirareas)uptothepresent. Althoughtheindicationswererecognized,thelinkbetweenmanifestationsofsolarvari- abilityandterrestrial,primarilygeomagnetic,effectswasmissing,becausesolarmagnetism IntroductiontoSolarMagnetism:TheEarlyYears 5 Fig.3 Thebutterflydiagramof thelocationofsunspotsin heliolatitude,shownasafunction oftime.Thepatternhastheclear periodicityofthesolarcycle. Upperpanel:Thebutterfly diagramasfirstpublishedby Maunder(1904b).Lowerpanel: Themodernbutterflypanelover anintervalofthesameduration, colourcodedwiththeareaofthe sunspots(courtesy:David Hathaway,NASA/MSFC) hadnotbeendiscovered.Infact,nolessanauthoritythanLordKelvindiscardedthepos- sibility of a remote effect of even a hypothetical magnetic field of the Sun on the Earth; thecausalagents(interactingsolarwindstreamofdifferentvelocities,X-raysandenergetic particlesassociatedwithflaresandCoronalMassEjections)remainedunknownuntilmuch later.And,inanycase,therewasnoproofthattheSunwasinanysensemagnetic. 2 GeorgeElleryHaleandtheDiscoveryofSolarMagnetism It is very satisfying to note that the discovery of solar magnetism was made by the new instrument,thespectroheliograph,inventedanddevelopedbyGeorgeElleryHaleinabout 1891(Hale1891,1929).Togetherwiththeverypainstakingobservationsthathelpedsolar physics make significant advances at the turn of the century, Hale started a new phase in solarresearch.Heobservedaverylargeflareor“eruption”ashecalledit,on15June1892; thissolareventwasalsofollowedwithinadaybyaverylargegeomagneticstorm.However, Haledidnotpursuetheassociation:thiswasdone,asalreadyrecounted,byEllis,Maunder andothers.ThediscoveryofsolarmagnetismisnowdatedfromHale’spaperonsunspots (Hale1908),whousedthespectralandimagingresolutionofhisinstrumentatMountWil- sontodetectZeemansplittinginanumberofspectrallinesofsunspotsthatcouldonlybe duetothepresenceofstrongmagneticfields. InHaleetal.(1919)thefindingsoftheearlierpaperaboutthediscoveryofthemagnetic fieldsweresummarizedasfollows: photographsofthehydrogenflocculimadewiththeHα lineshowedclearlymarked vortical structure in regions centering in sun-spots. This structure was found to be repeatedinhundredsofspots,leavingnodoubtastothegenerality.... Thesepho- tographs suggested the hypothesis that a sun-spot is a vortex, in which electrified particles,producedbyionizationinthesolaratmosphere,arewhirledathighveloc- ity.Thismightgiverisetomagneticfieldsinsun-spots,regardedaselectricvortices.
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