Large-scaleflowsinferredfromhelioseismologyrelatedtothesolarcycle

FrankHillNationalSolarObservatory

Boulder,CO

Helioseismology101- 1

• Thesunisanalogoustoahugemusicalinstrument– itringslikeabell• Acousticwavesaregeneratedbyconvectionatall

temporalfrequencies• Soundistrappedinsidethesunbytheinwardly

increasingtemperature– asonicmirage• Thedepthtowhichanacousticwavepenetrates

dependsonitspath• Onlymodesthathavenodesatthetwoturning

pointssurvivedestructiveinterference

Helioseismology101- 2

• About5,000,000modessurvivethedestructiveinterferenceinthetemperaturegradient• Sincetherearethuslotsofraypathspropagatingto

differentdepths,wecanusethemtoinferthephysicalconditionsbelowthephotosphereviainversions• Canusegloballyresonantmodesforglobally

averagedstudies,describedbysphericalharmonics• Alsousewavesthatarenotgloballyresonantfor

localizedstudies,i.e.inactiveregions.

WavesinaSphere:SphericalHarmonics

n – radialorder:0£ n £ 80

! – sphericalharmonicdegree:0£ ! £ 4000

m – azimuthaldegree:-! £m £ !

Fluxtransportdynamo

Twolarge-scaleflowsofimportance:differentialrotationandmeridionalflow

Internalrotation

• Rotationtransformsmagneticfieldfrompoloidal(N-S)totoroidal(E-W)• Earlydynamotheoriesassumedinternalsolar

rotationwas“constantoncylinders”• Iso-rotationcontoursparalleltorotationaxis• Taylor-Proudman theoremwassatisfied

• Helioseismologyshowedotherwise

Internalrotation

Intrigue!

• Differentialrotation(DR)persiststhroughtheCZ• Nearthebaseoftheconvectionzone,DR

transitionstosolid-bodyrotation(SBR)intheradiativezone(thetachocline)• TheDRataround50° latitudeisequaltotheSBR–

thisisalsothelatitudewheresunspotsfirstappearinthecycle• Dynamosrequirevelocityshear,andthereisalsoa

shearlayerimmediatelybelowthephotosphere,sotwodynamos?

Meridionalflow

• Afteractiveregionsemerge,reconnect,anddecay,asurfacepolewardmeridionalflow(MF)advects thefieldtothepoles• Presumably,theMFconvergesandsubmergesat

thepoles,takingthemagneticfieldwithit(notyetobserved)• Alsopresumably,asubsurfacereturn

equatorwardflowtransportsthemagneticfieldbacktowardslowerlatitudesandcompletesthetransitionfromtoroidaltopoloidalfield• Dynamomodelsassumeuni-cellularMFinthe

twohemispheresandindepth

Thesurfacemagneticfieldoverfoursolarcycles

MeridionalflowfromringdiagramsandGONGdata

Thespeedofthemeridionalflowchangesoverthecycleandiscorrelatedwiththestrengthofthenextcycle

FlowDivergence Blue:source/upflowYellow:sink/downflow

Highactivity:inflowsanddownflowsinactivelatitudes

Lowactivity– patternmuchweakerbutstillpresent

Threecellsinlatitudeineachhemisphere

Deepmeridionalflowestimates

Complexflow,maybemultiplecellsindepth,returnflowsurprisinglyshallow(80Mm)

BUTstilllittleagreementbetweenmethods,questionsofenergyandmassconservation

Torsionaloscillation

• Appearsasaveryweak“jetstream”zonalflowwithamplitude~5m/sabovebackgroundDR• Seenonthesurfaceanddownthroughatleast70Mm

depth• Movesinlatitudeduringthecycle• Twobranches:polewardandequatorward– total

lengthoftimeforaflowcycleis~22years• Solaractivityappearsonequatorwardsideofflow• EvolutionofflowtowardlowerlatitudesinC24was

correlatedwithlengthofcycleandextendedminimum• DoesthemagneticfieldcausetheTO,ordoestheTO

influencetheactivityevolution?

Surface“TorsionalOscillation”

1980 1990 2000

Therearenoobservationsofthesurfaceflowpast2009

TorsionaloscillationatR8=0.99

WhereisCycle25?

• Polewardflowshouldhaveappearedin2009• Equatorwardflowshouldhaveappearedin2015• WTF?• Onepossibility:backgrounddifferentialrotation

changedwithtime

Conclusions

• Helioseismologyhasrevealedthattheflowsdrivingthesolarcyclearemorecomplexthanassumedbydynamotheory• Newdynamomodelsarebeingconstructed

incorporatingthehelioseismicflows• Thetorsionaloscillationforthenextcycle(25)isvery

differentfrompredecessors– didtheSun’srotationprofilechangeoristhereagrandminimumcoming?• Butweonlyhavefourcyclesofobservationssofar(less

thantwowithhelioseismology)• Staytuned!