DTIC ADA522739: Solar Wind Sources in the Late Declining Phase of Cycle 23: Effects of the Weak Solar Polar Field on High Speed Streams

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Title:
Solar Wind Sources in the Late Declining Phase of Cycle 23: Effects of the Weak Solar Polar
Field on High Speed Streams
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2009
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Solar Wind Sources in the Late Declining Phase of Cycle 23: Effects of
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the Weak Solar Polar Field on High Speed Streams
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14. ABSTRACT
The declining phases of solar cycles are known for their high speed solar wind streams that dominate the
geomagnetic responses during this period. Outstanding questions about these streams, which can provide
the fastest winds of the solar cycle, concern their solar origins, persistence, and predictability. The
declining phase of cycle 23 has lasted significantly longer than the corresponding phases of the previous
two cycles.
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SolarPhys(2009)256:285–305
DOI10.1007/s11207-009-9354-5
STEREO SCIENCE RESULTS AT SOLAR MINIMUM
Solar Wind Sources in the Late Declining Phase
of Cycle 23: Effects of the Weak Solar Polar Field
on High Speed Streams
J.G.Luhmann·C.O.Lee·YanLi·C.N.Arge·
A.B.Galvin·K.Simunac·C.T.Russell·R.A.Howard·
G.Petrie
Received:4December2008/Accepted:1April2009/Publishedonline:21April2009
©TheAuthor(s)2009.ThisarticleispublishedwithopenaccessatSpringerlink.com
Abstract The declining phases of solar cycles are known for their high speed solar wind
streamsthatdominatethegeomagneticresponsesduringthisperiod.Outstandingquestions
about these streams, which can provide the fastest winds of the solar cycle, concern their
solar origins, persistence, and predictability. The declining phase of cycle 23 has lasted
significantlylongerthanthecorrespondingphasesoftheprevioustwocycles.Solarmagne-
STEREOScienceResultsatSolarMinimum
GuestEditors:EricR.Christian,MichaelL.Kaiser,ThereseA.Kucera,O.C.St.Cyr.
(cid:2)
J.G.Luhmann( )·C.O.Lee·Y.Li
SpaceSciencesLaboratory,UniversityofCalifornia,Berkeley,CA94720,USA
e-mail:[email protected]
C.N.Arge
AFRL/VSBXS,29RandolphRoad,HanscomAFB,Boston,MA01731,USA
e-mail:[email protected]
A.B.Galvin·K.Simunac
SpaceScienceCenter,UniversityofNewHampshire,MorseHall,Durham,NH03824,USA
A.B.Galvin
e-mail:[email protected]
K.Simunac
e-mail:[email protected]
C.T.Russell
InstituteofGeophysicsandPlanetaryPhysics,UniversityofCalifornia,LosAngeles,CA90024,USA
e-mail:[email protected]
R.A.Howard
SolarPhysicsBranch,NavalResearchLaboratory,4555OverlookAveSW,Washington,DC20375,
USA
e-mail:[email protected]
G.Petrie
NationalSolarObservatory,950NCherryAve,Tucson,AZ85719,USA
e-mail:[email protected]
286 J.G.Luhmannetal.
tographobservationssuggestthatthesolarpolarmagneticfieldisalso∼2–3timesweaker.
The launch of STEREO in late 2006 provided additional incentive to examine the origins
of what is observed at 1 AU in the recent cycle, with the OMNI data base at the NSSDC
available as an Earth/L1 baseline for comparisons. Here we focus on the year 2007 when
thesolarcoronaexhibitedlarge,long-livedmid-to-lowlatitudecoronalholesandpolarhole
extensionsobservedbybothSOHOandSTEREOimagers.STEREOprovidesinsitumea-
surementsconsistentwithrigidlycorotatingsolarwindstreamstructureatupto∼45°he-
liolongitudeseparationbylate2007.Thisstabilityjustifiestheuseofmagnetogram-based
steady 3D solar wind models to map the observed high speed winds back to their coro-
nal sources. We apply the WSA solar wind model currently running at the NOAA Space
WeatherPredictionCenterwiththeexpectationthatitshouldperformitsbestatthisquiet
time. The model comparisons confirm the origins of the observed high speed streams ex-
pectedfromthesolarimages,butalsorevealuncertaintiesinthesolarwindsourcemapping
associatedwiththiscycle’sweakersolarpolarfields.Overall,theresultsillustratetheim-
portanceofhavingaccuratepolarfieldsinsynopticmapsusedinsolarwindforecastmodels.
Atthemostfundamentallevel,theydemonstratethecontrolofthesolarpolarfieldsoverthe
highspeedwindsources,andthusonespecificconnectionbetweenthesolardynamoand
thesolarwindcharacter.
Keywords STEREOmission·Solarwind·Solarcycle·Solarmagneticfield·PFSS
model
1. Introduction
The launch of the Solar Terrestrial Relations Observatory (STEREO) twin spacecraft in
October2006providedanewsetofmultipointsolarwindmeasurementsthatcanbeusedto
understandthecoronalsourcesofthesolarwind,atatimewhenitisexpectedtobeexcep-
tionallystableandtraceable.Previousobservationsof∼27dayperiodicitiesininterplane-
taryplasmaandfieldfeaturesduringthedecliningphasesofsolarcycleshavesuggestedthat
slowlyevolvingconditionsofthecoronalfieldtopologyproduceanearlyrigidlycorotating
solarwindstructureatthesetimes.Thequasi-steadyconditionsoccurinspiteofthecontin-
uingemergenceofsomelatecycleactiveregionsandtheongoingredistributionanddecay
ofpreviouslyemergedphotosphericmagneticfields.Fromaspaceweatherperspective,the
associatedcorotatinghighspeedstreamsareknowntounderlieenhancementsintheEarth’s
relativisticmagnetosphericelectronpopulations,andrecurrentsmalltomoderategeomag-
neticstormsthroughthestreaminteractionregionstheyproduce(e.g.Bakeretal.,1997).
SincelateJanuary2007theSTEREOspacecrafthavebeenseparatingfromoneanother
at a rate of ∼45° year−1. This has allowed us to observe the near-ecliptic solar wind at
1 AU at increasing distances from the upstream (L1) location where conditions affecting
Earthareroutinelymonitored.Herewefocusona∼10monthperiodfromthefirstyearof
STEREOmeasurements,withtheOMNIdatabase(KingandPapatashvilii,1994)providing
L1comparisons,towardunderstandingthecharacterofthelatedecliningphasesolarwind.
Ofparticularinterestinthisstudyaretheeffectsoftheweaksolarpolarmagneticfieldsof
thiscycleonthesolarwindstructure.Tworecentpapersdescribesomeapparentdifferences
fromthepreviouscycledecliningphaseseeninUlyssesmeasurementsoftheinterplanetary
magnetic field (Smith and Balogh, 2008) and the solar wind mass flux (McComas et al.,
2008). An accompanying paper by Lee et al. (2009) investigates the nature of theweaker
interplanetarymagneticfieldseenintheeclipticthiscycle.Thepresentanalysisconsiders
thecomplementarysubjectofthesourcesoftherelatedhighspeedstreamsandthepossible
influenceoftheweaksolarpolarfields.
SolarWindSourcesintheLateDecliningPhaseofCycle23 287
We first show the STEREO Ahead and Behind solar wind and interplanetary field ob-
servations from 10 Carrington Rotations (2052–2062) in 2007 together with the OMNI
L1 data. Their similarities illustrate how well the assumption of steady corotating struc-
tureisadheredtooverbaselinesofupto∼45°(alsoseeSimunacetal.,2009),justifying
theuseofsteadymodelsofthecoronaandsolarwindtointerpretthedata.Wethenapply
the solar magnetogram-based potential field source surface (PFSS) model (e.g. Altschuler
andNewkirk,1969)toinvestigatetheinferredcoronalopenfieldsourcesofthehighspeed
streams. The WSA (Wang–Sheeley–Arge) coupled PFSS and steady solar wind model
(ArgeandPizzo,2000),basedondailyupdatingsolarmagneticfieldsynopticmaps,isthen
used to map the outflows from the corona to 1 AU. The comparisons with in situ obser-
vations reveal features of both the solar wind sources and the models. In particular they
illustrate the challenges faced by models even at times when the solar wind is ostensibly
at its best-behaved phase, as well as the control of the solar polar magnetic field over the
sourcesthatproducethehighspeedwinds.
2. STEREOandOMNIObservationsoftheSolarWindandItsSources
The STEREO observations described here were obtained from February through October
2007,whentheSTEREOAhead(-A)andBehind(-B)spacecraftwereintheirheliocentric
orbitsbetweenthepositionsshowninFigures1(a)and(b).CorrespondingOMNIdatawere
obtained from Advanced Composition Explorer (ACE) and WIND near the L1 point, ap-
proximately midway between the two STEREO spacecraft. The heliolongitude separation
ofeachSTEREOspacecraftfromtheEarthincreasesapproximately22.5°year−1.Although
theSTEREOspacecraftdeviateslightlyfrom1.0AUandfromtheeclipticplane,thesede-
viationsareconsiderednegligibleforthepurposesofthisstudyoflargescalefeatures.As
mentioned above, the period investigated corresponds to Carrington Rotations (CR) 2052
to 2062. Only a few Earth-directed coronal mass ejections (CMEs) with detected ICMEs
(InterplanetaryCMEs)occurredduringthistime,themostsignificantinthethirdweekof
MayduringCR2056(seeLietal.,2008;Kilpuaetal.,2009).
TwoinvestigationsonSTEREOprovidethemaininsitusolarwindobservations:Plasma
andSuprathermalIonComposition(PLASTIC)(Galvinetal.,2008)measurestheplasma
ionmoments,whiletheInsituMeasurementsofParticlesandCMETransients(IMPACT)
(Luhmann et al., 2008) measures the interplanetary magnetic field and the plasma elec-
trons.TheSTEREO dataareavailable throughwebsitelinksattheStereo Science Center
(http://stereo-ssc.nascom.nasa.gov),whiletheOMNIdatacanbedownloadedfromtheNa-
tional Space Science Data Center (http://omniweb.gsfc.nasa.gov). The OMNI plasma mo-
ments for this period are based either on ACE SWEPAM or WIND SWE measurements
(describedbyMcComasetal.,1998andOgilvieetal.,1995,respectively),withthemag-
netic fields mainly from the ACE magnetometer (e.g. Smith et al., 1998). For the present
studytheionvelocityandmagneticfieldpolarityaretheprimaryquantitiesusedforsource
mapping.
Figures2(a–c)provideanoverviewoftheSTEREO-AandBobservationsofthemag-
netic field magnitude and radial component at 10 minute resolution, and the plasma bulk
velocity,comparedtothecorrespondinghourlyOMNIdata.CarringtonRotationsaresep-
aratedbyverticallines.Thevelocities(Figure2(c))showhighspeedstreamswithtypical
maximum speeds of 500–700 kms−1 lasting several Carrington Rotations at a time. The
magneticfieldmagnitudeandradialcomponentsinFigures2(a)and(b)havefeaturescon-
sistentwithstreaminteractionregions,someofwhichcontainthepolarityreversalsofthe
288 J.G.Luhmannetal.
Figure1 Positionsofthe
STEREOspacecraftrelativeto
Earth/ACEinearlyFebruary
(a–upperpanels)andearly
December(b–lowerpanels).
ThelettersAandBidentifythe
locationsoftheSTEREO-Aand
STEREO-Bspacecraft,
respectively.TheACEspacecraft
iseffectivelyatthepositionofthe
Earth(greendot)onthisscale.
(a)
(b)
heliosphericcurrentsheet(e.g.Goslingetal.,1978).Theseobservationsreinforcetheidea
thatwhensolaractivityislow,thesolarwindevolvesinaquasi-steadymanner,suchthat
particularstreamscanbefollowed,sometimesformonths,atafixedheliosphericlocation,
Theycanalsoberecognizedatotherheliosphericlongitudesifappropriatecorotationlead
timesordelaysaretakenintoaccount(e.g.Simunacetal.,2009).Thisslowevolutioniskey
totheabilitytousemodelsforsourceidentification/mappingasdescribedbelow.
TheprevailingsolarsourcesofthesolarwindobservedbySTEREOandatL1in2007
areseenintheEUVimagesfromSOHOEIT(Delaboudiniereetal.,1997)andSTEREO
SECCHI-EUVI(Howardetal.,2008)inFigure3.Theimagesinthetoprowshownearly
identicaldisksobservedwhentheSTEREOspacecraftwereclosetoEarthinearlyFebru-
ary2007.ThebottomrowillustratesthemultiperspectiveviewsastheSTEREOspacecraft
becamesignificantlyseparatedfromeachotherandSOHOlaterintheyear.(Therightmost
SolarWindSourcesintheLateDecliningPhaseofCycle23 289
(a)
Figure 2 (a) Magnetic field magnitudes measured at L1 (OMNI hourly data) (top) and on STEREO-A
and-B.TheverticallinesmarkCarringtonRotationboundaries.
panelsinFigure3aredescribedbelow.)Thesolarwindsourcesintheseimagesarethedark
regionsonthediskscorrespondingtoopenfieldregionsorcoronalholes.Forasimpledipo-
larfieldcorona,twopolarcoronalholesprovidetheentireheliospherewithsolarwind(e.g.
PneumanandKopp,1971).However,thepresenceofactiveregionsatlowtomidlatitudes
introduceshigherordermomentsofthecoronalfield,andwiththesecomeamorecomplex
distributionofopenfieldregionsthatchangesasthesolarcycleprogresses(e.g.Luhmann
et al., 2002). The Earth’s solar wind on average maps to source regions about 45–60° to
theright(Westinthesolarimagingconvention)ofthecentralmeridian,butthesourcelat-
itudecanvarygreatlyandincludelowlatitudeopenfieldregionsatsomeheliolongitudes,
polarcoronalholeextensionsatothers,andtheedgesofthepolarcoronalholesatyetother
locationsastheSunrotates.
For the early months of 2007 two nearly diametrically opposed southern hemisphere
coronalholesdominatedthesolardisk.TheEUVsynopticmapinFigure4(a),createdby
theEUVIinvestigationteamfromSTEREO-AEUVIimagesforCarringtonRotation(CR)
2052,showsthesesourcesofthestrong,fastsolarwindstreamsseeninthefirstthreeCar-
ringtonRotationsinFigure2(c).Bylate2007,alargesouthernpolarcoronalholeextension,
withpatchesofbright(magneticallyclosed)fieldarcadeswithinit(Figure4(b)),provideda
recurrent,long-livedsourceofmorevariablehighspeedwind.Aconcentratedsourceregion
rootedaroundalowlatitudeoldcycleactiveregionnearCarringtonlongitude250°produced
290 J.G.Luhmannetal.
(b)
Figure2 (Continued.)(b)ComparisonsofmagneticfieldradialcomponentsmeasuredatL1(OMNIhourly
data)(top)andonSTEREO-Aand-B.
asmaller,slightlyslowerprecedinghighspeedstream.Inthefollowingsectionweusemod-
els,suchasthoseillustratedontherightsideofFigure3,toanalyzethesesourcesandtheir
implications.
3. SourceMappingwithModels
ArelativelysimpledescriptionofthecoronaltopologyisprovidedbyPotentialFieldSource
Surface(PFSS)modelsofthecoronalmagneticfield(e.g.seethereviewbySchatten,1999).
PFSSmodelsprovideasnapshotoftheopencoronalfieldregions,thegenerallypresumed
sourcesoffastsolarwind,basedonasynopticmapoftheobservedphotosphericmagnetic
fieldandtheassumptionthatthecoronalfieldiscurrentfreebetweenthephotosphericin-
ner boundary and a spherical outer boundary or source surface. The radius of the source
surface, together with the multipolar field content of the photospheric boundary map, de-
termine wherethemodel’s openfieldsoccur,makingtheconstructionofanaccurate map
fromfulldiskmagnetogramsacritical partofitssuccessfuluse(e.g.seeArgeandPizzo,
2000). Investigations using the PFSS model find the best source surface location for de-
scribing most observations is located at ∼2.5 solar radii, although values between ∼1.6
and3.25solarradiiaresometimesadopted.Whilethecurrent-freeandsphericalsourcesur-
faceassumptionscanbechallengedonaphysicalbasis,especiallyinthevicinityofthehe-
SolarWindSourcesintheLateDecliningPhaseofCycle23 291
(c)
Figure 2 (Continued.) (c) Comparisons of speeds measured at L1 (OMNI hourly data) (top) and on
STEREO-Aand-B.
Figure3 EUVimages(lefttoright:STEREO-BEUVI,SOHOEIT,STEREO-AEUVI)fromearly(top
row)andlate(bottomrow)2007.Thesourcesofhighspeedwindintheeclipticchangefromthetwosouthern
midlatitudecoronalholesinearly2007tothelargesouthernpolarholeextensionandasmallopenfieldregion
aroundanear-equatorialactiveregion(seennearwestlimb)inlate2007.GONGmagnetogram-basedPFSS
modelviewsfromEarthareatthefarright,showingareasofoutward(red)andinward(green)opencoronal
magneticfieldandtheclosedfieldlinesattheedgeofthestreamerbelt(blue).
292 J.G.Luhmannetal.
(a)
(b)
Figure4 SynopticmapsconstructedfromSTEREO-ASECCHIEUVIimagesat195Åwavelength,show-
ingthedarkareas(coronalholes)providingsolarwindduring(a–upper)CR2052and(b–lower)CR2062.
Bothlow-midlatitudeandpolarholesarepresent.
liosphericcurrentsheetandaroundactiveregions,PFSSmodelshavestoodthetestoftime
andapplicationstomanyproblemsrequiringrealisticlargescalecoronalfielddescriptions.
In particular, they have been shownto describe many of thegross morphological features
of coronal rays in white light eclipse and coronagraph images and the dark coronal holes
inX-rayandEUVimages,andtoreproducetheobservedsectorstructureorpolarityofthe
heliosphericmagneticfieldwhenmappedoutward(Hoeksema,Wilcox,andScherrer,1983;
HoeksemaandScherrer,1984).Oneexpectstheperiodaroundsolarminimumtobeapartic-