Solar radio bursts as a tool for space weather forecasting

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Solar radio bursts as a tool for space weather forecasting

Karl-Ludwig Klein, Carolina Salas Matamoros, Pietro Zucca

To cite this version:

Karl-Ludwig Klein, Carolina Salas Matamoros, Pietro Zucca.

Solar radio bursts as a tool for

space weather forecasting. Comptes Rendus Physique, Centre Mersenne, 2018, 19 (1-2), pp.36-42.

�10.1016/j.crhy.2018.01.005�. �insu-01744555�

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Contents lists available atScienceDirect

Comptes

Rendus

Physique

www.sciencedirect.com

Radio Science for Humanity/Radiosciences au service de l’humanité

Solar

radio

bursts

as

a

tool

for

space

weather

forecasting

Les sursauts radio solaires : un outil de prévision pour la météorologie de

l’espace

Karl-Ludwig Klein

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_Carolina

_{Salas Matamoros}

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,

e

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_{Pietro Zucca}

a

,

b

,

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a_LESIA,_Observatoire_de_Paris,_CNRS,₉₂₁₉₀_Meudon,_France

b_PSL_Research_University,_Université_{Pierre-et-Marie-Curie,}_Université_{Paris-Diderot,}_France c_Station_de_{radioastronomie}_de_Nançay,_Observatoire_de_Paris,_CNRS,₁₈₃₃₀_Nançay,_France d_PSL_Research_University,_Université_{d’Orléans,}_OSUC,_France

e_Space_Research_Center,_University_of_Costa_Rica,_San_Jose,_Costa_Rica f_ASTRON,₇₉₉₀_AA,_Dwingeloo,_The_Netherlands

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t

i

c

l

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Keywords:

Sun:particleemission Sun:radioradiation Sun:coronalmassejection Solar–terrestrialrelations

Mots-clés :

Soleil :émissiondeparticules Soleil :émissionsradio

Soleil :éjectioncoronaledemasse RelationsSoleil-Terre

Thesolarcoronaanditsactivityinducedisturbancesthatmayaffectthespaceenvironment ofthe Earth. Noticeable disturbances come fromcoronal mass ejections (CMEs),which arelarge-scale ejections ofplasmaandmagneticﬁelds fromthesolar corona,and solar energeticparticles(SEPs).Theseparticlesareacceleratedduringtheexplosivevariationof thecoronalmagneticﬁeldorattheshockwavedrivenbyafastCME.Inthiscontribution, itisillustratedhowfull Sunmicrowaveobservationscanleadto(1)anestimateofCME speedsand ofthe arrivaltimeoftheCMEattheEarth,(2)theprediction ofSEPevents attainingtheEarth.

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La couronne solaire et son activité peuvent perturber l’environnement spatial de la Terre.Leséjections coronalesde masse(CME)sont desinstabilités àgrande échellequi conduisentàl’éjection dansl’espaceinterplanétaireduplasmaetduchamp magnétique quile conﬁne. D’autres perturbationsviennent des particules solairesde haute énergie (SEP).Elles sontaccélérées aucoursde lavariationexplosive duchamp magnétiqueou par l’onde de chocqu’engendre une CMErapide. Dans cetarticle, onillustre comment desobservationsduSoleilentierenmicro-ondespeuventconduire(1)àestimerlavitesse d’uneCMEetsontempsd’arrivéeàlaTerre,(2)àlaprévisiondesévénementssolairesà particulesquiatteignentlaTerre.

*

Correspondingauthor.

E-mailaddress:Ludwig.Klein@obspm.fr(K.-L. Klein). https://doi.org/10.1016/j.crhy.2018.01.005

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1. Introduction:solaractivityandspaceweather

The solarcorona isaplasma structured by magneticfields.Theyemanatefromtheconvective zone belowthevisible photosphere. Because of the ubiquitous plasma motions in this zone, magnetic fields continually emerge into the solar atmosphereandinteractwithalreadyexistingstructures. Thissituation mayleadto thestorageandoccasional explosive releaseofenergy.Theeruptionofcoronalmagneticstructuresintotheinterplanetaryspace–coronalmassejections(CMEs) –andthereleaseofchargedparticleswithhigh,sometimesrelativisticenergies,aretypicalsignatures.Thesolarenergetic particles(SEPs)areacceleratedfromabout100 eVinthethermalcoronatoenergieswhichonoccasionmayexceed1 GeV forprotons.Thismayhappeninelectricfieldsinducedduringtheexplosivevariationofthecoronalmagneticfieldoratthe shockwavedrivenbyafastCME.

When intercepting the Earth, CMEs compress the magnetic field andtransfer energyinto the magnetosphere during a geomagnetic storm. The quantitative impact dependson whetheror not the orientation of the magnetic field within and around the CME enables magnetic reconnection withthe magnetic field of the Earth. There is therefore no unique parametertodescribethegeo-effectivityofaCME.Duringastronggeomagneticstorm,themagnetopause,whichisusually above10Earthradiiabovethegroundatthesubsolarpoint,canbepusheddowntoabout5Earthradii.Thedissipationof electricalcurrentsinduced intheionosphereheats andionisesthehighatmosphereinthepolarregions,disturbingradio communicationsandthenavigationofspacecraftandaircraft.

EnergeticparticlesmaycreateasupplementarystrongionisationofpolarregionsoftheEarth’satmosphereandthereby perturbradiocommunicationsduring severaldays.TheymayaffectthefunctioningofspacecraftoutsidetheEarth’s mag-netosphere or in polar orbits, launcher and space vehicle operations, and astronaut safety. Relativistic nucleons create secondaryparticlesintheatmospherethatalsocauseionisationandexcessradiation,whichmayreachaircraftaltitudes.

TheeffectsofCMEsandSEPsonhumanactivitiesarediscussedinmanypublications.Thereaderisreferredto[1]fora recenthistoricalaccount,to[2]foradetaileddiscussionofextremespaceweatherevents,andto,e.g.,[3]foranoverview ofthephysicalprocessesinvolvedinSun–Earthinteractions.

Methods to alert about thearrival ofsolar disturbances inthe space environment ofthe Earthare potentially useful toolstomitigatehazards.ThetraveltimesofCMEstotheEarthrangefromabout15 h[4]toafewdays.Protonsofafew tensofMeV travelthe distancewithina fewtensofminutes.Theseare typicaladvancewarningtimeswhenthe alertis triggeredbyoneoftheearlysignaturesoftheeruption,forinstanceelectromagneticemissions.Longerwarningtimescould be envisagedifone understoodthe details oftheeruption process andits precursors. Butno operationalschemeof this kindcouldbedevisedsofar.

Inthiscontribution,twoaspectsofradioemissionasaforecastingtoolwillbeillustrated,namelythearrivalofcoronal mass ejections andof energetic particles at 1 AU. We start withthe description of a simple scenario of solar eruptive activity.

2. Solareruptions,radioemission,andspaceweather

Transient enhancements of solarradio emission, calledradio bursts, are generated when electrons are acceleratedto energies well abovetheir thermal energyinthequiet corona. Thecartoons ofFig. 1depict major featuresof amagnetic eruption in the solar corona, which leads to a coronal mass ejection (CME) and a flare. The pre-eruptive configuration (Fig. 1a)isacurrent-carryingmagneticfluxrope,definedbythehelicoidalmagneticfieldlineswithinthefluxrope(light blue)andaround(black).TheLorentzforceofthisconfigurationisdirectedupward,sincethemagneticfieldlinesaremore densely packedbelow theflux ropethan above. Theupward Lorentz force is balanced inequilibriumby the downward-directedLorentzforceexertedbythesurroundingcoronalmagneticfield,whosefieldlinesareplottedinorange.Anexcess upwardforce canbegeneratedforinstancewhenplasmamotionsinthephotospheretwistthemagneticfieldinonefoot oftheflux rope.Whenthishappens,thefluxropeisliftedby theLorentz force(Fig. 1b),ambientcoronalplasmaandthe embeddedmagneticfieldareconvectedfrombothsidestowardstheregionwhereitwaslocatedbefore,andoppositely di-rectedmagneticfieldscanreconnect.ThisisillustratedinFig. 1bandcfortwofieldlines,withthereconnectionhappening inalimitedregionschematicallyindicatedbytheyellowsymbolofanexplosion.Newmagneticfieldisthenaddedtothe fluxrope(theupperpartofthefieldlinedrawninredcolour),andnewmagneticloopsforminthelowcorona.

A2D projection ofthissituationisdepictedinFig. 1d,together withtheconsequencesofthe magneticreconnection: chargedparticlesacceleratedintransientelectricfieldsinducedinandaroundthereconnectionregion,andelectromagnetic emissions excited by these particles in different regions of the erupting configuration. HardX-rays and gamma-rays are generatedrespectively by electrons andionsthrough collisional processes,which aremostefficient inthe dense low at-mosphere.Radioemissionisgeneratedbyenergeticelectronsindifferentregions.Microwavesaremostlygyro-synchrotron emission of electrons with energies of hundreds of keV to a few MeV in the newly reconnected loops below the CME (Fig. 1c,d),whichcontainalsohotplasmaemittingsoftX-rays.

Fig. 2isanillustrationoffull-SunsoftX-rayandradioemissionduringamajorsolarﬂare/CME.ThesoftX-rayemission inthebottompanel showstheslowlyevolving thermalplasma:thesuddenheating fromabouttwo millionto aboveten million K leads to the sudden rise, followedby a slowdecay, which maylast several hours. The top panel shows radio emission.Microwavefrequencies(e.g.,8800 MHz)areofinterestinthefollowing.

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Fig. 1. Cartoonscenarioofthemagneticfieldconfigurationaroundamagneticfluxropeinthesolarcorona(a),andofitsevolutionduringtheliftoffof acoronalmassejection(CME;b,c).Thewhiteandgrey-shadedareasindicateoppositemagneticpolaritiesinthephotosphere,thegreylineistheline wheretheverticalphotosphericmagneticfieldiszero.Figure(d)showsatwo-dimensionalcutof(c).From[5].

Fig. 2. Timehistoryofwhole-SunX-rayandradioemissionsduringasolarﬂare/CMEevent.Bottom:softX-raysintwowavelengthranges(GOESsatellites, NOAA).Top:radiowavesatselectedfrequencies(RSTN,USAirForce).FromMarquéetal.(2017,submittedtoJournalofSpaceWeatherandSpaceClimate).

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The closerelationship betweenthe CME liftoff, particleacceleration andelectromagneticemissions sketchedin Fig. 1

suggests that one might use one phenomenon that is easy to observe or is an early signature of the eruptive eventto inferinformationontheother.WefocusbelowontheuseofmicrowavestoforecastCMEspeedandtheprobabilityofSEP events.

3. Radioemissionasaspaceweatherforecastingtool

3.1. Speedsandinterplanetarypropagationtimesofcoronalmassejections

CMEs are usually observed in white light by coronagraphs, which artificially occult the bright emission of the solar disk. CMEs can hence only be observed in projection onto the plane of the sky. Those travelling at about right angles fromthe line ofsight are readily visible, andtheir propagation speed can be easily measured by localising thefront of theCMEinsuccessiveimages.Itismuch moredifficulttodiscernCMEs thatpropagatealong thelineofsight.Observing earthward-propagating CMEs witha coronagraphon theSun–Earth line, such astheLASCO instrumentaboardthe SoHO mission(ESA/NASA),is henceintrinsicallydifficult.No directmeasurement oftheearthwardspeed ofsuch aCME canbe obtainedwiththeclassicalcoronographicobservingtechnique.

WeestablishedanempiricalrelationshipbetweenthespeedofCMEs originatingnearthesolarlimbandthefluenceof the associatedsoftX-ray[6]andmicrowave bursts[7].The CMEspeed was measured inthe coronographicimages from SoHO/LASCO[8],andisavailable intheCMEcataloguegeneratedandmaintainedattheCDAWDataCenter byNASA and The Catholic University of America, in cooperation with the Naval Research Laboratory.1 Since the CMEs originate near the solarlimb, one can assume that they propagate atangles near 90◦ with the lineof sight,and that their speed can be measured withlittledistortion by foreshortening.The microwave datawere providedby the fourground-based RSTN observatories.2 The comparisonofthe datasets showsa correlation, withbroadscatter, betweenthe microwavefluence, especiallyatthehigherfrequencies (e.g.,8800 MHz), andtheCME speedintheplane ofthesky.Sucha relationshiphad been found by others, albeit rarely with a well-defined selection of limb-CMEs. It is not just empirical, butreflects the physicallinkbetweenCMEacceleration,plasmaheatingandelectronaccelerationbelowtherisingfluxropeinthecartoon scenario ofFig. 1. Aquantitative analysisof the physicalrelationship betweensoft X-rayflux and CME speed in such a modelwasconductedby[9].ThederivedempiricalrelationshipnowallowsustoestimateaCMEspeedwhenweknowthe fluenceofthesoftX-rayormicrowaveburst.ThisisespeciallypossiblewhentheCMEpropagatesearthward,becausethen theburstsourceisexpectedtobenearthecentreofthesolardisk,andwellvisiblefromtheEarth.

ItiswellknownfrompastobservationsthatfastCMEsinthecorona,withaspeedwellabovetypicalsolarwindspeeds (

∼

400 km

·

s−1),decelerateduringtheirinterplanetarypropagation.Thisisduetotheaccumulationofplasmainfrontofthe outward-propagatingmagneticobstacle.Empirically,thedecelerationwasrelatedinalinearwaytotheinitialspeedofthe CMEby [10].Wefed theCMEspeedinferredfromthemicrowaveandsoftX-rayﬂuences tothisempiricallawtopredict the arrival times at the Earth’s orbit of eleven Earth-directed CMEs originatingon the solar disk. The predicted arrival timeswerecomparedwiththosederivedfromtheinsitumeasurementsoftheplasmaandmagneticﬁeldparameters.The magneticstructureisoftenprecededbyashockwaveandaturbulentsheathplasmabehindit.

ThepredictedandobservedCMEarrivaltimesare comparedinFig. 3.The solareventsareorderedbytheheliographic longitudewheretheCMEsoriginate.Theordinateshowsthedifferencebetweenpredictedandobservedarrivaltimes.The zero value corresponds to the exact prediction,positive values tocases where the CMEis observed to arrive before the predictedtime.Theblackvertical linesshowthetime intervalsbetweenthearrivalsoftheshockwaveandthemagnetic obstacleoftheCME.Theredhorizontallinesmarkpredictionerrorsof

±

12 h.Thedifferentsymbolsandcoloursdistinguish theoriginoftheCMEspeedestimate:microwavefluenceat9 GHz(redfilledsquares),softX-rayfluence(greenasterisks), andanempiricallycorrectedspeedderivedfromcoronographicobservations(bluefilledcircles).Thepredictionsusingsoft X-raysandmicrowavesareofcomparablequality. Notablefeaturesarethat(1)onaveragetheCMEarrivalispredictedtoo early (biastowards thelower halfof thefigure), and(2)thepredictionstend tobe betterforCMEs originatingfromthe westernsolarhemisphere(rightpartofthefigure).Thisisnotacoincidence:adetailedanalysisshowsthat,inmostcases (7/11),theEarthinterceptsthe flankofthe CME(events labelled“F”),andonly infoureventsisthe vicinityofthe nose seenbythespacecraft(eventslabelled“N”).AtthetimewhentheflankisdetectedattheEarth,thenoseisalreadybeyond theEarth’sorbit.Thisisconsistentwiththeearlyprediction.Thesystematicrelationshipbetweentheearlyarrivalandthe eastern heliographiclongitudeoftheparentactivitysuggestsfurthermorethat thepredictioncan becorrected.Adetailed description ofthis work isin [7]. The comparisonwith other predictionmethods and withthe observationsshows that softX-rayormicrowavefluenceisavaluabletooltopredicttheCMEarrival,withthesupplementaryadvantagethatthese fluences are knownatthetime whenthe CMEis stillbehind the occultingdiskofcontemporarycoronographs. Thefirst warningcanhencebeissuedveryearly,whentheCMEisstillclosetotheSun.

1 _http_:/_/cdaw.gsfc_.nasa_.gov_/CME_{_list}_/index_.html_.

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Fig. 3. DifferencebetweenobservedandpredictedCMEarrivaltimesattheEarthforelevenCMEsorderedbytheheliographiclongitudeoftheirorigin. From[7].

3.2. Solarenergeticparticle(SEP)events

SEP eventsare associatedwithsolarﬂares andCMEs, whichwe collectivelycall eruptiveevents inthefollowing. The onlypracticableforecastingstrategyispresentlytoinfertheSEPstocomefromtheﬁrstobservationsoftheeruptiveactivity inthecorona.Severaldifferentbutcomplementaryapproacheshavebeendeveloped.

TheUMASEP scheme,developedattheUniversityofMálaga[11],combinesthemonitoringofsolarsoftX-rayemission andofsolarprotonsatenergiesbetweenafewMeVandafewhundredsofMeV,usingGOESmeasurements.Simultaneous rises in the soft X-ray flux and the particle intensity are considered as an indicator that an SEP event is to occur. We conductedanexploratorystudytoseeifthesoftX-raydatacanbereplacedorcomplementedby microwaveobservations referring tothegyrosynchrotronemissionofmildlyrelativisticelectronsacceleratedintheassociatedflare.Themotivation istwofold:fromaphysicsviewpoint,microwaveemissionproducedbynon-thermalelectronsmaybeexpectedtobemore closely related to SEP accelerationthan soft X-rays, which are emitted by the plasma heatedduring the solar eruption. Fromanempiricalviewpoint,thederivativeofthesoftX-raytimeprofileisknowntomimicthetimeprofileofmicrowave emission fromnon-thermalelectrons [12].Thiscanbeseenduring thesoftX-rayburstinthebottompanel ofFig. 2.The microwave emission(first burst)is strongduring therise,andhasdecayedtobackgroundnearthemaximumofthe soft X-rayburstinthe(0.1–0.8) nmband.

Weconstructedanuninterruptedseriesofmicrowavefluxdensitiesat4995and8800 MHzduringa13-monthsinterval fromDecember2011toDecember2012.TheinputdatawerethedailylightcurvesobservedatthefourRSTNobservatories. Data werecleanedsuchastoreducediscontinuitiesatthetransitionbetweentwoobservatories,whicharedueto calibra-tion problems,incorrectantenna pointingandother instrumentfailures.The light curvesare shownin Fig. 4. Numerous solar burstsare visible.Duringthisperiodnine SEPeventswithoriginon thewestern hemispherewere detected by the GOESsatellites.ThemicrowavetimeprofileswerefedtotheUMASEPpredictionschemeinsteadofthesoftX-rayderivative. ThekeyfindingsforthisthirteenmonthsperiodwithnineSEPeventsaresummarisedasfollows.

•

Theprobabilityofdetection(POD)is7/9events.ItisthesameasinthetraditionalUMASEPscheme,wherethe deriva-tiveofthesoftX-raytimeproﬁleiscorrelatedwiththeSEPintensity.

•

Thefalsealarmrate(FAR)iszeroforthemicrowavedataatbothfrequenciesconsidered,1/8whensoftX-raysareused.

•

Thewarningtime,i.e. thetimebetweentheforecastandtheinstantwheretheSEPintensityexceedstheoﬃcialevent thresholdusedbyNOAA,isslightlyimprovedwiththemicrowavelightcurvesoverthesoftX-rays(30.7vs. 26.4 min).

This showsthatmicrowave dataimprovethe prediction,especiallybecause theyare rarerphenomena than softX-ray bursts. The soft X-ray bursts reveal the heating of coronal plasma during a flare – a process that may or may not be accompanied by particle acceleration.Major microwave bursts need the acceleration ofelectrons to relativistic energies. UsingmicrowaveburststhereforeavoidsnumeroussmallandpurelythermalfluctuationsofthesoftX-rayemission.Itisto be noted,however,that afewmoderatelystrongSEPeventsarenotrelatedtoconspicuousparticleaccelerationinflaring

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Fig. 4. TimeproﬁleofthewholeSunmicrowaveemissionduring13months,composedfromdailyobservationsofthefourRSTNobservingstationsofthe USAirForce.From[13].

active regions, andmaybe missedby a predictionthat relies onlyon non-thermalmicrowave bursts.The details ofthis studyconductedincollaborationwiththeUniversityofMálagacanbefoundin[13].

4. Discussionandconclusion

It has been known since the 1950s that solar radio bursts are closely related to interplanetary plasma disturbances and energetic particles (see [14] for a review). But the advent and easy availability of soft X-ray monitoring with the GOESsatellitesoperatedbyNOAAhasdiminishedtheinterest forthisaspectofradiomonitoring.The USAir Forceisthe onlyinstitutiontoprovide24-hmonitoringoftheSun,usingfourstations aroundtheEarth.Theseradio observationsare conductedwithrathersimplepatrolinstruments,whichmonitorthewholeSunﬂuxdensityusingparabolicantennaswith a typicalsize of1 m. Such dataare presently not provided inrealtime, butthere isnotechnical obstacleto doso. Ifa reliablecalibrationandstableandreliableantennaoperationscanbeachieved,microwavepatrolobservationsappeartobe a signiﬁcantaddition toourability toforecast theoccurrenceofSEPeventsandthe interplanetarytravel timesofCMEs. Inaddition tobeingeasy to handleandrelativelycheap, radio observationshavethe advantagethat theinstruments are protectedbytheEarth’satmosphereandmagnetosphereagainstspaceweatherhazards.

Acknowledgements

This work was supported by the ANR/ASTRID project ORME (Observationsradioastronomiquespourlamétéorologiede l’espace,contractNo.ANR-14-ASTR-0027),bytheHESPERIAproject,fundedbytheEuropeanUnion’sHorizon2020research andinnovationprogramundergrantagreementNo. 637324,andbythe“CentreNationald’EtudesSpatiales”(CNES)(grant No.W-EEXP/10-01-01-05).

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