part 2: cme analysis with stereocatfor space weather ......but now let's say the user chooses...

Part2:CMEAnalysiswithStereoCAT forSpaceWeather:

Limitations

BarbaraThompson(NASA/GSFC)[email protected]

SOHO/LASCO C2

June2017

CMEMeasuringgeometriesCMEsaretypicallyviewedusingacoronagraph.CMEsarelargeandusuallyseenasasemi-transparentcloud,sowhatyouseeistheCMEintegratedalongthepathlengthoftheviewer(likeyou'dseeacloudofsmokeorfog).

Itisa2-Dprojectionofa3-Dobject,andthechallengeistotakethose2-Dobservationsanddeterminewhatthe"real"3-Dstructureis.

However,coronagraphsfunctioninawaythatisdifferentfromyoureyesora"typical"imager.Coronagraphsviewphotonsscatteredoffofelectronsinthecorona,andthescatteringefficiencyofanelectrondependsonthelocationoftheobserver.Thiscomplicatesthingsquiteabit!

CMEMeasuringgeometriesFactorsinvolved:1)Basedonviewingangle,whereistheapparentleadingedgeof

theCMErelativetothe*real*leadingedge?2)Howcloseistheapparentleadingedgeasseenfromone

spacecrafttotheapparentleadingedgeasseenfromanother?3)BasedonThomsonscatteringgeometry,howbrightwillthe

leadingedgeoftheCMEberelativetotherestoftheCME?(Forangles>60° outoftheplaneofthesky,themassisbasicallyinvisible,andyouwillonlyseethepartoftheCMEclosertotheimageplane.)

4)Howcanwedeterminewhichspacecraftpairsaremostidealandwillproducethemostaccuratemeasurementsfordifferentviewinggeometries?

Warning: StereoCAT shouldnotbeusedforhaloCMEs,andforwideCMEs (>50degreeshalfwidth)measurementscanbeveryinaccurate.

StereoCAT isausefulandversatiletoolformeasuringCMEs.However,StereoCAT will*always*giveyouananswer,evenifitmakesnosense!Youcanclickanywhereanditwillgiveyouananswer.Therefore,StereoCAT isapowerfultoolinthehandsofaknowledgeableuser,butyoumustalwaysbemindfulofthemeasurementsandtheresultsthatyoureceive,andkeepyoureyesopenforresultsthatappear"suspicious.”

Let’sgothroughafewexamples…

CMEMeasuringgeometries

ForSTEREOlocationson2014-01-16

STEREO-A:151° longitude

STEREO-B:-154° longitude

DiagramofaWestlimb(90°)CMEat14RSun(Sunnottoscale)

Thearrowsindicatethe"leadingedge",ortangentialline,fromtheviewpointofeachofthecoronagraphs.

Howcloseistheapparentleadingedgeasseenfromonespacecrafttotheapparentleadingedgeasseenfromanother?

Measuringangles(tangentpoints)foraWestlimbCMEat

14RSun

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SOHOSTEREO-ASTEREO-B

Best*combination:STEREO-AandSOHO.Apparentleadingedgesareclosest.Secondplace:STEREO-BandSOHO.Nextclosestleadingedges.Worst:STEREO-AandSTEREO-B. Theyeachseeverydifferentleadingedges,neitherclosetothetrueleadingedge.

*Ofcourse,youdon'tknowwhattheCMElongitudeiswhenyoustartmeasuring,sohowareyousupposedtoknowwhichpairwillproducethebestresults?Perhapsconsideringsourcelongitude ontheSuncanhelp.

ZoominDiagramofaWestlimb(90°)CMEat14Rsun

Diagramofa45°CMEat14RSun



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Bestcombination:STEREO-AandSOHO.Secondplace:STEREO-B*andSOHO.Worst:STEREO-AandSTEREO-B.

*NoteCMEisfarfromimageplaneinSTEREO-B.Seediscussionofscatteringamplitudesbelow.

Zoomin



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Bestcombination:STEREO-BandSOHO.Second/thirdplace:STEREO-B+STEREO-Aifyouwantbetterestimateofleadingedgespeed,butSTEREO-A+SOHOifyouwanttobetriangulatingthesamepoint.*

*Again,seediscussionofscatteringamplitudes.EventheflankthatSTEREO-Aismeasuringisfaroutoftheimageplane,butyourintegrationpathislonger,soitwillbemuchbrighterthanthetrueleadingedge.


Note:ForEastlimbCMEs,theSTEREOA/Bgeometryisreversed.


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Example:50° half-widthEarth-directedhaloCME

(Sunnottoscale)

Example:50° half-widthCME,haloCMEpointingatEarthStereoCAT userclicksonapparentleadingedgefromthepointofviewofSOHO,STEREO-AandSTEREO-B.

ActuallocationofCMEfront:45Rsun -1° longitude(soCMEisslightlyfastertoeastfromLASCOPOV)

Red/Blue(A-B)triangulationpoint: 74Rsun 4° longitude(mostaccuratelongitude,but65%offbyheight)Red/Green(A-SOHO)triangulation:117Rsun -10° longitude(>150%offinheight!)Green/Blue(SOHO-B)triangulation:49Rsun -46° longitude(bestheight,butonlytheCMEflankhasbeentriangulated).276pixelstotaldistance37pixelsCMEheightTriangle:Width41height17

Nowlet’slookattheeffectofCMEwidth:

Sameexample:50° half-widthCME,haloCMEpointingatEarthUserclicksonapparentleadingedgefromthepointofviewofSOHO,STEREO-AandSTEREO-B.

Butnowlet'ssaytheuserchoosestheothersideinSTEREO-Bastheleadingedge(forsomestrangehypotheticalreason).

ActuallocationofCMEfront: 45RSun -1° longitude

Red/Blue(A-B)triangulationpoint:40RSun 50° longtiudeRed/Green(A-SOHO)triangulation:117RSun -10° longitude

(>150%offinheight!)Green/blue(SOHO-B)triangulation:55RSun -143° longitude(!!)

Itisimportanttounderstandhowcoronagraphswork.Consider:WhatpartoftheCMEcanbeseen?

Afeature30° outoftheplaneoftheskyhasonlyhalftheoverallscatteringamplitudeofafeatureintheplaneofthesky.Forpoints>45° ,scatteringamplitudeisverylow.

Features>60° awaywillnotbeseenincoronagraphimages,andareeffectivelyinvisible.

IftheCMEisoutoftheplaneoftheimagemorethan60degrees, youdefinitelycannotseetheleadingedge.Youaremeasuringfarintotheflank.

Anytriangulation resultgivingaviewingangle>40° foreitherofthespacecraftshouldbeviewedwithgreatsuspicion.Theslopeishighandfeaturesclosertotheplanewillstilltendtodominate.

A40° anglefromtheplaneoftheskywillgiveyou~25%increaseinyourcalculated3Dspeed.Therefore,anything>25%increaseinoneoftheline-of-sightCMEspeedsshouldbeexaminedclosely.

Argh!Whatcan wedoaboutthis?

Icecreamconereprojection:Lon=-15v=670km/s

Lon=-4meansCMEleadingedgeis65° outofthePOSforSTEREO-Aand60° forSTEREO-B.Leadingedgeisinvisible!

SpeedismuchfasterthanLOSspeeds.Suspicious...

"IceCreamCone"Model

Handyconversionbetweentheapparentspeedofasphericalbrightshellofvaryinganglesanditsobservationangle.

Otherweirdresults– makesureyouexaminetheanswers!

3DspeedislowerthanSTEREO-A??

Thebestofbothworlds:SWPC_CAT

Thebestofbothworlds:SWPC_CATSWPC_CATperformsthetriangulationmeasurementslikeStereoCAT does,butitusesa3Dprojectiongeometrysimilartotheconeprojectionmodel.Additionally:

- itbeusedtocanfithaloCMEs (butbecareful!)- youcanuse1,2or3viewpoints- theimagetimesdonothavetomatch,afterinitialfit

Justrememberhowacoronagraphworks:featuresfaroutoftheplaneoftheskyareinvisible,soyou'llonlyseetheedgesofahaloCMEandnottheleadingedge.

Misc.information

Wehavealittlemoreinformationaboutanalysisthatmayhelpguideyouinthefuture.:)

[email protected]

RememberthatmanyCMEs(particularlyfastones)decelerate.SpeedsenteredintoDONKIshouldbeasclosetothet@21.5speedsaspossible.Ifyoucannotdetermineaspeedcloseto21.5RSun,youmaywanttodeterminedecelerationthroughthemeasurementfieldandcorrectforthat.

LASCOpitfalls:Missingdataarefilledinwithpreviousimages.Iftriangulationdoesn'tmakesense,itmightbebecausetheyfillinmissingdatawiththepreviousimage(s)!

Example:Theseimagesarenearlyidentical.Theredarrowshowswherethe"real"11:30UTimageends.

Bewareoffilled-inimages!You'llbeshownthelocationofwhereaCME"was"insteadofwhereit"is."Checkthedifferencepaneformissingdata,orchecktomakesureimageadvanceswitheachframe.

RealLASCOimagefrom11:30UT

Filled-inLASCOimagefrom11:30UT

LASCOpitfalls:

Featuresbehindthepylonwillbehardtosee.Pylonwilleitherbeinupperrightorlowerleft(dependingonspacecraftorientation).

LASCOC2andtheSTEREOcoronagraphsalsohavepylons– theeffectisn'taspronounced,butitisstillsomethingyoushouldkeepinmind.

Pylonlocation


STEREOpitfalls:STEREOdoesn'tfillinmissingdata,butthegraininessofthebeaconimagesmeansthatsomefeaturescanmimicCMEshapes.Thereisarapidfalloffinbrightnessneartheedgeoftheimage,atcertainangles- thesecanlooklikealeadingedge.The"real"leadingedgemaybebeyondtheFOV– whatyouthinkisafrontmaybethedropoffinbrightnessconvolvedtheremainingpartoftheCME.


ThisisnotaCME. It'safanofstreamerfeatures,andthebrightnessdepressionsatthetopandtherightsidemakeitlookmorelikealargeCME.

Conclusions- ChoosingviewinganglepairscangreatlyinfluencetheresultingCME

speedtriangulation.- EvenifyouareabletomeasurethesamepointonaCME,projection

anglecanhaveahugeimpactonthemeasuredspeedrelativetothetruespeedoftheleadingedge.Ingeneral,thewidertheCME,thelessaccuratethemeasuredspeed(forasphericallyexpandingCMEapproximation).

- However,forawideCMEitismorelikelythatmuchofitwillbeinvisiblebecauseofThomsonscatteringlimitations.Aleadingedge30° outoftheplaneoftheskywillhavehalfthescatteringefficiencyrelativetofeaturesintheplaneofthesky.Features>60°outoftheimageplaneareinvisible.

- WhenyouonlyhaveimagesfromoneviewpointyoucanusethesourcesignaturelocationasanapproximationoftheCMElat/lon.

part 2: cme analysis with stereocatfor space weather ......but now let's say the user chooses...

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