Microfilament-Eruption Mechanism for Solar Spicules
AbstractRecentstudiesindicatethatsolarcoronaljetsresultfromeruptionofsmall-scalefilaments,or “minifilaments”(Sterlingetal.2015,Nature,523,437;Panesar etal.ApJL,832L,7). Inmanyaspects,thesecoronaljetsappeartobesmall-scaleversionsoflong-recognizedlarge-scalesolareruptions thatareoftenaccompaniedbyeruptionofalarge-scalefilamentandthatproducesolarflaresand coronalmassejections(CMEs). Incoronaljets,ajet-basebrightpoint(JBP)thatisoftenobservedtoaccompanythejetandthatsitsonthemagneticneutrallinefromwhichtheminifilament erupts,correspondstothesolarflareoflarger-scaleeruptionsthatoccursattheneutrallinefrom whichthelarge-scalefilamenterupts. Large-scaleeruptionsarerelativelyuncommon(~1/day)andoccurwithrelativelylarge-scaleeruptingfilaments(~10^5kmlong). Coronaljetsaremorecommon(>~100s/day),butoccurfromeruptingminifilaments ofsmallersize(~10^4kmlong). Itisknownthat solarspiculesaremuchmorefrequent(manymillions/day)thancoronaljets. Justascoronaljets aresmall-scaleversionsoflarge-scaleeruptions,herewesuggestthatsolarspiculesmightinturn besmall-scaleversionsofcoronaljets;wepostulatethatthespiculesareproducedby eruptionsof``microfilaments’’oflengthcomparabletothewidthofobservedspicules(~300km). Aplotoftheestimatednumberofthethreerespectivephenomena(flares/CMEs,coronaljets, andspicules)occurringontheSunatagiventime,againsttheaveragesizesoferupting filaments,minifilaments,andtheputativemicrofilaments,resultsinasizedistributionthatcan befitwithapower-lawwithintheestimateduncertainties. Thecounterpartsoftheflaresof large-scaleeruptionsandtheJBPsofjetsmightbeweak,pervasive,transientbrightenings observedinHinode/CaII images,andtheproductionofspiculesbymicrofilamenteruptions mightexplainwhyspiculesspin,asdocoronaljets. Theexpectedsmall-scaleneutrallinesfrom whichthemicrofilamentswouldbeexpectedtoeruptwouldbedifficulttodetectreliablywith currentinstrumentation,butmightbeapparentwithinstrumentationofthenearfuture. A fullreportonthisworkappearsinSterlingandMoore2016,ApJL,829,L9.
Overview
Atleastmanycoronaljetsappeartobeminiatureversionsoflarge-scaleeruptions. Specifically,bothresultfromeruptionoffilament-likefeatures:normalfilamentsinthecaseoflarge-scaleeruptions,andsmaller-scale``minifilaments’’inthecaseofcoronaljets. Also,bothphenomenahavebrightenings attheirbases:solarflaresinthefilamenteruptions,andJBPsatthebaseofthecoronaljets. Thissuggeststhatthetwophenomena,filamenteruptionsandflaresontheonehand,andcoronaljetsandJBPsontheother,arethesamephenomenonbutondifferentsize scales.
Besidessizescale,anotherdifferencebetweenthesetwofeaturesistheiroccurrencerate:thenumber ofcoronaljetsismuchlargerthanthenumberoflarge-scaleeruptions. Spiculesarealsotransient, jet-likephenomena,andarefarmorecommonthaneitherlarge-scaleeruptionsorcoronaljets. And,recentintriguingrecentideasfortheirformationnotwithstanding(e.g.DePontieu etal.2004, Martinez-Sykora etal.2017;see,e.g.,Beckers 1998andSterling2000forolderideas),afull explanationfortheircauseisstilloutstanding.
Anaturalquestiontoaskiswhethertherelationshipbetweensize-scaleandoccurrencerateofthe relatively-smallnumberoflarge-scaleeruptionsandthelargernumberofsmaller-scalecoronaljets, extendstothemuch-more-frequentandsmaller-size-scalespicules. Weexplorethisideabyplottingthesizescales(orexpectedsizescales)ofthefilament-likefeaturestheerupttoformCMEs,jets,andspicules,againstthemeasuredoccurrencerateofCMEs,jets,andspicules. Wethenseewhetherthisrelationshipbetweenthesequantitiescanbefitwithapowerlaw.
References
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Coronaljets(“minifilament eruptions”)
CoronaljetsareareseeninX-rayandEUVcoronalimages. Theyhaveageyser-likeappearanceandcanreach>50,000kmwithwidthsof~8000km,withlifetimesof~10min(Savcheva etal.2007);thesenumbersareforpolarcoronal-holejets,buttheyareseenallovertheSun(e.g.,Shimojo etal.1996).Recentinvestigationsindicatethatmany,ifnotall,coronaljetsresultfromeruptionsofsmall-scalefilaments(“minifilaments”)ofsize~8000km(Sterlingetal.2015).(Figure1.) Thesejetshaveabrighteningattheirbase(jet-basebrightpoint,JBP),analogoustoflares,andthejets/minifilamenteruptionsoccuronmagneticneutrallines(e.g.Huangetal.2012, Panesar etal.2016),analogoustolarge-scalefilamenteruptions. Jet-producingerupting miifilaments havesizes~8000km;andbasedonobservedratesandlifetimesinpolarcoronalholes (Savcheva etal.2007),weestimatethatthereare~5jetsoccurringontheSun onaverageatanygivenrandomtime.
Athay,R.G.1959,ApJ,129,164Beckers,J.M.1968,SolarPhys.,3,367Bernasconi,P.N,etal.2005,SolarPhys.,228,97Chen,P.F.2011,LRSP,8,1DePontieu,B.etal.2004,Nature,430,536Huang,Z.,etal.2012,A&A,548,A62Lynch,D.K.,etal.1973,SolarPhys.,30,63Martinez-Sykora,J.etal.2017,Science,356,1269
Spicules(“microfilamenteruptions”??)
Spiculesareextremelycommonchromospheric jet-likefeaturesreachingheights~5000km. Wespeculatethat,iftheyresultsfromeven-smaller-scaleeruptionsoffilament-likefeatures (“microfilaments”),theerupting-microfilamentswouldhavewidthssimilartospiculewidths,or~300km(Pereiraetal.2012). HistoricalmeasurementsplacethenumberofspiculesontheSunatanygiventimeas~(9.3—50.0)x10^4(Athay 1959,Lynchetal.1973).
Acknowledgements
ThisinvestigationwassupportedbyNASA’sHGIprogram,andtheHinode project. WethankT.Tarbell forassistancewiththeSOTimages.
ResultsandDiscussion
Figure2showsthenumberoferuptionoffilament-likefeaturesontheSunatanygiventime,plotted againstthesizeofthefilament-likefeatures,wherethenumbersareasdetermined/estimatedinthepreviouspanels.Wefindthat,withintheuncertaintyranges(Sterling&Moore2016),thevaluesallfallonthesamepowerlawline. Thissupportsthatthefilament-like-eruptionmechanismthatdrives large-scaleeruptionsandcoronaljets,coulddrive~10%— 100%ofspicules,andstillbeconsistentwithfittingthepower-lawdistribution. Acounterpartoftheputativespicule-producingmicrofilamenteruptionstotheflaresandtheJBPsof,respectively,filamentandminifilament eruptions,mightbebrightenings sometimesseeninnear-limbCaIIimages(Figure3).
Sofarhowever,theexistenceofspicule-producingmicrofilamentsistotallyspeculative,andthe natureoftheCaIIbrightenings isuncertain. DKISTandfutureSun-observingspacemissionsshouldimproveourknowledgeofthenatureofsolarphenomenasuchaslarge-scaleeruptions,coronaljets,andspicules.
Figure3.(a)–(c)Hinode SOTCaIIcolor-reversedimages(�01pixel−1resolution)ofapolarcoronalholenearthelimb,showingon-disk“CaIIbrightenings”andspicules(white/blackarrowsshowbrightenings/spicules).Yellowarrowsshowexamplesofadditionalbrightenings thatmightalsobeatrootsofspicules.Brightenings betweenwhitearrowsin(a)evolvetoamore-concentratedbrighteningindicatedbyarrowin(b),whichfadessignificantlyby(c).Blackarrowsin(b)and(c)showastrandofaTypeIIspiculeemanatingfromtheCaIIbrightening.(d)–(f)Sameas(a)–(c),butforaquiet-Sunregionnearthesouthpolelimb.Arrowsinaccompanyinganimationsshowadditionalbasebrightening/spiculeexamples.(g)–(i)ImagesfromHinode/XRT,showingX-rayjet#12inSterlingetal.(2015,ExtendedDataFigure2ofthatpaper);here,weshowthesameanimationasinthatpaper,butwithcolorsreversedtoemphasizemorphologicalsimilaritieswiththeSOTfeaturesin(a)–(f);white/blackarrowsshowJBPexamplesandajetspire,respectively,wheretheJBPin(g)isofaweakerjetinthebackground.Northisupwardandwestisright.Intensityscaledtohighlightfaintfeatures,resultinginsaturationofsomefeatures.(AnimationsavailableinSterling&Moore2016.)
Figure2.Distributionofestimatednumberoferupting-filament-likefeaturesontheSunatanygiventime,asafunctionofthesizeofthoseeruptingfeatures.Right,middle,andleftpointsareforfilamenteruptionsdrivingCMEsandtypicalsolarflares,minifilamenteruptionsdrivingcoronaljetsandJBPs,andpostulatedmicrofilamenteruptionsthatwoulddrivespicules.“Error”barsshowmeasuredorestimatedrangesoftheplottedvalues.Weused±50%oftheplottedvaluesasguessesfortheuncertaintiesinthenumber-on-Sunvalues,excepttheuppernumberfortheeruptingmicrofilamentsisfromLynchetal.(1973).Seethetextforuncertaintiesinsizes.Theline(slope=−2.7)istheleast-squaresbestfittothethreepointswithoutconsiderationofuncertaintyweights.
Figure1.Schematicofminifilamenteruptionleadingtoacoronaljet(modifiedversionofSterlingetal.2015;Figure2).Blacklinesshowmagneticfields,witharrowsindicatingpolarities.Redlinesindicatefieldsthathaveundergonemagneticreconnection,withredcrossesshowingreconnectionlocations.(a)Initially,aminifilament(blue)sitsinacompactshearedbipoleadjacenttoalargerless-shearedbipole,allinsidesurroundingopenfield.(b)Anunspecifiedagenttriggerstheminifilamentfieldtoeruptlikealarger-scalefilamenteruption.Reconnectionattheleft-sideredXbetweentheenvelopeoftheerupting-minifilamentfieldandtheopencoronalfieldresultsinahotjet(shaded-orangestrip),visibleasahotter-EUV(e.g.,211A)orX-rayjet.Reconnectioninteriortotheexplodingminifilamentfieldattheright-sideredXresultsintheJBP(boldredsemicircle),analogoustoasolarflareinlarge-scalefilamenteruptions.(c)Iftheminifilamenteruptsfarenoughintotheopposite-polarityopen-fieldregion,thentheleft-sidereconnection“eatsaway”enoughoftheenvelopingouterfieldsurroundingtheminifilamentinthecoreforminifilamentmaterialtobeexpelledalongtheverticalfield,resultinginacool(e.g.,EUV304A)jetcomponent.SeeSterlingetal.(2015,2016)fordetails.
Large-ScaleEruptions(“filamenteruptions”)
By“large-scaleeruptions,”wemeanthetypicalmagneticeruptionsthatmakeoftenresultin coronalmassejections(CMEs)andsolarflares. Typicallythesebeginwitheruptionofafilament,whichrangeinsize3x10^4- 1.1x10^5km(Bernasconi etal.2005),fromamagneticneutralline. Aflarethengrowsalongtheneutrallinefromwhichthefilamenterupted. Wecanmakearoughestimateforthenumberoflarge-scaleeruptionsontheSunatanygiventimeinthisway:thereare0.5—6CMEs/day(Yashiroetal.2004;Chen2011),andthedurationofastrong-flare’sdurationis~20min(Verong etal.2002);fromthis,wededucethatthereare~0.03CME-producing typicalfilamenteruptionsoccurringontheSunatanygivetime. (Thatis,lookingattheSunrandomly1/0.03~33independenttimesaboutaday’stimescaleshouldshowonaverageonelarge-scale filamenteruptionoccurring.)
Panesar,N.K.etal.2016,ApJ,828L,9Pereira,T.M.D,etal.2012,ApJ,759,18Sterling,A.C.2000,SolarPhys.,196,79Sterling,A.C.etal..2015,Nature,523,437Sterling,A.C.,&Moore,R.L.2016,ApJL,828,L9Savcheva,A.etal.2007,PASJ,59,771Shimojo,M.,etal.1996,PASJ,48,123Veronig,A.,etal.2002,A&A,382,1070Yashiro,S.etal.2004,JGRA,109,7105
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