THEASTROPHYSICALJOURNAL,2010,INPRESS PreprinttypesetusingLATEXstyleemulateapjv.03/07/07 TANGLEDMAGNETICFIELDSINSOLARPROMINENCES A.A.VANBALLEGOOIJENANDS.R.CRANMER Harvard-SmithsonianCenterforAstrophysics 60GardenStreet,MS-15,Cambridge,MA02138,USA DraftversionJanuary15,2010 ABSTRACT Solarprominencesareanimportanttoolforstudyingthestructureandevolutionofthecoronalmagneticfield. 0 Here we considerso-called“hedgerow”prominences, whichconsist of thin verticalthreads. We explorethe 1 possibilitythatsuchprominencesaresupportedbytangledmagneticfields. Avarietyofdifferentapproaches 0 areused. First,thedynamicsofplasmawithinatangledfieldisconsidered. Wefindthatthecontortedshape 2 ofthefluxtubessignificantlyreducestheflowvelocitycomparedtothesupersonicfreefallthatwouldoccur n in a straight vertical tube. Second, linear force-free models of tangled fields are developed, and the elastic a responseofsuchfieldstogravitationalforcesisconsidered. Wedemonstratethattheprominenceplasmacan J besupportedbythemagneticpressureofatangledfieldthatpervadesnotonlytheobserveddensethreadsbut 5 alsotheirlocalsurroundings. Tangledfieldswithfieldstrengthsofabout10Gareabletosupportprominence 1 threadswithobservedhydrogendensityoftheorderof1011cm- 3. Finally,wesuggestthattheobservedvertical threadsaretheresultofRayleigh-Taylorinstability.Simulationsofthedensitydistributionwithinaprominence ] R threadindicatethatthepeakdensityismuchlargerthantheaveragedensity. We concludethattangledfields provideaviablemechanismformagneticsupportofhedgerowprominences. S . Subjectheadings:MHD—Sun:corona—Sun:magneticfields—Sun:prominences h p - 1. INTRODUCTION o r Solar prominences (a.k.a. filaments) are relatively cool t s structures embedded in the million-degreecorona at heights a wellabovethechromosphere(seereviewsbyHirayama1985; [ Zirker 1989; Priest 1990; Tandberg-Hanssen 1995; Heinzel 1 2007). Above the solar limb, prominences appear as bright v structures against the dark background, but when viewed as 7 filamentsonthesolardisktheycanbebrighterordarkerthan 5 their surroundings, depending on the bandpass used to ob- 7 serve them. Magnetic fields are thought to play an impor- 2 tantroleinsupportingtheprominenceplasmaagainstgravity, . and in insulating it from the surrounding hot corona. Most 1 0 quiescent prominences exhibit intricate filamentary struc- 0 tures that evolve continually due to plasma flows and heat- 1 ing/cooling processes (see examples in Menzel&Wolbach : 1960; Engvold 1976; Malherbe 1989; Leroy 1989; Martin v 1998). In some cases the threads appear to be mostly hori- i X zontal,whilein othercasestheyareclearlyradiallyoriented (“hedgerow”prominences).Figure1showsseveralexamples r a ofprominencesobservedinHαattheBigBearSolarObser- vatory(BBSO)andtheDutchOpenTelescope(DOT).Theex- FIG. 1.—Quiescentprominencesabovethesolarlimb: (a)Hαimageof prominenceobservedatBigBearSolarObservatory(BBSO),November22, amplesinFigs.1aand1bshowmainlyverticalthreads,while 1995;(b)Hα,BBSO,1970;(c)Hα,DutchOpenTelescope,September15, theprominenceinFig.1cshowshorizontalthreads. Off-limb 2006;(d)HeII304Å,STEREO/EUVI,2008April20at00:06UT. observationsinHeII304Åindicatethattherearehigheralti- tudepartsthatareopticallythininHαandthereforenotvisi- der 10–70 km s- 1 (Menzel&Wolbach 1960; bleonthedisk(oratleasthavenotbeenclearlyidentifiedin Engvold 1976; Zirker,Engvold&Martin 1998; diskobservations). Figure1dshowsaprominenceabovethe Kucera,Tovar&DePointieu 2003; Lin,Engvold&Wiik solarlimbasobservedinHeII304ÅwiththeSECCHI/EUVI 2003; Okamotoetal. 2007; Bergeretal. 2008; Chaeetal. instrument(Howardetal. 2007) on the STEREO spacecraft. 2008). Recent high-resolution observations of filaments on Theupperpartsoftheprominenceconsistofverticalthreads thesolardiskindicatethattheyconsistofacollectionofvery withanintricatefine-scalestructure.Moviesequencesofqui- thinthreadswithwidthsofabout200km,atthelimitofreso- escentanderuptingprominencescanbefoundattheSTEREO lutionofpresent-daytelescopes(Lin,Engvold&Wiik 2003; website1. Linetal. 2005a,b; Lin,Martin&Engvold 2008; Linetal. Prominence plasma is highly dynamic, ex- 2008). Individualthreads have lifetimes of only a few min- hibiting horizontal and vertical motions of or- utes, but the filament as a whole can live for many days. It seemslikelythatthesethinthreadsarealignedwiththelocal 1http://stereo.gsfc.nasa.gov/gallery/selects.shtml magneticfield.High-resolutionimagesofprominencesabove 2 VANBALLEGOOIJEN&CRANMER thelimbhavebeenobtainedwiththeSolarOpticalTelescope ported against gravity? Many authors have suggested (SOT)onboardHinode. Forexample,Okamotoetal. (2007) that quiescent prominences are embedded in large-scale observed horizontal threads in a prominence near an active flux ropes that lie horizontally above the polarity in- region, and studied the oscillatory motions of these threads. version line (Kuperus&Raadu 1974; Pneuman 1983; Heinzeletal. (2008) observed a hedgerow prominence vanBallegooijen&Martens 1989; Priest,Hood&Anzer consisting of many thin vertical threads, and they used 1989; Rust&Kumar 1994; Low&Hundhausen 1995; multi-wavelength observations to estimate the amount of Aulanieretal. 1998; Chaeetal. 2001; Gibson&Fan absorptionand “emissivity blocking”in the prominenceand 2006; Dudiketal. 2008). The prominence plasma surrounding cavity. Bergeretal. (2008) observed another is thought to be located near the dips of the helical hedgerow prominence and found that the prominence sheet field lines. The magnetic field near the dips may be is structured by both bright quasi-vertical threads and dark deformed by the weight of the prominence plasma inclusions. The brightstructuresare downflowstreamswith (Kippenhahn&Schlüter 1957; Low&Petrie 2005; velocity of about 10 km s- 1, and the dark inclusions are Petrie&Low 2005; Heinzel,Anzer&Gunár 2005). Oth- highly dynamic upflows with velocity of about 20 km s- 1. ers have suggested that the magnetic field in hedgerow The downflow velocities are much less than the free-fall prominences is vertical along the observed threads, and speed,indicatingthattheplasmaissomehowbeingsupported that the plasma is supported by MHD waves (Jensen 1983, against gravity. Bergeretal. (2008) proposed that the dark 1986; Pecseli&Engvold 2000). However, relatively high plumes contain relatively hot plasma that is driven upward frequencies and wave amplitudes are required, and it is bybuoyancy.Chaeetal.(2008)observedapersistentflowof unclearwhysuch waveswouldnotlead to strong heatingof Hαemittingplasmaintoaprominencefromoneside,leading the prominence plasma. Dahlburg,Antiochos&Klimchuk to the formation of vertical threads. They suggested that (1998) and Antiochosetal. (1999) showed that the promi- the vertical threads are stacks of plasma supported against nence plasma can be supported by the pressure of a coronal gravity by the sagging of initially horizontal magnetic field plasma lower down along an inclined field line; however, lines. this mechanism only works for nearly horizontal field lines Direct measurements of the prominence magnetic field (also see Mackay&Galsgaard 2001; Karpenetal. 2005; can be obtained using spectro-polarimetry (see reviews Karpen,Antiochos&Klimchuk 2006; Karpen&Antiochos by Leroy 1989; Paletou&Aulanier 2003; Paletou 2008; 2008). Furthermore, hedgerow prominences are located LópezAriste&Aulanier 2007). A comprehensive effort to in coronal cavities where the plasma pressure is very low. measure prominence magnetic fields was conducted in the Therefore, it seems unlikely that the verticalthreads seen in 1970’s and early 1980’s using the facilities at Pic du Midi hedgerow prominences can be supported by coronal plasma (France)andSacramentoPeakObservatory(USA).Thiswork pressureonnearlyverticalfieldlines. showedthat (1) the magneticfield in quiescentprominences In this paper we propose that hedgerow prominences are has a strength of 3–15 G; (2) the field is mostly horizon- embeddedinmagneticfieldswithacomplex“tangled”struc- tal and makes an angle of about 40◦ with respect to the ture. Such tangled fields have many dips in the field lines longaxisoftheprominence(Leroy1989;Bommier&Leroy where the weight of the prominence plasma can be coun- 1998; Paletou&Aulanier 2003); (3) the field strength in- teracted by upward magnetic forces. Our purpose is to creasesslightlywithheight,indicatingthepresenceofdipped demonstrate that such tangled fields provide a viable mech- field lines; (4) most prominences have inverse polarity, i.e., anism for prominence support in hedgerow prominences. thecomponentofmagneticfieldperpendiculartothepromi- Casini,MansoSainz&Low(2009) recentlyinvokedtangled nence axis has a direction opposite to that of the potential fieldsintheinterpretationofspectropolarimetricobservations field. Theseearlierdatalikelyincludedavarietyofquiescent of an active region filament. While such filaments are quite prominences, some with predominantly horizontal threads, different from the hedgerow prominences considered here, others with more vertical threads. In more recent work, thisworkshowsthattangledfieldshaveimportanteffectson Paletouetal. (2001) reported full-Stokes observations of a themeasurementofprominencemagneticfields. Sucheffects limbprominenceinHe I 5876Å (He D ), andderivedmag- willnotbeconsideredinthispaper. 3 netic field strengths of 30–45 G, somewhat larger than the Thepaperisorganizedasfollows.InSection2wepropose values reported in earlier studies. Casinietal. (2003) pub- that hedgerow prominences are supported by tangled mag- lishedthefirstvector-fieldmapofaprominencewithaspatial netic fields, and we discuss how such fields may be formed. resolutionof a few arcseconds. Theyfoundthat the average In Section 3 we presenta simple modelfor the dynamicsof magneticfieldinthisprominenceismostlyhorizontalwitha plasmaalongthe tangledfield lines, and we show thatweak strengthofabout20Gandwiththemagneticvectorpointing shock waves naturally occur in such plasmas. In Section 4 20◦ to 30◦ off the prominence axis, consistent with the ear- wedevelopamagnetostaticmodeloftangledfieldsbasedon lier studies. However,the map also showsclearly organized thelinearforce-freefieldapproximation,andinSection5we patches where the magnetic field is significantly stronger study the response of such fields to gravitational forces. In than average, up to 80 G (also see Wiehr&Bianda 2003; Section6wesimulatethedistributionofplasmainacylindri- LópezAriste&Casini 2002, 2003; LópezAriste&Aulanier calprominencethread. InSection7wediscusstheformation 2007;Casini,Bevilacqua&LópezAriste2005). Itisunclear ofverticalthreadsbyRayleigh-Taylorinstability. Theresults howthesepatchesarerelatedtothefinethreadsseenathigher oftheinvestigationaresummarizedanddiscussedinSection spatial resolution. Recently, Merendaetal. (2006) observed 8. He I 10830 Å in a polar crown prominenceabove the limb, 2. TANGLEDFIELDSINPROMINENCES andfoundevidenceforfieldsofabout30Gthatareoriented only25◦fromtheverticaldirection. The spectro-polarimetric observations of prominences de- How is the plasma in hedgerow prominences sup- scribed in Section 1 are consistent with the idea that quiescent prominences are embedded in coronal flux ropes that TANGLEDMAGNETICFIELDSINPROMINENCES 3 We suggest that the tangled field may be formed as a re- sult of magnetic reconnection, not the twisting or stress- ing of field lines. Quiescent prominences are located above polarity inversion “lines” that are often more like wide bands of mixed polarity separating regions with dom- inantly positive and negative polarity. In these mixed- polarity zones, magnetic flux elements move about ran- domlyandoppositepolarityelementsmaycanceleachother (e.g.,Livi,Wang&Martin1985). Newmagneticbipolesfre- quently emerge from below the photosphere. These processes causes the “recycling” of the photosphericflux about (a) (b) once every 2 to 20 hours (Hagenaar,Schrijver&Title 2003; Hagenaar,DeRosa&Schrijver2008),andthecoronalfluxis FIG. 2.— Magnetic support ofsolar prominences: (a) by alarge-scale coronalfluxrope,(b)byatangledmagneticfieldinacurrentsheet. recycled even faster (Closeetal. 2005). It is likely that the interactionsbetweenthesefluxelementsproduceacomplex, lie horizontally above the polarity inversion line (PIL). Fig- non-potentialmagneticfieldinthelowcorona.Withinthisen- ure 2a shows a vertical cross-section through such a flux vironmentmagneticreconnectionislikelytooccurfrequently rope. The magnetic field also has a componentinto the im- atmanydifferentsitesinthecoronaabovetheinversionzone. age plane, so the field lines are helices, and the plasma is Each reconnection event may produce a bundle of twisted assumed to be located at the dips of these helical windings. field,andthetwistedfieldsfromdifferenteventsmaycollect A dip is defined as a point where the magnetic field is lo- intolargerconglomeratestoformatangledfield. Thetangled cally horizontaland curvedupward. As indicatedin the fig- fieldmayrisetolargerheights(asaresultofitsnaturalbuoy- ure,themagneticfieldmaybedeformedbytheweightofthe ancy), and may collect into a thick sheet that is sandwiched prominence plasma, creating V-shaped dips. The magnetic betweensmootherfields,asillustratedinFigure2b. Theob- field near such dips is well described by the Kippenhahn- served prominenceconsists of plasma that is trapped within Schlüter model (Kippenhahn&Schlüter 1957), and several this sheet. New tangled field is continuallyinjected into the authors have developed local magnetostatic models of the sheetfrombelow,producingverticalmotionswithinthesheet. finestructuresobservedinquiescentprominences(e.g.,Low We suggestthatthe “darkplumes”observedbyBergeretal. 1982; Petrie&Low 2005; Heinzel,Anzer&Gunár 2005). (2008)maybeamanifestationofsuchverticalmotionsofthe However,recentobservationsof“darkplumes”(Bergeretal. tangledfield. 2008)androtationalmotions(Chaeetal.2008)withinpromi- 3. FLOWSALONGTHETANGLEDFIELD nencesremindusagainthatprominenceshavecomplexinter- nalmotions,anditisnotclearhowsuchmotionscanbeex- Thespatialdistributionofplasmawithintheprominenceis plainedintermsofasinglelargefluxrope. Perhapsthemag- determinedinpartbythedynamicsofplasmaalongthetan- neticstructureofhedgerowprominencesismorecomplicated gledfield lines. Figure3 showsthe contorted(butgenerally thanthatpredictedbythefluxropemodel(Figure2a). downward)pathofanindividualfieldlineinthetangledfield. In this paper we propose an alternative model, Notethatthereareseveral“dips”wherethefieldlineishori- which is illustrated in Figure 2b. Following zontalandcurvedupward,and“peaks”wherethefieldishor- Kuperus&Tandberg-Hanssen (1967), we suggest that izontaland curveddownward. Tracingthe field line upward hedgerow prominences are formed in current sheets that from a dip, one always reaches a peak where the field line overlie certain sections of the PIL on the quiet Sun. Unlike againturnsdownward.Therefore,thequestionariseswhether those previous authors we suggest that the current sheet theplasmacollectedinthedipswouldremaininthesedipsor extendsonlytolimitedheight( 100Mm),andmayextend besiphonedoutofthedipsviathepeaksofthefieldlines. only a limited distance along∼the PIL. Furthermore, we Toanswerthisquestion,weconsiderasimplemodelforthe proposethattangledmagneticfieldsarepresentwithinthese motion of the prominence plasma along the magnetic field. current sheets. A tangled field is defined as a magnetic For simplicity we assume that the flow takes place in a thin structure in which the field lines are woven into an intricate tubesurroundingtheselectedfieldline(i.e.,thedivergenceof fabric,andindividualfieldlinesfollownearlyrandompaths. neighboringfield lines is neglected), and the cross-sectional We suggest that the field is tangled on a spatial scale of areaofthistubeistakentobeconstant. Weassumeasteady 0.1–1Mm,comparabletothepressurescaleheightH ofthe flow is established along the tube. Let v(s) and ρ(s) be the p prominenceplasma(H 0.2Mm). Theprominenceplasma plasma velocity and density as functionsof position s along p is assumed to be locate≈d at the many dips of the tangled the tube, then conservation of mass requires ρv = constant. field lines. The tangled field is confined horizontallyby the Theequationofmotionoftheplasmais verticalfieldsoneithersideofthesheet,andverticallybythe dv dp dz weightoftheprominenceplasma. ρv =- - ρg , (1) ds ds ds Akeyfeatureofatangledfieldisthattheplasmaandfield where p(s)istheplasmapressure,z(s)istheheightabovethe are in magnetostatic equilibrium, i.e., the Lorentz force is photosphere,andgistheaccelerationofgravity.Theequation balanced by the gas pressure gradients and gravity. There- ofstateiswrittenintheformp=Kργ,whereγandKarecon- fore, a tangled field is quite differentfrom “turbulent”mag- stants(weuse γ <5/3todescribenon-adiabaticprocesses). neticfieldsinwhichlarge-amplitudeAlfvénwavesarepresent Eliminating p(s) and ρ(s) from equation (1), we obtain the (e.g.,thesolar wind). Ina tangledfieldthe magneticpertur- followingequationfortheparallelflowvelocity: bations do not propagatealong the field lines. In this paper weexaminethebasicpropertiesoftangledfields,andwein- c2 dv dz v- =- g , (2) vestigatetheirabilitytosupporttheprominenceplasma. (cid:18) v (cid:19)ds ds 4 VANBALLEGOOIJEN&CRANMER (a) (b) (c) (d) FIG.4.—Modelforplasmaflowalongasinglefieldlineinatangledfield. (a)Thesolidcurveshowstheheightz(s)asfunctionofpositionsalongthe fieldline[inunitsofthedistanceΛbetweenneighboringpeaks,seeequation (3)].Thedashedcurveshowstheoveralldownwardtrendoftheflowtube.(b) Thesolidcurveshowstheparallelflowvelocityv(s)(inunitsofc0)forγ=1.5 and Hp,0 =0.15Λ. The dashed curves show the subsonic and supersonic solutionsofequation(6).Thesonicpointsarelocatedatthepeaksinthefield line(s=0ands=Λ),andtheshockislocatedats=0.325Λ.(c)Similarplot forthesoundspeedc(s)(inunitsofc0).(d)Thesolidcurveshowstheplasma densityρ(s)alongthetube(inarbitraryunits). Thehydrostaticequilibrium densityisshownbythedashedcurve. FIG. 3.—Flowsalongahighlydistortedfieldlineinthetangledmagnetic Thesonicpointswillthenbelocatedats=0ands=Λ.Figure fieldofasolarprominence. Thefieldlineisindicated bythesolidcurve, andtheblack dotsindicate “dips” inthefieldlinewherecoolprominence 4a shows the height z(s) for A=0.15Λ and φ0 =0.2 rad, so plasmacancollect. Thearrows show thedirection ofsubsonic(blue) and thatC=0.187. Let p ,ρ andc bethepressure,densityand 0 0 0 supersonic(red)flows. Sonicpointsarelocated atpeaks inthefieldlines soundspeedatthesonicpoints, thenK =γ- 1c2ρ1- γ, andthe (blackvertical bars), andshockwavesoccurwheresupersonic flowsslow 0 0 downbeforereachingadip(magentabars). soundspeedcanbewrittenas wherec(s)isthesoundspeed(c2 γp/ρ). Theaboveequa- c(s)=c [ρ(s)/ρ ](γ- 1)/2=c [v(s)/c ]- (γ- 1)/2. (5) 0 0 0 0 ≡ tion has a critical point where the flow velocity equals the Insertingthisexpressionintoequation(2),weobtain sound speed (v=c). Therefore, a transition from subsonic to supersonic flow can occur only at points where the RHS du 1 dz of this equationvanishes, dz/ds=0. These sonic points are (u- u- γ) =- , (6) ds γH ds locatedatthepeaksofthefieldlineswheremattercanbesi- p,0 phonedoutofonedipanddepositedintoanotherdipatlower where u(s) v(s)/c , and H p /(ρ g) is the pressure 0 p,0 0 0 ≡ ≡ height. The resulting flow pattern is indicated in Figure 3. scaleheightatthesonicpoints.Equation(6)canbeintegrated Asthesupersonicflowapproachesthenextdip,itmustslow asfollows: downtosubsonicspeeds,whichcanonlyhappeninashock. 1 z(s)- z(s ) Therefore, the tube has a series of subsonic and supersonic 1[u2(s)- 1]+ u1- γ(s)- 1 =- 0 , (7) 2 γ- 1 γH flowsseparatedbyshocksandsonicpoints. Theroleofthese (cid:2) (cid:3) p,0 shocksistodissipatethegravitationalenergythatisreleased wheres isthepositionofasonicpoint. Equation(7)canbe 0 bythefallingmatter. solved for u(s) by Newton-Raphson iteration. When γ =1, Thepositionandstrengthoftheshockscanbecomputedif thereis ananalytic solutionforu(s)in termsof the Lambert theheightz(s)oftheflowtubeisknown. Neighboringpeaks W function(see Cranmer 2004); in the presentpaper we as- are generally not at the same height. Therefore, each sec- sumeγ=1.5. Thesupersonicsolutionv (s)iscomputedwith 1 tion between neighboring peaks is approximated as a large- s =0,andthesubsonicsolutionv (s)iscomputedwiths =Λ. 0 2 0 amplitudesinusoidal perturbationsuperposedon a generally ThedashedcurvesinFigure4bshowv (s)andv (s),andFig- 1 2 downwardpath: ure4cshowsthecorrespondingsoundspeedsc (s)andc (s). 1 2 z(s)≈Acos(cid:16)2πΛs - φ0(cid:17)- Cs, (3) Hathmeerpfleliotwwuediesasogsfiuvtmehneedbfiyealds-clainleehdeisigtohrttiHonps,0.=Th0e.1M5Λa,cheqnuuaml btoerthoef wheresisthepositionalongtheflowtube,Λisthedistance betweenneighboringpeaks(asmeasuredalongtheflowtube), M(s)=v(s)/c(s)=[u(s)](γ+1)/2. (8) Aistheamplitudeoftheperturbationinheight,φ isaphase 0 Theshockislocatedatthepoints wheretheMachnumber angle, and C is the background slope. The phase angle is sh M beforetheshockandtheMachnumberM aftertheshock chosensuchthatthepeaksintheflowtube(wheredz/ds=0) 1 2 arelocatedats=0ands=Λ,thentheslopeisgivenby satisfythefollowingrelationship: C=2πΛAsinφ0. (4) M22= 22γ+M(2γ-- (1γ)M- 112), (9) 1 TANGLEDMAGNETICFIELDSINPROMINENCES 5 whichfollowsfromtheRankine-Hugionotconditionsforpar- Ifgravityandplasmapressuregradientsareneglected,then allelshocks(Landau&Lifshitz 1959). Therefore,the actual F 0, so the magneticfield B(r) must satisfy the force-free ≈ flowvelocityv(s)betweenthetwosonicpointsisgivenbythe condition: fullcurvein Figure4b, andthesoundspeedc(s) isgivenby B=αB, (14) thefullcurveinFigure4c. ∇× Theplasmadensityρ(s) alongtheflow tubeis determined where α(r) may in generalbe a function of position. In the bymassconservation(ρv=constant),andisplottedinFigure specialcase thatα is constantthroughoutthe volume, equa- 4d. Note thatthere is a strongpeak in the density at the dip tion (14) becomes a linear equation for B(r), and the solu- inthefieldline,s 0.56Λ. ThedashedcurveinFigure4d tionsarecalledlinearforcefreefields(LFFF).Woltjer(1958) dip showsthedensitypr≈ofilethatwouldexistiftheplasmawere hasshownthatinaclosedmagneticsystemwithaprescribed inhydrostaticequilibrium(HE): magnetichelicity(H A BdV, whereAisthevectorpo- ≡ · tential) the lowest-energRy state is a LFFF. Therefore, in this ρ (s)=ρ exp - z(s)- z(sdip) , (10) paper only LFFFs are considered, and α is treated as a free HE dip (cid:20) H (cid:21) parameter. We find that LFFFs can be tangled. The typical p lengthscaleofthetangledfieldisgivenbytheinverseofthe whereρdipandHparethedensityandpressurescaleheightat αparameter,ℓ α- 1. Inthefollowingwefirstconsiderthe thedip, H =0.254Λ. Thedeviationsfromhydrostaticequi- ≡| | p casethatℓissmallcomparedtothedomainsizeLinanydi- librium are significant only in those regions where the flow rection, and then consider the boundaryeffects. Section 4.3 velocityiscomparabletothesoundspeed.Wedefineanaver- describestangledfieldsinacylindricaldomain. ageflowvelocityv¯by Λρ(s)v(s)ds Λ 4.1. TangledFieldinaLargeVolume v¯≡ R0 Λρ(s)ds = Λv- 1(s)ds. (11) Intheabsenseofboundaryconditions,thesolutionofequa- 0 0 tion(14)canbewrittenasasuperpositionofplanarmodes: For the case shownRin Figure4 weRfind v¯=0.6c , so the av- 0 N erageflowspeedislessthanthesoundspeed. Therefore,the B(r)= B ê cos(k r+β )- ê sin(k r+β ) , (15) contortedshapeoftheflowtubesignificantlyreducestheflow n 1,n n· n 2,n n· n Xn=1 (cid:2) (cid:3) velocitycomparedtothesupersonicfreefallthatwouldoccur inastraightverticaltube. whereNisthenumberofmodes,k αê isthewavevector n 3,n ≡ The cooler parts of the prominence are thought to have a (n=1, ,N),B isthemodeamplitude,β isaphaseangle, n n temperatureT 104K.Assumingahydrogenionizationfrac- and[ê··,·ê ,ê ]areunitvectorsthataremutuallyorthogo- 1,n 2,n 3,n ∼ tion of 10%, a helium abundance of 10 % and γ =1.5, the nalandformaright-handedbasissystem: soundspeedc 10kms- 1,andwepredictanaverageflow velocityv¯ 0.06≈c 6kms- 1. Theverticalcomponentofthis ê1,n=cosθn(cosφn ˆy+sinφnˆz)- sinθnˆx, (16) 0 velocity is≈v¯ - C≈v¯ - 1.1 km s- 1, less than the observed ê =- sinφ ˆy+cosφ ˆz, (17) z 2,n n n verticalveloci≈tiesinp≈rominencethreads(5–10kms- 1). Note ê =sinθ (cosφ ˆy+sinφ ˆz)+cosθ ˆx. (18) 3,n n n n n thatthepredictedvelocityisrelativetothepatternofthetan- gled field, therefore, if the tangled field expands in the ver- Here θn and φn are the direction angles of the wave vector tical direction it will push the prominence plasma upward. relativetotheCartesianreferenceframe. Wespeculatethattheobservedupwardmotionsinhedgerow Figure5showsanexampleofafieldwithN=100modes, prominences(e.g., Bergeretal. 2008) are due to such large- anisotropicdistributionofdirectionangles(θn,φn),andran- scalechangesinthetangledfield. domly selected phase angles βn. The starting points of the field linesare randomlyselected fromthe centralpartofthe 4. LINEARFORCE-FREEFIELDMODELS box, and the field lines are traced until they reach the box In this section simple models for tangled fields are devel- walls. Note that individual field lines follow random paths, oped.AvolumeV inthecoronaisconsidered,andtheplasma andthatdifferentfieldlinesaretangledtogether. insidethisvolumeisassumedtobeinmagnetostaticequilib- Now consider an ensemble β,N of fields with different E rium,- p+ρg+F=0,wherepistheplasmapressure,ρisthe (randomlydistributed)phaseanglesβn,butwithafixednum- density,∇g isthe accelerationofgravity,andF isthe Lorentz berof modesN, and with fixedmode amplitudesBn and di- force. All quantities are functions of position r within the rection angles (θn,φn). The phase angles are assumed to be volume.TheLorentzforceisgivenby uniformlydistributedintherange[0,2π],andanglesfromdif- ferentmodesnandn′ areassumedto beuncorrelated. Then 1 1 theensembleaverageofthemagneticfieldvanishes, F j B= ( B) B, (12) ≡ c × 4π ∇× × =0, (19) β where j is the electric currentdensity and B is the magnetic field. Usingtensornotation,equation(12)canalsobewritten where < > denote the average over phase angles β . β n ··· as F = ∂T /∂x , where T is the magnetic stress tensor, a Also,theaverageofthetensorBBisgivenby i ij j ij specialcaseofMaxwell’sstresstensor(Jackson1999): N B2 BB = 1 B2 ê ê +ê ê T - δ + i j. (13) β 2 n 1,n 1,n 2,n 2,n ij≡ 8π ij 4π Xn=1 (cid:0) (cid:1) The first term describes magnetic pressure, and the second N termdescribesmagnetictension. Inatangledfieldbothpres- = 1 B2 I- ê ê , (20) 2 n 3,n 3,n sureandtensionforcesareimportant. Xn=1 (cid:0) (cid:1) 6 VANBALLEGOOIJEN&CRANMER latedwiththedirectionangles,and=B2/N,whereB n 0 0 isaconstant.Then=0,sothemeanmagneticfieldvan- ishes. Furtheraveragingofequation(20)showsthat isanisotropictensor: N = 1 I- <ê ê > = 1B2I. (22) 2 n 3,n 3,n 3 0 Xn=1 (cid:0) (cid:1) It follows that =B2, so B equals the r.m.s. value of 0 0 thetotalfieldstrength.Theensembleaverageofthemagnetic stresstensor,equation(13),isgivenby B2 =- 0 δ , (23) ij ij 24π which is also isotropic. Note that the diagonal components of are negative, so the effects of magnetic pressure ij dominateovertheeffectsofmagnetictension. Therefore,the isotropictangledfield hasa positivemagneticpressure, p = t B2/(24π). The averageenergydensity is E =B2/(8π). The 0 t 0 relationship E = 3p is similar to that for a relativistic gas t t (e.g.,Weinberg1972). 4.2. BoundaryEffects The tangled field must be confined within a certain volume (e.g., a current sheet, see Figure 2b), and the confine- FIG.5.—Tangledfieldobtainedbysuperpositionof100randomlyselected ment must be effective for a period much longer than the modesofthelinearforce-freefieldinalargevolume. Alfven travel time across the volume. What are the condi- whereIistheunittensor(I=ˆxˆx+ˆyˆy+ˆzˆz). Expression(20)is tionsforsuchconfinement? Toanswerthisquestionwemust independentofpositionr,sothemagneticfieldisstatistically consider the boundary region between a tangled field and a homogeneous,butisnotnecessarilyisotropic. smooth field. The tangled field is assumed to be character- We study the statistical properties of field lines in models izedbyahighvalueof α, andthesmoothfieldpresumably | | with different values of the mode number N. For each N hasamuchlowervalueof α. Tolastalongtime,themag- | | we construct a series of models (m= 1, ,M) with differ- neticfieldneartheboundarymustbeapproximatelyinequi- entphaseangles,butwithconstantvalues·o·f·themodeampli- librium(non-linearforce-freefield). Theforce-freecondition tudesB anddirectionangles(θ ,φ ). Themodeamplitudes (14)impliesthatαisconstantalongfieldlines,sotherecan- n n n aretakentobethesameforallmodes(B =1).Foreachreal- not be many field lines that pass from the smooth region to n izationmofthephaseanglesβ wetraceoutthefieldlinethat thetangledregion.Therefore,oneimportantconditionforthe n startsattheorigin(x=y=z=0),andwemeasurethesquare survivalofthetangledfieldisthatthetworegionsarenearly oftheradialdistancer2(s)asfunctionofpositionsalongthe disconnectedfromeachothermagnetically. Anotherrequire- m fieldline: mentisthatthetworegionsareapproximatelyinpressurebal- r2(s)=x2(s)+y2(s)+z2(s). (21) ance. m m m m Toshowthattheseconditionscanbesatisfied,wenowcon- WethenaveragethisquantityoverM=100realizationsofthe sider a simple modelforthe boundaryregion. The interface phaseanglestoobtainthemeansquaredistancer2(s).ForN= between the tangled and smooth fields is approximatedby a 3 both mutuallyorthogonaldirections(θn,φn) and randomly planesurface,heretakentobetheplanex=0inCartesianco- chosen directionsare considered. In both cases we find that ordinates.Theabove-mentionedconditiononthelackofcon- the field lines follow long helical paths, and r2(s) increases nectivity between the smooth and the tangled fields requires quadraticallywiths. Therefore,forN=3thefieldlinesdonot B (0,y,z)=0at x=0. Thetangledfield inx 0is assumed x behaverandomly.ForN 4onlyrandomlychosendirections tobeaLFFFwithaspecifiedvalueofα. The≥solutionofthe ≥ are considered. For N = 4 some of the field lines are long LFFF equation is again written as a superposition of planar helices, while others have more random paths, and for N = modes. However,in thepresentcase themodesare grouped 5 all field lines seem random, however, in both cases r2(s) into pairs with closely related wave vectors k and k′, and n n is not well fit by a power law. True random walk behavior withthesameamplitudeB andphaseβ : n n of the field lines, as indicated by a linear dependence of r2 N/2 on distance s, is found only when the number of modes is B(r)= B ê cos(k r+β )- ê sin(k r+β ) n 1,n n n 2,n n n increasedtoN 10. InthelimitoflargeN,r2(s) 10s/α. Xn=1 (cid:2) · · ≥ ≈ | | phWaseenaonwglecsoβnsnibduertaallsaorgtheeremnosedmeabmlepElNituindewshBicnhanndotdoirnelcytitohne ··· - ê′1,ncos(k′n·r+βn)+ê2,nsin(k′n·r+βn) . (24) angles(θn,φn)areallowedtovary.Fromnowon< >will HereN isthetotalnumberofmodes,ê1,n andê2,nared(cid:3)efined denoteanaverageoverthislargerensemble.Thedire··c·tionan- in equations (16) and (17), k′ αê′ is the modified wave glesareassumedtohaveanisotropicdistribution,i.e.,thean- vector,andtheunitvectorsê′n ≡andê3′,n aredefinedby 1,n 3,n gleφ isuniformlydistributedintherange[0,2π],andcosθ isuninformintherange[- 1,+1],sothat= 1. Furn- ê′1,n=- cosθn(cosφnˆy+sinφnˆz)- sinθnˆx, (25) thermore,themodeamplitudesB areassumedtonbeun3corre- ê′ = sinθ (cosφ ˆy+sinφ ˆz)- cosθ ˆx. (26) n 3,n n n n n TANGLEDMAGNETICFIELDSINPROMINENCES 7 Note that ê′ differs from ê only in the sign of 3,n 3,n the x-component, whereas ê′ has the sign of the y- 1,n and z-components reversed. Therefore, the unit vectors [ê′ ,ê ,ê′ ] again form a right-handed basis system. The 1,n 2,n 3,n magneticfieldattheboundary(x=0)isgivenby N/2 B(0,y,z)=2 B cosθ (cosφ ˆy+sinφ ˆz) n n n n Xn=1 cos[αsinθ (ycosφ +zsinφ )+β ], (27) n n n n × whichsatisfiesB (0,y,z)=0. Therefore,itispossibletocon- x struct a tangled field that is disconnected from its surroundings. WenowconsiderthestatisticalaverageofthetensorBBat x=0. Averagingoverphaseanglesβ ,weobtain n N/2 =2 B2cos2θ (cosφ ˆy+sinφ ˆz) β n n n n Xn=1 (cosφ ˆy+sinφ ˆz) atx=0, (28) n n × andfurtheraveragingovermodeamplitudesanddirectionan- glesyields FIG. 6.—Tangledmagneticfieldsobtainedbysuperpositionofmodesof = 16B20(ˆyˆy+ˆzˆz) atx=0. (29) twhiethliinneaarhefdogrceer-ofwreeprfioemldiniennscidee.Tahceylrianddiearl,cwomhipchonreenptreosfetnhtesfiaevlderBtircavlanthisrehaeds atthecylinderwall. Thethreepanelsshowmodelswithdifferentvaluesof Hereweassumeanisotropicdistributionofdirectionangles, a≡|α|R,whereαisthetorsionparameterandRisthecylinderradius: (a) andweuse=B2/N,whereB isther.m.s.fieldstrength a=3.0;(b)a=4.5;(c)a=6.0.Incase(a)thefieldhasonlyasinglemode, n 0 0 butiscases(b)and(c)therearemultiplemodes(N=4andN=6),some intheinteriorofthetangledfield(seeSection4.1). Notethat ofwhicharenon-axisymmetric,resultingintangledfieldlines. Eachpanel at the boundary =B2/3, while in the interior showsthreefieldlines(red,greenandbluecurves).Inpanels(b)and(c)the 0 =B2, so the r.m.s. field strength at the boundary is magentadotsshowdipsinthefieldlines. 0 reduced by a factor 1/√3 compared to that in the interior. 5. DEVIATIONSFROMTHEFORCE-FREECONDITION Themagneticpressureatx=0isgivenby Theabovemodelsfora tangledfield are purelyforce-free B2 B2 and do not have any magnetic forces to support the promi- ext = 0 = p, (30) nence plasma against gravity. To include such effects, we t 8π ≈ 8π 24π nowconsiderthe“elastic”propertiesofthetangledfield,i.e., where pt istheaveragepressureintheinteriorofthetangled itsresponsetoexternalforces. Specifically,theweightofthe field [see equation(23)]. Equation(30) showsthat itis pos- prominencecauses the tangled field to be compressedin the sible to maintain pressure balance between the tangled field verticaldirection,resultinginaradiallyoutwardforceonthe anditssurroundings. plasma. Also, shearing motions may occur within the tangled field as dense plasma moves downward and less dense 4.3. TangledFieldinaCylinder “plumes”moveupward(e.g.,Bergeretal.2008). Thisresults Here an infinitely long cylinder with radius R is consid- insheardeformationofthetangledfieldandassociatedmag- ered. We adopt a cylindrical coordinate system (r,φ,z), and neticstressesthatcounteracttheplasmaflows. Inthefollow- we assume that the radial componentof magnetic field van- ingbothoftheseeffectsareconsideredinsomedetail. ishes at the cylinder wall, B (R,φ,z) = 0. In the Appendix r we analyze the eigenmodesof the LFFF equationin the do- 5.1. CompressionalEffect mainr Rsubjecttotheaboveboundarycondition. Wefind Wefirstconsidertheeffectsofgravityonalayeroftangled ≤ thatthiseigenvalueproblemhasadiscretesetofmodes,and magneticfield.Themagnetostaticequation(- p+ρg+F=0) the number of modes depends on the dimensionless param- cannotbesolvedanalyticallyforatangledfie∇ld,sowemake eter a αR. Figure 6 shows the resulting magnetic fields thefollowingapproximation: ≡| | fora=3.0,4.5and6.0. Inthefirstcaseonlytheaxisymmet- ric(Lundquist)modeispresent,sothefieldlinesarehelical. B′(r) 1B(r)e- z/HB = B(r)- 1 ˆz B(r) e- z/HB. Assumingthecylinderaxisisvertical,therearenodipsinthe ≡∇×(cid:20)α (cid:21) (cid:20) αH × (cid:21) B fieldlines.Ifcoolplasmaweretobeinjectedintosuchastruc- (31) ture, it would spiral down along the field lines and quickly Here B(r) is the LFFF given by equation (15), and H is B reachsupersonicspeeds. Incontrast, fora=4.5 anda=6.0 the magnetic scale height of the modified field (we assume therearemultiplemodesoftheLFFF,andtherandomsuper- αH >2). Note that B′ =0 as required. This modified B positionofthesemodescreatesatangledfieldwithmanydips fi|e|ld B′(r) is no longer∇fo·rce-free,but has the followingsta- where prominence plasma can be supported. The field-line tisticalproperties: dips(i.e.,siteswhereBz=0andB·∇Bz>0)areindicatedby =0, (32) magentadotsinthemiddleandrightpanelsofFigure6. We willreturntothismodelinSection7. = 1B2 (1+ǫ2)(ˆxˆx+ˆyˆy)+ˆzˆz e- 2z/HB, (33) 3 0 (cid:2) (cid:3) 8 VANBALLEGOOIJEN&CRANMER where ǫ (αH )- 1 < 1. Therefore, the magnitude of the that create magnetic stresses in the tangled field. The mag- modified≡fiel|d|drBopsoff2exponentiallywith heightz. Let T′ netic couplingbetweenthe prominenceand its surroundings ij bethemagneticstresstensorofthemodifiedfield. Takingits causes the weight of the dense prominenceto be distributed statisticalaverage,wefindforthenonzerocomponentsofthe overa widerarea. Ineffect, theprominenceplasmaisbeing stresstensor: supportedby the radial gradientof the magnetic pressure of the tangled field over this larger area. In the following we B2 ==- 0 e- 2z/HB, (34) estimatethemagneticstressesandverticaldisplacementsre- xx yy 24π sultingfromtheseforces. B2 The tangled field is modeled either as a vertical slab with =- 240π(1+2ǫ2)e- 2z/HB. (35) half-width R, or as a vertical cylinder with radius R. The prominence is located at the center of this slab or cylinder, Note thatfor ǫ< 1 the stress tensor is nearlyisotropic. The andhasahalf-widthorradiusr ==0, (36) avg≈ 0(cid:16)R(cid:17) B2 whereρ0isthedensityoftheprominence,andweneglectthe = 0 (1+2ǫ2)e- 2z/HB. (37) mass of the surroundings. The exponent n=1 for the slab z 12πH B model or n= 2 for the cylindrical model. As discussed in Notethattheaverageforceactsinthepositivezdirection,i.e., Section 5.1, observationsof hedgerow prominencesindicate themagneticforcecounteractstheforceofgravity. Ineffect, ρ0 2 10- 13gcm- 3(e.g.,Engvold1976),andtoexplainthe ≈ × theplasmaisbeingsupportedbythemagneticpressureofthe observedheightsofsuchprominenceswithB0=10G,were- tangledfield. The tangledfield actslike a hotgasthat hasa quireρavg/ρ0<0.05.Accordingtoequation(39),thisimplies significantpressurebutnomass. Theaveragedensityofthe R/r0 20fortheslabmodel,orR/r0 4.5forthecylindri- ≥ ≥ plasmathatcanbesupportedbythetangledfieldisgivenby calmodel. Theobservedthreadshavewidthsdowntoabout 500km(Engvold1976),whichcorrespondstor 250km. ρ (z)= = B20 (1+2ǫ2)e- 2z/HB, (38) Therefore, the magnetic coupling by the tangle0d≈field must avg g 12πgH extendtoasurroundingdistanceofatleast5Mmfortheslab B model,or1.1Mmforthecylindricalmodel. Moregenerally, wheregistheaccelerationofgravity. equations(38)and(39)yieldthefollowingexpressionforthe ThehorizontalcomponentsofLorentzforce,F′andF′,do x y magneticscaleheightoftangledfield: not vanish for any particular realization of the tangled field, andcannotbewrittenasthegradientsofascalarpressure p. B2 R n H 0 . (40) Thereasonisthatexpression(31)isnotanexactsolutionof B≈ 12πgρ (cid:18)r (cid:19) 0 0 the magnetostatic equilibrium equation. However, equation Therefore, the maximum height of the prominence depends (36) shows that the horizontal forces vanish when averaged stronglyonthemagneticfieldstrengthB . overthefluctuationsoftheisotropictangledfield. Therefore, 0 Accordingto the presentmodel, magneticstress buildsup expression (31) is thoughtto give a good approximationfor inthetangledfieldasaresultofthedifferenceingravitational theeffectsofgravityonthetangledfield. forcesbetweenthethreadanditssurroundings. Canthefield We now applythe abovemodelto the verticalthreadsob- supportsuchshearstress? Toanswerthisquestionweexam- served in hedgerow prominences. To explain the observed ine the effect of vertical displacements on the tangled field. heights of such prominences, we require that the magnetic For simplicity we neglect the mean vertical force given by scale height H is at least 100 Mm. The size ℓ of the mag- B equation (37), and we focus on relative displacements. Let netic tangles is assumed to be in the range 0.1–1 Mm, so r′ be the new position of a fluid parcel originally located at ǫ=ℓ/H 1. For B =10 G and H = 100 Mm, we find B 0 B ρhyavdgr≈og1en0-≪d14engsictmy-n3,which5cor1re0s9pocmnd-s3.toTahnisaviseroangley(atobtoaul)t dpeofsoitrimonedr.fieInldthBe′laitmthiteonfeawppeorfseitcitolnyrc′oinsdguicvteinngbpylasma,the 0.05 times the densHit,ayvgρ≈0 ×2 10- 13 g cm- 3 or nH 1011 B′= Bj ∂x′i, (41) cm- 3 typically observedin≈hed×gerowprominences(E≈ngvold i J ∂x j 1976,1980;Hirayama1986).Thiscomparisonshowsthatthe where B(r) is the originalfield, and J is the Jacobian of the pressureofthetangledfieldinsideaprominencethreadisnot transformation(e.g.,Priest1982). Weassume sufficienttosupporttheweightoftheprominenceplasma.To supportthe plasma with tangled fields, we need to take into x′=x, y′=y, z′=z+h(x,y), (42) account the magnetic coupling between the vertical thread whichyields anditssurroudings. Suchcouplingisneglectedintheabove plane-parallelmodel. B′ =B , B′ =B , B′ =B +B ∂h+B ∂h, (43) x x y y z z x∂x y∂y 5.2. ShearStressEffect where h(x,y) is the verticaldisplacement. The originalfield We now assume that the tangled field pervades not only B(r) is assumed to be a realization of the isotropic tangled theobservedverticalthreadsbutalsotheirlocalsurroundings. fieldgivenbyequation(15). Usingequation(22),weobtain Thedensityinthesurroundingsislessthanthatinthethreads, ′ =0, (44) so the force of gravity is also much lower. This difference in gravitational forces leads to vertical motions (downflows = 1B2 ˆxˆx+ˆyˆy+fˆzˆz+∂hˆxˆz+∂hˆyˆz , (45) in the dense threads, upflows in the tenuous surroundings) 3 0(cid:20) ∂x ∂y (cid:21) TANGLEDMAGNETICFIELDSINPROMINENCES 9 where ∂h 2 ∂h 2 f(x,y) 1+ + . (46) ≡ (cid:18)∂x(cid:19) (cid:18)∂y(cid:19) This yields the following expressions for the off-diagonal componentsofthestresstensor: B2 ∂h B2 ∂h = 0 , = 0 . (47) xz 12π∂x yz 12π∂y TheLorentzforceisgivenbyF′=∂T′/∂x ,andsince i ij j zz isindependentofz,theaverageverticalforceisgivenby B2 ∂2h ∂2h = 0 + =g∆ρ(x,y), (48) z 12π(cid:18)∂x2 ∂y2(cid:19) where∆ρ(x,y)isthedensityperturbation(∆ρ ρ- ρ ).Us- avg ≡ ingthisequation,wecandeterminetheverticaldisplacement h(x,y) for a given density variation ∆ρ(x,y). In the follow- (a) ingsubsectionswesolvetheaboveequationfortheslaband cylindermodels. 5.2.1. SlabModel Wefirstconsideraslabwithinfiniteextentinthe+y,- yand +zdirections. Thecoordinatexperpendiculartotheslabisin therange- R

Tangled Magnetic Fields in Solar Prominences

Similar Tangled Magnetic Fields in Solar Prominences