1988Apj. . .333. .735B the Astrophysical Journal, 333:735-742

1988ApJ. . .333. .735B 1 4-3 luminosity. closer tothefilamentsmight bethecauseofsomuchArch center, thereisconspicuously lessluminosityontheside opposite theArch.Itseems plausiblethatanotheragency loops apparentlyexpandsymmetrically fromthecenterandso ments requiresaspecialviewingangle.Further,althoughtheir meet aheaviermedium.Toobservetheirshocksaslinear fila- should strikethedenseouter mediumonbothsidesofthe luminosity itmustinvokeasetofshockwaveswheretheloops center, inordertoexplainthewell-orderedfilamentsandhigh explains somemorphologicalandenergeticpuzzlescloseto the center throughexpandingmagneticloops.Whilethismodel model ofenergytransferfromtheinnerparsecGalactic collisions. peratures therefurthersuggestthatshockdissipationorsome other unusualprocessdoesthisheating,ratherthangas-dust gas suggeststhattheyareionizedbyexternalsources(Serabyn and Gusten1986).Lowdusttemperaturehighgastem- Gusten, Walmsley,andPauls1981;ArmstrongBarrett km s”)thantheircounterpartsinthegalacticdiskafew peratures of50-100K),andlargerlocallinewidth(20-40 emission seenthroughouttheclouds),warmer(gastem- kiloparsecs fromthecenter(GustenandDownes1980; edges. Thesecloudsaredenser(~10cm,byCSandNH appear tolieneargiantmolecularcloudsandoftenoverlapthe sion showthattheArchfilamentsarethermalemitters.They dynamic modelfortheseobservedbrightelements. toward theGalacticcenter.Thispaperattemptsanelectro- weakly echoedatnegativeGalacticlatitude,suggestingan overall structureconnectingtheendsofstraightfilaments which turnbacktowardGalacticcenter.Thesefeaturesare 1985; Baileyetal.1987).Distributionofionizedandmolecular Arch—thermally emittingfilamentsabovetheGalacticplane Arc, whichhastwoverydifferentsegments:straightnon- thermal filamentsperpendiculartotheGalacticplane,and structures visibleintheradiobands.Mostprominentis 3 The AstrophysicalJournal,333:735-742,1988October15 © 1988.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. Heyvaerts, Norman,andPudnitz(1988)recentlyproposed a Observations ofrecombinationlinesandthermaldustemis- Near Galacticcenterlieseveraltypesofordered,luminous 3 luminosities. Subject headings:galaxies:nuclei—hydromagneticsinterstellar:molecules where high-densityregionsshouldbemostluminous.Electronscanaccelerateintheinducedfieldstorela- trodynamic decelerationofcloudscanpowerfullyincreaseaccretiontowardgalacticcentersandenhancetheir tivistic energies,yieldingtheradioluminosity.Electrodynamicflaresmayoccuronyeartimescales.Suchelec- organize intolarger,long-livedstructures.Ohmiclossesareenergeticallyimportantinthemolecularclouds, resistance inthecircuit.Smallmagneticpinchesformwhicharegenerallykinkunstablebutcan ordered magneticfields,£~10~G.Suchcurrentpathsdrivelow-levelionacousticturbulence,providinga Galactic center.ThevxBinducedfieldcandrivecurrentsoverequivalentcircuits~100pcalongthehighly © American Astronomical Society • Provided by the NASA Astrophysics Data System Molecular cloudsmovingatKeplerianvelocitiescaninducestrongelectricfieldsinionizedregionsnearthe I. INTRODUCTION AN ELECTRODYNAMICMODELOFTHEGALACTICCENTER Received 1987November19;accepted1988April12 University ofCalifornia,Irvine Gregory Benford ABSTRACT -43 -1 the straightfilamentssuggests someorderingprinciple,sinceit plane, whichareseparatedby ~1It-yr.Theregularpatternof each other.Theyresemblethe individualfilamentsnearthe They furthersuggestthatsome filamentsgentlytwistabout with acommonaxisatthe mean positionofthefilaments. constant pitchangleandradiusofcurvature~9pc.They see three distincthelicalsegments crossinginfrontofthefilaments, ments. Yusef-ZadehandMorris(1987)detectapparenthelical from adecreaseinexternalgaspressureawaytheplane, (Seiradakis etal.1985;Tsuboi1986).Thiscouldarise structure windingaboutthesystemoflinearfilamentswith lobes alignedwiththefilamentsandplacedsymmetrically or anadjustmentofpossiblemagneticconfinementthe fila- plane theybecomediffuseandbroad,leadingtopolarized galactic center,alonglociofconstantangularvelocity. index ofnonthermalfilamentaryemissionisflat,implying young, monoenergeticelectrons(Yusef,Morris,andChance plane. Somedepolarizationalongtheirlengthimpliesinterven- (1987) showthatpolarizationpeaksattheoverlapbetween tion ofnonthermalmatteralongourlinesight.Thespectral smoothly luminousfor30pcperpendiculartotheGalactic ation alongthestraightfilaments,whichremaincoherentand particle accelerationthere.Theirworkrevealslinearpolariz- inward towardGalacticcenter. straight filamentsandtheArch,suggestinginteraction propose thattidaldisruptionforcesportionsoftheseclouds straight filaments,especiallysinceYusef-ZadehandMorris tric orbits(Ballyetal1987).SerabynandGusten(1986) mate amilligaussmagneticfieldinthisregion,andperhaps molecular lineemission.Yusef-ZadehandMorris(1987)esti- of galacticrotationathighvelocity,perhapsonhighlyeccen- tivistic electrondensityofatleast4x10cm. ently interactwiththeArch.Acloud40kmsvelocity throughout theentirevolume.Emissionmeasuresgivearela- 1984). Thestraightfilamentscurveslightlyconcavetoward seems toprotrudeintotheArch,judgingfromcontourmapsof As thestraightfilamentsextendawayfromGalactic Two prominentclouds,G0.1+0.08andG0.18—0.04,appar- This unusualconfigurationsuggestsaconnectionwiththe Further, thesegiantcloudsmoveoppositelytothedirection 1988ApJ. . .333. .735B 736 is unlikelythatweareinaprivilegedobservingpositionand This orderingvanishesintheArch.FurtherfromArchand less than0.5pc. are witnessing,forexample,afamilyofshocksseensidewise. thin, nonthermalthreadsextendover30pcwithdiametersof straight filaments,whichtogethercomprisetheArc,isolated, current paths.TheArc,Arch,threads,andfilamentsremind invoking electrodynamicalcouplingandformationoflarge are heatedbycurrentsandshapedbothexternal one powerfullyofplasmadischargesinthelaboratorywhich poloidal fieldwithin50pcofGalacticcenter(Yusef-Zadehand self-generated magneticfields.Giventheevidenceforastrong these orderedfields,illuminatingindividualpathsbysynchro- energy release.(Heyvaerts,Norman,andPudnitz1988). regions nearerthelargemolecularclouds.Forconveniencelet tron emissioninthefilamentsandbythermalradiation Morris 1987),itseemsnaturaltosupposecurrentsrunalong us termallsuchstructures“strands,”implyingthattheymay (Yusef-Zadeh etal1986)suggeststhatrelativisticparticle these morphologiessuggeststrongmagneticorderingofthe effects. have deeper,smallerstructure,butallarewovenbysimilar inferred fromradiorecombinationlinesis~1eV,butioniza- center (Jacobson1982).Theslim“threads”(MorrisandYusef- range oflocaltemperaturesorelsesomeionizingagency, tion insomeregionsrequires10-100eV.Thisimpliesawide acceleration andorderedflowsarecommonnearGalactic intervening molecularclouds.Withthesehints,weattempt naturally explainedbyfieldlineswhichcarrycurrentbutmay Zadeh 1985)speakforextremelylocalizedheatingwhichis filaments thusfarstudiedarealignedalongthefield.Along here adiscussionintermsofanoverallcircuitequationforthe have beenseparatedfromthefilamentfamiliesbynearby with evidenceofhelicalstructureinsomesouthernfilaments, such ascurrentflow.Polarizationmeasurementsshowthatall region. exist, withresistivityrjandelectronpressureP. rents Jflow,electrostaticpotentials0andvectorA regions is where weneglectelectroninertiaandwrite The integrationshereareperformedoveranypathin the plasma. Theleftsideofequation (1)istheappliedvoltageF, capacitive, andinductivevoltage drops.ThequantityEcon- and thetermsonrightside are,respectively,theresistive, tains bothsolenoidalandelectrostatic terms,butasmalldis- which isproportionaltothegradient ofelectronpressure,VP. the capacitivepartinequation (1)canceltheelectrostaticterm, placement ofelectronscarrying negligiblecurrentswillmake e 0 e All theseobservationssuggestamodelfortheentireregion The low-energyjetemanatingfromtheGalacticnucleus There isfurthersuggestiveevidence.Thegastemperature Consider anelectrodynamicplasmaregioninwhichcur- The circuitequationforelectrodynamicallyconnected - j£•d/=i/7à/+JV<^irf/,(1) 0 £=-V4>--—; E=-\P-——' 0e © American Astronomical Society • Provided by the NASA Astrophysics Data System c dtne 1 dA^™vxB II. BASICCIRCUIT BENFORD (2) 2 1/2 -3 2 paths oflengthL(Tiandv9C(T/7¡),withCtheionsound waves givesrisetomomentum-transfercollisionfrequenciesof will permanentlyexistwhereverarelativedrifttakesplace for theenhancedscattering.Extensiveexperiencewithlabor- with phasevelocity>i\.Sinceweknowsolittleofconditions electrostatic turbulence,E,describedbyW=(Ely/4nnT. with theappropriatecollisionfrequencyvsetbylevelof cm. Onlyslightbendingofafilamentinteractingwiththe some linearfilaments.AfieldofB_«1wouldproviderough and largemolecularcloudswithtwicethisspeedappearnear latitudes (BallyandYusef-Zadeh1988)appearatcloudedges. their soft“collisions”withthestrong,orderedmagneticfield. Thus levels withW<£1canpersistandstilldominateoverclassical low levelofelectrostaticturbulence,sinceinasteadystate and Forslund1981).Althoughionheatingcanshutoffthis speed. Ioncyclotronmodeswithv>lOt;*canalsoproduce atory plasmasuggeststhatalowlevelofion-acousticwaves at galacticcenter,weshouldseekabroadlyplausibleagency spedd, weselectthenonlinearscatteringratefromfieldsE B_3 >1.Therewillbeasteadyelectrondriftinthemeanfield “sickle” (Yusef-ZadehandMorris1987)bysuchgasimplies of equation(6), theory (Borisetal1970;Schrijver1973;Papadopoulos1977) We shalltakeaformsuggestedbyextensivesimulation and resistivity, whichforourconditionswouldyieldv«10 co. v ~3x10-2\0~co(Schrijver1973)inconditions Since i;willtypicallybecomparablewiththeionthermal W «10“-10seemsplausiblefromlaboratoryexperiments. eD p eDsc e 3 D e p p D We donotneedoutright,constantinstabilitytoproducea 21/2 v V — =1.5x10nsin^_ W_1, d 2ß-^B-s A v,i ~W^nTi ' 42 © American Astronomical Society • Provided by the NASA Astrophysics Data System 4 v =3x10 D v d — v &œW,(9) p 12 Wn 47raev ’ sin ^cms (10) (11) BENFORD (8) -3 1 -3 5 linear filaments.Thismeans a filamentiscongregationof Note that~10G,whichisnecessaryforstable, scale (perhaps nearlyforce-free)equilibrium. Wecancon- self-pinched elementswhich may cooperatetoformalarger very smallcomparedwiththe 0.3pcobservedwidthofthe shows thatapinchsupported bygaspressurewith7~1is the orderedregionsofilluminated strands.Equation(12) long-lived pinches,asweshall discuss. while themagneticfieldseennearanysmallpinch,B,is still small, The totalresistanceofNpinchesinparallelisR=R/N, and from equation(1)willdoubtlessflowthroughalargenumber total currentflowingis of smallpinchesinparallel.Thedrivingvoltageis With manysuchpinchpathsavailable,thevoltagederived large, asequation(5)yields density isn(0)ev.Usingequation(10)forvyields structure. Thenarrownessofthepinchmeansitsresistanceis discernible onlyiftheycongregate,contributingtoalarger suggests immediatelythatanyself-pincheddischargeswillbe This isaverysmallobjectforastrophysicalconditions,andit Here n(0)istheplasmadensityonaxis,wherecurrent velpiece 1973;Spitzer1962).Thisyields speed andtheplasmapressuretobeconfined(KrallTri- thermal pressureandtheBq/Sticonfiningpressure.Theradius of acylindricalpinchisthenfixedbyspecifyingthenetdrift Galactic center,andsoleaveWanopenparameter. siderations, givenourlevelofignoranceconditionsat eV) ~1,ncm,andtheotherquantitiesareoforderunity. ate withtheAlfvénvelocity,v,andsufferseverepitchangle The firstconditioninsuresthatdriftingelectronscannotreson- scopic lossesaswell.Wecannotplausiblyincludesuchcon- the plasmastrikesasteadystate,fixingW,dependsonmacro- yield highv,self-consistentlyproducingtheresultantW.How The conditionv>lOi^fortheioncyclotroninstabilitythus occurs ifW_0-3.Lowlevelsofturbulence,W~10~,thus speed, canexciteioninstabilitiesinregionswhere7](in scattering. Thesecondimpliesthatv>theionthermal e t D A D D 2 Dti R = So farwehavenoideahow many smallpinchescomprise A self-constrictedpinchachievesabalancebetweeninner 9W1/2 a =2x10(-—-,,)rcm.(12) Pn V =ß-4 jß_ß_ sin 43 N^/nWT ßfp —/. N Qp III. ENERGETICS ev\l 7tn(0) D « 6X1CTVnTG.(16) c l2(T+73 e £ —Y in ipj\a p 18 sin i/j10volt. 10 2 x10amp, 7 5 x10ohm. Vol. 333 (14) (15) (13) 1988ApJ. . .333. .735B 2 31 71/2 8 3 Where B=21/caisthemagnetic fieldatthesurfaceof 1/2 / =j7ta.Forr>a,the“cocoon” region,externalplasmacan flow, r=a,assumingjisindependent ofrfor=G.Wecanthen equation (21)demandsforL=100a*,/a.Stability A plasmaflowinahelicalmagneticfieldisunstabletopin- 0 3 i »5x10 8 r *7x10 e 2 >2naIn(b/a).(23) Z B >2naBJL.(21) e B > d In (L/a)’ B z (22) (20) (19) 739 1988ApJ. . .333. .735B 2 2 2 6 2 easy, nonadiabaticscatteringforfreshelectronstoenterpinch into thepinchesfromotheraccelerationsitesorareproduced configurations. Themeanfreepathtoscatterarelativisticelec- by localprocesses.Thelevelofplasmaturbulenceprovides electrons canenterandilluminatethecompressedpinchcores. where Vfisthesizeofscatteringcenterinunits10 tron ofv/c=/?æ1throughonegyroradiusis(Benford clouds throughthepoloidalfield. The averageBofthesepinchesisproducedbythespatial plasma Debyelengths.Thisshortlengthmeansrelativistic morphology innonthermalfilaments,soperhapslocallythis evidence ofanymorecomplexmorphologythanlinear envisioned currentpath,though,doesnotdisplayanyobvious evolution toforce-freeequilibriaoccurs.Mostofour a largernearlyforce-freeconfigurationintime~L/v average ofthevxBinducedlocallybypassageconducting 740 This suggestselectromagneticattractionbetweencurrents. have nkT>B/Sn. pinches. TheArchfilamentscannotbeforce-freesincethey field Bgivestheconfiguration“backbone”againstdestabi- changing thepitchangleofhelicallyentwinedcurrentscan lizing Lorentzforceontheright-handside.Perturbations kinetic energydestabilizetheforceequilibrium,whileaxial neglecting termsoforder(a/d)1.BothBandthedrift dz/da =cot0,forcebalanceis(Achterberg1987) Consider twopinches,eachcarryingcurrent/,adistanced straight synchrotronfilamentstwistslightlyabouteachother. grow inatime apart, inhelices180°outofphase.Withaveragepitchangle 1986). 1000 yr.Yusef-ZadehandMorris(1987)argueforsomehelical which islimitedbygalacticrotationto~10yr.Observations This mutualtwistingcouldgrowwithinthestrandlifetime 0 runaway heating.Ifonepathheatsandtherebylowersitsresis- of suchformswouldsupportourpicture. 0 could arisefromadifferentinductiveregion,theremaybe no lead tothisinstability,soanetofpathswillpersistevenif they path dominatesthecircuit,eliminatingothers.Detailed tivity, itcandrawcurrentfromnearbypaths.Eventually this A possibility ofsuchinstabilities. are allconnectedtothesamevoltage.Sinceeachfilament treatment showsthatthecollisionrateofequation(9)does not pinches weenvision,andchanges mayoccurmorerapidlythan e written astheproductof free spacecapacitanceofalength analogy, notethatthecapacitance ofequation(5)canbe our instabilityconditionssuggest. Returningtothecircuit so C=dc/4nv.Also,theinductance canbewrittenincgs A 2 The familyofsmall,Bennett-sizedpinchescanorganizeinto Yusef-Zadeh andMorris(1987)observethattwoofthe Networks ofcurrent-carryingpathscanbeunstable to There isnotruelong-termequilibriumforthemagnetic c* =y/4n,andthelow-frequency dielectricconstant,c/vj L9 2 ~d~ 2na © American Astronomical Society • Provided by the NASA Astrophysics Data System ^mnvj) — 3/2 s~ tN$W- 2 tan /?o( sin6) 4n 16n) 2 sin 0=—-cos26, 4 10~ pc, cd (24) BENFORD 2 9-1/2 2 5 -1 -3 circuit isvery“lossy”andmaysuppresssignalsalongit. The smallratioaJLforcesQtolowvalues.Thismeansour units as4L/nc.Thisyieldsameasureofcircuitquality, filaments providesuchasite.Theyshouldbemonitoredto ratio ofplasmaenergytofieldislow.Thewell-ordered will relax,dissipatingenergyinatimedeterminedbyboth intermittent change.Ifconditionsalter,thecircuitasawhole clouds, andofresistivityinsustainedturbulence,arguesfor inductive processesinthemolecularclouds.Whilefieldcon- effective collisionfrequencytov*—eitherfromincreasedturb- global andlocalcharacteristics.Asharpriseinresistivity state, buttheoriginofelectricfieldsinpassingmolecular figurations canalteronlyatspeedv=2x10£_n cm delayed bylight-traveltime)withflaresoralterationsinthe study rapidluminositychanges,perhapscorrelated(although where xisthedistanceoverwhichmagneticenergydis- this occurstheglobaltimeconstantisapproximately ulence, orfromintrusionofdenseplasmaintothecircuit.If series withthecurrent,forexample,couldfollowarisein electrodynamically byohmicallydissipatingitsvelocityv into R 2dv*/(cox),orvLvd(a/x)inwhichcaseatime sipated locally;typicallyx&a.Tworegimesemerge:(a) including filamentsnearSagC(Liszt1985). complex mayexistonlyrelativelybriefly.Perhapsthisis why less thanthedynamicaltimeof~10yr.Thusentire luminosity Lis s ohmicdissipationcanpropagateatnearlylightspeed. we seeonlyafewsuchluminousfeaturesnearGalacticcenter, pinch radiusadoesnotchangethroughout.ForL/d—100 the regiond.Herewehavewrittenv*=gœandassumed Two conditionsarenecessary: enoughionizationtoproduce circuit interpretation. and g=10,n100cmtis20months.Observationof cloud crosses.Probablyneither conditionholdsforcloudsin the induction,andastrong,ordered magneticfieldwhichthe such rapidchangesinthethinfilamentswouldsupporta would beawarmingofclouds attheiredges.Mostordinary the Galaxygenerally,although asignofsomeinteraction of thegloballystoredenergydissipates,givenby the bulkofcircuitenergystoredinductivelydissipatesin A3 c p c p p9 g 2 We haveconsideredtheGalacticcentercircuitassteady Effective dissipationoccurs,asinsolarflares,whenthelocal For comparison,thetimetodecelerateamolecularcloud Why donotallmolecularclouds sufferthisdeceleration? Case bholdswhenvL>v*d(a/x)andonlyasmallfraction p G2 h t = R+(2dv*/ax)’ p Vol. 333 1988ApJ. . .333. .735B 341 2 outflow. Itflowsintothecorotationregionofinnermagne- This currentispresumablythereturnofpolar that aradialcurrentmustflowthereofmagnitude disk asitssource,wenotethattoreverseBintherequires object ofsuchasize. radio sourceemits-2x10ergss“,makingitthebrightest the sizeofobservedradiopointsource(Lo1986).This G atadistanceof20AUfromthehole,whichcorrespondsto black holesandappliedbyRees(1982).ThisimpliesB»100 the generalschemeproposedbyReesetal(1982)formassive the disk,whichshouldbeoforder400dynecm“,following typical valueforB%/Snwillnotexceedthethermalpressureof since presumablytwinjetsflowfrombothpolarregions.A B whichreversessignaswepassaxiallythroughthedisk, disk (Fig.2),asinCamenzind(1986).Weenvisionafield model presentedhere? e center blackhole,ifitexists,figuresstronglyinthecircuit electrodynamically harvestingenergyinblackholesandaccre- tion diskstofuelGalacticjets.Canweexpectthatthe black holeatdynamicalGalacticcenter(GenzelandTownes e finally driveinductivecircuits. they arefirstheatedbycollisions,thenionizedsomewhat,and the Galacticrotationseemtocauseluminousfilaments; partially asaplasma.Thismaybewhycloudsmovingopposite e 1987). Mucheffortofthelastdecadehasexploredmethods requires unusuallevelsofheatingtomakeacloudactatleast cant energylosscanoccur.Thefirstconditionprobably No. 2,1988 value ofsini//inourequationsisverysmallbeforeanysignifi- clouds probablyencounterexternalfieldsthataredisordered over scalesofafewcloudradii,sothemeantime-averaged To envisionthecircuitthatmightoperatewithsuchahot Probably not.Considerasimplecircuitmodelofholeand The discussionsofarconspicuouslyignoredthepossible © American Astronomical Society • Provided by the NASA Astrophysics Data System V. CONCLUSIONS GALACTIC CENTERMODEL 401 18 5 31 9 emission. Itseemsunlikelythattheblackholemagnetosphere, some tensofparsecsaway,playsamajorroleindrivingthe below the10ergss“requiredtofuelmolecularcloud seeming jetcouldeasilybeconfinedbyexternalgaspressure. compares wellwiththeestimateofequation(27).However,this magnetic fieldwithradiusR,iflj«10(Ryi0pc)A,which models. IftheextendedridgeofemissionseenbyYusef-Zadeh et al(1986)issuchajet,itwouldbeself-confinedbyitsown with presentunderstandingofsimpleelectrodynamicdisk estimated 10“Gequipartitionmagneticfield. factor neededtoyieldthe160MHzemissioninjetat (Yusef-Zadeh etal.1986).Ifthisfeatureisindeeddrivenbythe we requirey,«1000.ThisiscomparablewiththeLorentz black holeaccretiondiskthroughelectrodynamicprocesses, energy jetvisibleat160MHzofluminosity~10ergss“ current. (WeassumetheaxialfieldBæingenerating fixed bycomparingwiththeobservationofabroad,low- region forsimplicity.)TheLorentzfactorofthejet,ymaybe Here ajetradiusr,oforder10cmwillcarrytheneeded ze h and luminositywillbe relativistic speeds,throughanareaAj.Theoutflowingcurrent Electric fieldswilldrivethisdensityawayfromthepolesat exist : order tomaintaincorotationaGoldreich-Juliandensitymust tosphere andconnectswiththehighlatituderegion,wherein The mainpointisthattheseexpectedluminositiesarefar These roughestimatessuggestalow-energyjetiscompatible Lj=x 1030ergs1(2?) Gi¿ a);*• h ~ tîgj^cAj nas1410 30111 100 G (lOOg)(i0cm)• B. 9 5 x10cm 17 3 x10A, 741 1988ApJ. . .333. .735B jet mayderiveelectrodynamically,itprobablyemitsmostof 742 .1983,Ap.J.,269,690. checked byobservation,arethefollowing: model forourGalacticcenter.Anysuchpicturemustfinally leaving littletocontributethemoreluminousfeatures100 circuit flowingthroughtheArc.Althoughweak,one-sided Gregory Benford:Department ofPhysics,UniversityCalifornia,Irvine,CA 92717 .1987,inAstrophysicalJetsandTheirEngines,ed.W.Kundt(Boston: .1986,J.PlasmaPhys.,35,165. many ofthematlowluminosity. filaments, althoughsometransverseconfiningfieldsmaybe clouds, luminosityshouldfollowhigh-densityridges. clouds, filamentsshouldbemostcommonlyseenthere.Within paths. Sinceinductivevoltagesoccurattheedgesofmolecular pc away(Uchida,Shibata,andSofue1985). the availableenergyinanaccretiondisk-poweredcircuit, force-free equilibria.Inthisviewthestraightfilamentscorre- into thelargerstructuresobserved,perhapslocallyforming with alargenumberNofsmallpinches.Thesecanorganize tive potentialofthecloudsisunscreenedbyplasma.This disrupted, producingcapacitor-likeregionswheretheinduc- by theinductionofpassingmolecularclouds.Thereshouldbe away fromthemoreactiveregionsbymagnetictension,driven the intense,straightfilaments. helical force-freegeometries. stand orfallonitsinherentplausibilityandpredictions. Armstrong, J.T.,andBarrett,A.H.1985,Ap.Suppl,57,535. Achterberg, A.1987,inAstrophysicalJetsandTheirEngines,ed.W.Kundt region wherevxBinducedelectricfieldsprovidethedynamo total thermalluminosityfromregionsnearmolecularclouds tron radiationaresuchregions. sites betweendiffuse,thermalemissionandordered,synchro- should produceflat-spectrumsynchrotronemission.Probably observable intheouterregionsofapinch. other fromcurrentcoupling,andlocallymayformtypical Jacobson, A.S.1982,inAIPConf. Proc. 83,TheGalacticCenter,ed.G.R. Gusten, R.,Walmsley,CM.,andPauls,T.1982,Astr.Ap.,103,197. Gusten, R.,andDownes,D.1980,Astr.Ap.,87,6. Genzel, R.,andTownes,C.H.1987,Ann.Rev.Astr.Ap.,25,377. Camenzind, M.1986,Astr.Ap.,156,137. Benford, G.1981,Ap.J.,247,792. Bally, J.,andYusef-Zadeh,F.1988,Ap.J.{Letters),inpress. Bally, J.,Stark,A.A.,Wilson,R.W.,andHenkel,C.1987,Ap.J.Suppl,65,13. of thecircuit. spond tomagneticallyorderedpinches,whiletheArchis Heyvaerts, J.,Norman,C,andPudnitz,R.E.1988,Ap.330,718. Felber, F.S.,Hunter,R.O.,Periera,N.R.,andTajima,T.1982,Appl. Phys. Borovsky, J.E.1986,Ap.J.,306,451. Boris, J.P.,Dawson,M,Orens,H.,andRoberts,K.V.1970,Phys. Rev. Krall, N.A.,andTrivelpiece,A.W. 1973, PrinciplesofPlasmaPhysics(New Lo, K.Y.1986,Science,233,1394. Liszt, H.1985,Ap.J.(Letters),293,L85. (Boston: Reidel),p.211. Letters, 41,705. Letters, 25,706. York: McGraw-Hill). Reidel), p.197. Riegler andR.D.Blandford(NewYork: AIP),p.123. 4. Magneticfieldsshouldbepolarizedalongsynchrotron This paperattemptsaqualitativeunderstandingofcircuit Our principalinferencesfromthemodel,whichmaybe The electrodynamicpicturecanaccountfortheobserved 6. Strongaccelerationoccurswheresmoothcurrentflowis 2. Straight,synchrotronfilamentsmaytwistabouteach 3. Flaresduetochangesinresistivitycanlastyearsamong 5. “Threads”arecurrentpathswhichhavebeenpulled 1. Luminosityfromohmicheatingshouldfollowcurrent © American Astronomical Society • Provided by the NASA Astrophysics Data System BENFORD ported bytheAirForceOfficeofScientificResearch. the refereeforusefulsuggestions.Thisworkwaspartlysup- Tsuboi, M.,Inoue,Handa,T.,Tabara,H.,Kato,Sofue,Y.,andKaifu, Rees, M.J.1982,inAIPConf.Proc.83,TheGalacticCenter,ed.G.R.Riegler molecular cloudsthrougharegionnearertheGalacticcenter. Wessel, F.,andRobertson,C.1981,Phys. Fluids,24,739. Stringer, T.E.1964,PlasmaPhys.,6,267. Spitzer, L.,Jr.1962,PhysicsofFullyIonizedGases(NewYork:Interscience). Serabyn, E.,andGusten,R.1986,Astr.Ap.,161,334. Seiradakis, J.H.,Lasenby,A.,Yusef-Zadeh,F.,Wielebinski,R.,andKlein, U. Schrijver, H.1973,Physica,70,358. Schmidt, G.1966,PhysicsofHighTemperaturePlasma(NewYork: Rees, M.J.,Begelman,€.,Blandford,R.D.,andPhinney,E.S. 1982, Peter, W.,andRostoker,N.1982,Phys.Fluids.,25,730. Papadopoulos, K.1977,Rev.Geophys.SpacePhys.,15,113. Morris, M.,andYusef-Zadeh,F.1985,A.J.,90,2511. Mitchell, H.G.,Fedder,J.A.,Huba,D.,andZalesak,S.T.1985,Geophys. F. Wessel,andE.Serabynforstimulatingconversations, Mark Morris.TheauthorthanksM.Morris,R.Blandford, emission depolarizedbyinterveningmatter,asisseenalong plasma initiallypinchesinward,drivinganelectricfieldina current loop.Thiscouldbeaccomplishedinductively,as flows inahaloaroundthefilamentitself,soeachistight current flowmaypassthroughtheplaneandreturnto Yusef-Zadeh, F.,Morris,M.,Slee, O. B.,andNelson,G.J.1986,Ap. Yusef-Zadeh, F.,Morris,M.,andChance, D.1984,Nature,310,557. Yusef-Zadeh, F.,andMorris,M.1987, A.J.,94,1178. Yusef-Zadeh, F.1986,Ph.D.thesis,Columbia University. Uchida, Y.,Shibata,K.,andSofue,Y.1985,Nature,317,699. forming processesfurtherin.Theoriginofthisfieldinourown field whichmustarisefromthediskandcouldaffectjet- contributing tothebroadlineregioninQSOsthroughohmic fueling andalsoincreasesluminosityoftheregion,perhaps though. valid nomatterwhattheactualgeometryofcircuit, paths. Theenergeticswehavesketchedhereshouldremain the linearfilamentsintermittently. return currentswouldbeundetectable,however,andtheir and returnthroughsomenonluminouspath(Fig.1).Diffuse they runalongtheorderedfields,throughGalacticdisk, to thestrongfieldlinesnearmolecularcloudsofArc return path.Thisapparentlymeansthatoncecurrentsattach until theycanbeconductedthrougha(presumablydenser) rents generatedalongastrongmagneticfieldwillcontinue teristic 0.3pcseparationofthefilaments.Oncestarted,cur- surrounding denserplasma.Thismightaccountforthecharac- It ispossiblethatthereturncurrentforeachlinearfilament structure belowtheGalacticplanesuggeststhatordered Galaxy isanoutstandingmystery. heating. Thecrucialingredientisalarge,orderedmagnetic vides astrong,orderedmagneticfield.Thisaidsblackhole aid toaccretionofcloudstowardthecenter,ifdiskpro- nuclei. Generally,electrodynamicbrakingcanbeapowerful Academic). Nature, 295,17. and R.D.Blandford(NewYork:AIP),p.25. Res., 90,11091. (Letters), 300,L47. N. 1986,Astr.Ap.,inpress. 1985, Nature,317,697. Several oftheideasinthisworkcamefromdiscussionswith The modelmayalsoapplytotheinnerregionsofGalactic How thecircuitclosesisnotclear,butobservedfilamentary Further observationsmaybeabletodetailprobablereturn