The$Macrophysics$&$ Microphysics$of$Cosmic$Rays$

Transcription

1 The$Macrophysics$&$ Microphysics$of$Cosmic$Rays$ Ellen$Zweibel$ University$of$Wisconsin=Madison$ Erin%Boe)cher,%Chad%Bustard,%Cory%Co)er,%Elena%D Onghia,% John%Evere),%Jay%Gallagher,%Peng%Oh,%Mateusz%Ruszkowski,% Josh%Wiener,Karen%Yang,%%Tova%YoastFHull% % Supported%by%the%NSF%&%the%UW%

2 Cosmic$Rays$are$Pervasive$

3 Tools+&+Systems+ Direct+probes+ In+situ+par/cle+detectors+ Telescopes+from+radio+to+γ6ray++ ν+detectors+ Indirect+probes+ Astrochemical+modeling+ Geology+ Natural+and+lab+plasmas+ Space+&+solar+ Diffuse+and+HED+

4 Highly$isotropic$&$ resident$for$~10myr$

5 (An)Isotropy! Left: The distribution of cosmic ray arrival directions is highly isotropic, up to the knee. Right: Weak fluctuations at TeV energies have been discovered recently & challenge theory.' Hillas 1984! Abbasi et al. 2010!

6 Cosmic+Ray+Accelera/on+ Blue:+x6rays+ +Supernova Curved+shock+front+ (red),+fluid+flow+in+ frame+of+shock+(black+ arrows),+magne/c+ fieldlines+in+purple.+ NASA+image+

7 Diffusive+Shock+Accelera/on+ Par/cles+are+scaVered+back+and+ forth+across+the+shock+by+ waves+and+turbulence+they+ generate+themselves,+resul/ng+ in+a+power+law+spectrum+that+ depends+on+the+shock++ compression+ra/o.+ Maximum+E+is+set+ by++shock+evolu/on+ &+geometry.+ E "2$ for$strong$ shock$

8 Overarching+Ques/ons+ How+does of+interstellar+par/cles+come+to+ have+as+much+energy+as+the+thermal+pool?+ Observa/onal+probes+ Accelera/on+&+propaga/on+processes+ How+does+this+virtually+collisionless+ component+interact+with+the+thermal+gas+&+ magne/c+field?+ Kine/c+instabili/es+ Fluid+treatments+

9 Extend$to$Other$Galaxies$ Fits$to$the$γ=ray$spectrum$of$M82:$LeO$is$best$fit$to γ=ray$spectrum,$

10 Yoast=Hull$l$et$al.$2015$

11 Milky$Way$Central$Molecular$Zone$

12 $Coupling$to$the$Thermal$Gas$ $$$$$$$$$$$$$$Collisional$ Chemistry$ γ=rays$from$π 0 $decay$&$ inverse$comp@on$ Bremsstrahlung$ Light$element$$produc@on$ $$$$$$$$$$$$Collisionless$ Momentum$&$energy$ exchange$mediated$by$the$ ambient$magne@c$field.$ Fermi,$Parker,$Kulsrud$ The$subject$of$this$talk$

13 Plan$of$This$Talk$ Classical$Cosmic$Ray$Hydrodynamics$ Generalized$Cosmic$Ray$Hydrodynamics$ Beyond$Alfven$Waves$ Going$to$extremes$ Unfinished$business$

14 Steady,$spherically$ symmetric,$pressure$driven$ ouclow$a%la%chevalier$&$ Clegg$1985$but$extended.$ Cory$Cofer$&$Chad$Bustard$ %%%%%%(Bustard,%EZ,%D Onghia%2015)$ $ Rela@vis@c$$fluid$cools$more$ slowly$&$drives$a$faster$ wind.$ $ $$

15 Milky$Way$Wind$

16 Disk$ Yang,%EZ%in%preparaVon.%

17 Perpendicular$Dynamics$are$Easy$ Pressure$gradient$introduced$ through$lorentz$force$

18 $$$$$$Parallel$Dynamics$are$Subtle$

19 Gyroresonant Scattering Orbits$follow fieldlines and short wavelength fluctuations average out. Gyroresonant fluctuations (Doppler shifted frequency kv parallel = ω cr ) scatter in pitch angle cos -1 µ. 5 resonant$ 0 nonresonant$

20 Gyroresonant$Streaming$Instability$ $resonance$$$$$$ damping$ Here$&$elsewhere$I m$interested$in$the$bulk$ cosmic$rays$w$γ$~$1$

21 Fokker$ 0$ Pitch$angle$scafering$(D µµ )$dominates:$$ Scafering$frequency$$$$$$$ν$~$ω c (δb/b) 2 $$ Small angle Scattering by nearly periodic randomly phased waves D pµ $=$D µp $are$order$(v A /c)$ D pp $is$order$(v A /c) 2$

22 integrate$over$momentum$space:$ $ Energy$density$$$$$Energy$flux$$$$$$$$$$$$$Energy$transfer$to$waves$

23 frictional heating

24 From$Dewar s$theory:$

25 Fluid$Treatment$ Classical$Cosmic$Ray$Hydrodynamics$(CCRH)$ on$self=confinement$model$ Stream$down$pressure$gradient$at$v A $rela@ve$to$ thermal$gas$(care%needed%in%implemenvng%this).% Transfer$momentum$through$pressure$gradient$ = Heat$gas$at$ v A# grad#p c.$ = Diffusion$along$B$with$diffusivity$κ v 2 /ν Αpplied$to$shocks,$galac@c$winds,$ISM$hea@ng,$ intracluster$medium.$

26 Include$grad$P c,$advect$with$fluid,$ignore$ stochasvc%on%gyroradius%scale).% No$need$to$ensure$streaming$is$down$grad$P c.$ A $by$thermal$ sound$speed$v S.$ Same$advantages$as$previous$bullet.$ These are the main variants in the literature

27 Generalized$Cosmic$Ray$ Hydrodynamics$(GCRH)$ Account$for$non=cosmic$ray$sources$of$waves.$ MHD$turbulence,$Boldyrev$ group$

28 Composite$streaming$velocity$ Pressure$gradient$force$is$unchanged$

29 cosmic$ray$$ driving/damping$ extrinsic$ driver$

30 Balance$Driving$&$Damping$ Simple model This is easily solved

31 Transport$Velocity$ w#)>#0#when#external#driving#dominates## w#)>#v A #when#cosmic#ray#driving#dominates# # Cosmic#ray#hea9ng#is#reduced#but# compensated#by#turbulent#damping#

32 Extrinsic$Turbulence$Model$ Advect$cosmic$rays$at$ the$fluid$speed$v.$ Neglect$$cosmic$ray$ Retain$pressure$ gradient$force.$ $ $All$hold$in$the$limit$of$ strongly$driven,$balanced$ turbulence.$ But,$Alfven$turbulence$ produces$anisotropic,$ field$aligned$diffusion.$ Isotropic$diffusion$is$ produced$by$a$small$ scale,$stochas@c$field.$

33 Beyond$Alfven$Waves$ $High$β For β=$p G /P M $>>$1$ Affects$waves$which$ scafer$cosmic$rays$with$ µ$>$µ c$ ω~ω ci /β 1/2$ Demands$very$weak$ fields,$e.g$b$<$$10 =12 G$in$ galaxy$clusters.$ Enforces sub-alfvenic streaming

34 Nonresonant Instabilities When U cr /U B > c/v D there is a new, nonresonant instability driven by the electron current that compensates the cosmic ray current. Conditions are met at shocks, and possibly in young galaxies. Evereyy$&$EZ$2010$

35 Diffusive+Shock+Accelera/on+ Par/cles+are+scaVered+back+and+ forth+across+the+shock+by+ waves+and+turbulence+they+ generate+themselves,+resul/ng+ in+a+power+law+spectrum+that+ depends+on+the+shock++ compression+ra/o.+ Maximum+E+is+set+ by++shock+evolu/on+ &+geometry.+ E "2$ for$strong$ shock$

36 Rapid Growth to Nonlinear Amplitude resonant$ 10 3 nonresonant PIC simulation showing magnetic field growth in a shock layer Ω i rad s kr cr Linear growth rates (Zweibel & Everett 2010) Riquelme & Spitkovsky 2010

37 Beyond$Alfven$Speed$ streaming$&$require$extremely$weak$fields.$ ray$fluxes$$ Growth$rates$comparable$to$frequency$require$ nonstandard$treatments$

38 Summary$&$Prospects$ Cosmic$rays$appear$in$diffuse$plasmas$ everywhere,$in$defiance$of$thermodynamics.$ They$exchange$momentum$and$energy$with$ the$background$medium,$mediated$by$ feedback.$