Cosmic Rays and Particle Acceleration

1 Cosmic rays and Particle AccelerationNick MurphyHarvard-Smithsonian Center for AstrophysicsAstronomy 253: Plasma AstrophysicsApril 14 16, 2014 These lecture notes are based off of P. Carlson (2012), Zweibel (2013), Kulsrud, and slides by P. Blasi andothers. The derivations for first and second order Fermi Acceleration followHigh Energy AstrophysicsbyLongair (3rd ed). This book contains several very nicely written chapters on relativistic Particle dynamics, Cosmic rays , and Particle and importance of Cosmic raysIDetecting Cosmic raysIProperties of Cosmic raysISecond order Fermi accelerationIFirst order Fermi Acceleration (diffusive shock Acceleration )IUltra-high energy Cosmic raysIntroductionICosmic rays (CRs) are highly energetic particles that mostlyoriginate outside of our solar systemICosmic rays are in rough energy equipartition with ourgalaxy s magnetic field, the Cosmic microwave background,and starlightIThe most fundamental questions of Cosmic ray research are:IWhat are the source regions of Cosmic rays ?

2 IHow are Cosmic rays accelerated?IHow do Cosmic rays propagate in the galaxy?IParticle astrophysicists have made progress on all of thesequestions but much work remainsIGalactic and extragalactic Cosmic rays are not the onlyenergetic particles that we care about: there are also solarenergetic particles (SEPs)!History of Cosmic ray researchIResearch question in early 1900s: what is the source ofatmospheric ionization?IExperiments by Coulomb in 1785 showed that a chargedmetallic sphere left alone in air gradually loses its chargeICosmic rays were first discovered by Victor Hess in balloonexperiments in 1912 IThe amount of atmospheric ionizationincreasedwith heightIOrigin of atmospheric ionization is extraterrestrial!ISea voyages by Clay (1927) and Millikan (1932) showed adependence of Cosmic ray flux on latitudeIModulation by Earth s magnetic fieldIEffect more pronounced for lower energy Cosmic raysICosmic rays are (mostly) charged particles !

3 IMore CRs came from the west than the east (late 1920s to1930s)IMost CRs are positively charged! Cosmic ray research led to the discoveries of several newparticlesIBefore Particle accelerators, CRs were the best way to studyhigh energy Particle physics!ICloud chamber observations of CRs by Anderson led to thediscovery of the positron (1932) and the muon (1936)IPions were discovered in CR observations in 1947 IThe first CR antiproton was discovered in 1979 IThese are secondaries resulting from collisions of primarycosmic rays with other nucleiWhat effects do Cosmic rays have on astrophysicalplasmas?IVertical support of ISM in galaxiesIIonization and heating of ISM through collisionsIAllows weak ionization in molecular clouds, protoplanetarydisks, and the cold neutral mediumIImpacts chemistry in the ISM and molecular cloudsILow energy CRs (.)

4 10 MeV) are most responsible for thisIProbable driver of galactic windsIAmplification of magnetic fieldsIUpstream and downstream of shocksICosmic ray driven dynamoIModification of astrophysical plasma processesIShocks, dynamos, jets, reconnectionWhy should humans care about Cosmic rays ?IContribution to atmospheric chemistryIProduction of14 Cin the atmosphere through the neutroncapture reactionn+14N 14C+p(1)IHistorical spikes of14 Cproduction ( , 774 775 CE) possiblydue to Cosmic ray eventsISource of background radiationIAirline pilots have a higher risk of cataracts due to CRsICRs and SEPs are a health hazard for interplanetary spacetravel (in particular, outside of Earth s magnetosphere)IImpact on Earth and space based electronicsICosmic rays cause bit flips ( , error on Voyager 2)IIncreases with altitude in atmosphereIError correction schemes can mitigate these effectsWhy should Particle physicists care about Cosmic rays ?

5 IThe maximum energy of Cosmic rays is of order 1020eVIOrders of magnitude greater energy than accessible by theLarge Hadron Collider (LHC) at CERN!IPossible to investigate Particle physics at otherwise inaccessibleenergiesIHigher energy Cosmic rays induce Particle showers inatmosphereIDifficulties:ILack of experimental controlILack of cross-checks with experiment at these energiesIRarity of most energetic eventsPrimary Cosmic rays collide with molecules in theatmosphere and yield secondary particles in an air showerICommon products: pions, electrons, positrons, neutrons,kaons, muons, and neutrinosIParticles produce Cherenkov radiation that is detectableParticle profile: pionsIPions are mesons: they consist of a quark and antiquark +:u d 0:u uord d :d uICharged pions have a mass of MeV/c2. The mostcommon decay routes are + ++ (2) + (3)with a characteristic decay time of 26 nsINeutral pions have a mass of MeV/c2and usually decayinto two photons with a characteristic time of 10 17s: 0 2 (4)IGamma rays from pion decay are detectable byFermiParticle profile: muonsIMuons ( , +) are leptons with a mass of MeV/c2(about 200 times the mass of an electron)IMost naturally occuring muons are Cosmic ray secondariesresulting from pion decayIMuons have a mean lifetime of sIThat s really long!

6 ITime dilation allows muons to propagate longer distances inour frame before decayingIThe most common decay routes are e + e+ (5) + e++ e+ (6)How are Cosmic rays observed?ISynchrotron radiationIGamma rays (pion decay bump)ICloud chambers and bubble chambersIImportant historically; mostly obsoleteIMagnetic spectrometersIHigh precision measurements of bending of Cosmic ray pathsby an applied magnetic fieldIUseful at relatively low energiesICherenkov radiationIParticles traveling faster than the speed of light in a mediumAlpha Magnetic SpectrometerIA magnetic spectrometer1on theInternational Space StationIAbove atmosphere so it measures primary Cosmic raypopulationIMeasuring electron-to-positron ratioIProvides limits on annihilation of dark matter particlesIRequired a special act of Congress to get it launched!1 The first rule of tautology club is the first rule of tautology Auger ObservatoryIObserves air showers of secondary Cosmic ray particlesIRate of ultra-high energy Cosmic rays is 1 km 2century 1 IEffective area is 3000 km2 IFluorescence detectors to observe air showers in atmosphereICherenkov detectors to detect secondary particles on groundICombination of both methods reduces systematic errors thatresulted during prior workIA key result: anisotropy of UHECRs (more later)What are the properties of Cosmic rays ?

7 IEnergy spectrumIAbundancesIConfinement/lifetime sIIsotropy/anisotropyCosmic ray research is a great way to get practice withrarely used SI prefixes!Energy spectrum of Cosmic raysEnergy spectrum of Cosmic raysIKneearound 3 1015eVIAnklearound a few 1018eVEnergy spectrum of Cosmic raysIFlux of lowest energy Cosmic rays is uncertain due toheliospheric modulationISolar energetic particles present at lower energies ( keV to 100 MeV)IFrom 109to 1015eV the power law index is part of the spectrum contains most of the Cosmic rayenergy densityIAbove the knee the index steepens to spectrum starts to flatten again around the ankle at afew 1018eVWhy do the knee and ankle exist in the Cosmic ray powerspectrum?IChanges in the spectral index may indicate a transition in theacceleration or confinement mechanism and/or compositionIThe knee is postulated to be an upper limit associated withthe Acceleration mechanism for galactic Cosmic raysICan diffusive shock Acceleration in supernova remnants reachthis energy?

8 IDoes the knee have anything to do with Particle transport orescape?ICosmic rays above the ankle are extragalactic in originIGyroradius is comparable to size of galaxyINeither the knee nor the ankle is well-understoodIQuestion: how could we determine if Cosmic rays between theknee and ankle are galactic or extragalactic?Abundances of Cosmic raysAbundances of Cosmic raysIRelative abundances are more or less representative ofinterstellar plasmasIHandHeare somewhat underabundantI 89% are protonsI 10% are particlesIOverabundance of3He,Li,Be, andB(by 5 7 orders ofmagnitude at 1 GeV)I1 2% are electronsISteeper slope because of radiative lossesIVery low density of Cosmic rays maintaining quasineutralityis not a problemIVery small fraction of CRs are positrons or antiprotonsINo antihelium nuclei yet discovered in Cosmic raysAre Coulomb collisions significant?

9 IRecall: Coulomb collisions occur when a Particle interactswith another Particle via electric fieldsIThe Coulomb cross section of a 1 GeV Particle is 10 30cm2:tiny!IFor 1 GeV Cosmic rays propagating in the ISM (n 1 cm 3),the mean Coulomb collision rate isn v 10 1(7)This corresponds to a 1% chance of a Coulomb collisionduring a Hubble time!ICoulomb collisions can therefore be neglected completelySpallation reactions provide limits on Cosmic ray lifetimesISpecies such as3He,Li,Be, andBare very overabundant ingalactic Cosmic raysISpallation occurs whenC,N,O,Fenuclei impact interstellarhydrogenIThe larger nuclei are broken up into smaller nucleiIThe abundance enhancements of the lighter elements showthat spallation reactions are somewhat importantIMust have passed through&3 g cm 2of the ISMIH eavier elements such asFeare only mildly depletedICannot have passed through more than 5 g cm 2of materialIThis corresponds to a Cosmic ray lifetime of 3 Myr in the disk(or significantly longer in the halo)IRadioactive isotopes provide an additional diagnosticIExample.

10 10 Behas a half-life of MyrCosmic rays are effectively confined if the Larmor radius ismuch smaller than the system sizeIRelativistic momentum:p= mvIThe relativistic Larmor radius isrL=p |q|B= mvZeB(8)This can be written for a proton asrL=pc/eV300(B/G)cm(9)How effectively are Cosmic rays confined in differentsystems?ICalculate the Particle energy associated with a gyroradiusequal to the characteristic length scaleIHeliosphere:L 100 AU,B 10 G EL 4 1012eVILow energy Cosmic rays are modulated by heliosphericmagnetic fieldsIISM:L 100 pc,B 5 G EL 4 1017eVIThe highest energy Cosmic rays are barely deflected by themagnetic field of the Milky WayIsotropy of Cosmic rays below the kneeIBelow 1014eV, Cosmic rays are highly isotropicICosmic rays propagate along field lines in galaxyIPropagation and confinement make it difficult to determinesource regionsIScattering and diffusion play an important roleIWeak anisotropy (above) is not well understoodWhat properties of Cosmic rays must an accelerationmechanism explain?