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Proton Injection & Acceleration at Weak Quasi-parallel ICM shock
Β Hyesung Kang (Pusan National University) Dongsu Ryu, Ji-Hoon Ha (UNIST, Korea) SN1006 Supernova remnant shock Solar wind bow shock Radio Relic shocks Galaxy clusters
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ICM: IntraCluster Medium
πΈ β₯ : πΈ β₯ : 2-3 20-30 Previous studies on collisionless shocks focus mainly on low b (<1) plasma (e.g. solar wind & ISM). Physics of weak shocks in b=100 ICM could be quite different (e.g. various microinstabilities).
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: Requirement for shock crossing
DSA: Fermi first order process at πΈ β₯ shocks U2 U1 Shock front particle downstream upstream B mean field : Requirement for shock crossing Kinetic plasma processes are crucial for pre-acceleration/injection problem Focus of this talk
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Particle Energy Spectrum: Injection Problem
Maxwellian DSA power-law spectrum Kinetic processes PIC/Hybrid simulations π πππ
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Q: Can weak ICM shocks accelerate CR protons/electrons ?
Shocks in Structure Formation Simulations (Ryu et al 2003) Shocks in ICM Weak internal shocks with Ms<3 are dominant and energetically important inside hot ICM. Cosmic-rays are accelerated via Fermi-I just like Earthβs bow shock & supernova remnants Q: Can weak ICM shocks accelerate CR protons/electrons ? 25h-1 Mpc Quasi-parallel ( πΈ β₯ ) shocks in the ICM ο CR protons ο collision with ICM ο p0 decay g rays (not detected) Quasi-perpendicular ( πΈ β₯ ) shocks in the ICM ο CR electronsο radio synchrotron emiss (observed) Ji-Hoonβs talk Accretion Shocks ICM= Intracluster Medium (π= K)
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subcritical supercritical Low π΄ π¨ πΈ β₯ shocks 1D hybrid simulations
π΄ π¨ = 1.5 : shock is steady & smooth. π΄ π¨ > 2.8 : shock is unsteady & undergoes self-reformation. Subcritical weak shocks: shock transition is smooth, lacking an overshoot. Ion reflection is inefficient, maybe no particle acceleration supercritical 1994
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Earth Bow shock: Q-perp shock
-relative drift between reflected ions and incoming particles excites various waves via micro-instabilities in the shock foot. upstream Shock criticality & ion reflection are crucial for the shock structure & particle acceleration. Collisionless shock is not a simple MHD jump. Dissipation is mediated by collective EM interactions instead of inter-particle collisions. ο¨ complex kinetic processes are involved, e.g. microinstabilities.
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Two ways to reflect protons/electrons at the shock
(1) magnetic mirror reflection due to compressed magnetic fields B n QBn shock mirror force due to gradient of B dominant at πΈ β₯ shock Caprioli & Spitkovsky (2) Shock potential barrier: -decelerates ions but accelerates electrons -ions are reflected by overshoot dominant at πΈ β₯ shock Both magnetic field compression & shock potential drop depends on π΄ π ο Reflection fraction increases with increasing π΄ π
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Kinetic Plasma Simulations: PIC (Particle In Cell)
-can follow kinetic plasma processes: e.g. wave-particle interactions -provide the most complete pictures, but very expensive / = Large mass ratio ο large time scale ratio
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1D/2D PIC simulations for πΈ β₯ shocks (proton injection to DSA)
Ha et al. 2018 downstream upstream B in x-y plane Simulation frame = downstream rest frame
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First critical Mach number : π π β β2.3
Time evolution of shock structure: Stack plot Supercritical subcritical -Time-varying overshoot in eF & B -cyclic reformation of the shock -No overshoot, smooth transition First critical Mach number : π π β β2.3
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Ms=3.2 supercritical with a beam of reflected ions
Ms=2.0 subcritical without reflected ions Overshoot ~ 200
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Time Evolution of energy spectrum Injection to DSA
DSA power-law spectrum extends to higher energy in time. Obliquity dependence Early stage of DSA in Q-par shocks Only SDA (no DSA) in Q-perp shocks q=63 q13
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Mach number dependence
Injection momentum Injection fraction Ms~2.25 π πππ = 2 π πππ
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SUMMARY 1. internal shocks driven by mergers and chaotic flows: π π β€3, 2. In high b ICM, only supercritical πΈ β₯ shocks with π΄ π β₯π.π may inject suprathermal protons to DSA and accelerate CR protons. So most of ICM shocks do not accelerate CR protons. 3. The injection fraction, x(t), decreases with time. Long-term evolution of x(t) can be studied with other methods. Key words: Shock criticality, π΄ π β βπ.π, Ion Reflection, Ion Injection to Fermi-I process, Weak ICM Shock
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πΈ β₯ πΈ β₯ Key elements for proton injection to DSA at πΈ β₯ shocks
(1) Reflection at the shock & energy gain near the front via SDA (2) Backstreaming of ions upstream along B0 ο¨relative drift between reflected ions & incoming particles: free E source (3) Self-excitation of upstream waves: e.g. whistlers or Alfven waves ο¨ Scattering leads to Injection to Fermi I acceleration πΈ β₯ πΈ β₯ (3) self-excitation of waves (2) (1) after 1/2 gyro-orbit, advect downstream Scattering by upstream waves Backstreaming upstream
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Particle Acceleration Processes operating at collisionless shocks
Diffusive Shock Acceleration (DSA): Fermi 1st order process - effective at quasi-parallel ( πΈ β₯ ) shocks - scattering off MHD waves in the upstream and downstream region (2) Shock Drift Acceleration (SDA) - effective at quasi-perpendicular ( πΈ β₯ ) shocks - drifting along the convective E field (grad B) at the shock front (3) Shock Surfing Acceleration (SSA) - reflected by shock potential, scattered by upstream waves - moving along the convective E field, while being trapped at the shock foot (4) Turbulent Acceleration: Fermi 2nd order process, stochastic acceleration - much less efficient than DSA - could be important only in turbulent plasma (5) Magnetic Reconnection There are three main acceleration processes operating at collisionless shocks.
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Shock Drift Acceleration (SDA) at πΈ β₯ shocks
downstream Electrons drift anti-parallel to E field. upstream shock surface in x-y plane upstream Trajectory of an electron upstream From Guo, Sironi, & Narayan 2014 SDA drift along - z -perpendicular component of B is compressed at shock -motional Electric field is induced -grad(B) drift of electron in -z direction (anti-parallel to E) protons in +z direction (parallel to E) in the shock transition layer ο gain energy pre-acceleration of protons/electrons ο multi-cycles of SDA ο injection to DSA reflected
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CR proton acceleration efficiency from Hybrid simulations
Caprioli & Spitkovsky 2014 πΈ β₯ πΈ β₯ No proton acceleration at πΈ β₯ shocks High beta cases π½ π ~36 Caprioli (KAW9)
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according to hybrid simulations, b~1
2016 Fermi LAT upper limits on gamma-ray flux from individual clusters To be compatible with Fermi upper limits, the CR proton acceleration efficiency should be h <0.1% for 2<M<5. according to hybrid simulations, b~1 Different DSA efficiency models
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Signatures of shocks in ICM: X-ray shocks in merging clusters
The Bullet Cluster Markevitch 2006 Shimwell et al. 2015 Cluster A665 Dasadia et al. 2016 shock Structure formation shocks have been detected by temperature or surface brightness discontinuities in X-ray observations, as shown in this Bullet cluster and also in the cluster A665. These are among the strongest X-ray shocks detected in the ICM, with the sonic Mach number ranging from 2.5 to 3.
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Radio relics: diffuse radio sources in merging clusters
1RXS J PLCKG shock Sausage relic Shock Mach numbers estimated from spectral index, based on DSA model, π΄ π ~πβπ ZwCl0008.8 Radio relics are diffuse radio source found in the outskirts of merging clusters. They are interpreted as synchrotron emissionβ¦. The shock Mach number can be estimated from the radio spectral index, based on diffusive shock acceleration model.
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Key Physical Processes in ICM: shocks, turbulence, magnetic fields,
& particle accelerationο¨ nonthermal radiation Gravitational energy X-ray shocks: observed supersonic flows Heat Shock Quasi-perpendicular shocks: CR electrons (Radio relics): observed Relativistic particles Quasi-parallel shocks: CR protons pp collision ο p0 decay g rays (not detected) DSA (Fermi I) Ji-Hoonβs talk Heat turbulence decay Vorticity Turbulence turbulent acceleration (Fermi II) turbulence dynamo MHD/Plasma waves CR 2ndary electrons (Radio halos) Magnetic fields CR protons ο¨ secondary ptls merging clusters dissipation scale
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Hybrid simulation: Proton acceleration at shocks
Caprioli & Sptikovsky 2014 upstream downstream At Q-par shocks: stream of accelerated ions into upstream self-generated waves B amplification B n At Q-perp shocks: No backstreaming ions into upstream No turbulent waves n B
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DSA beyond injection ? Amplitude of the DSA power-law should decrease in time. πΎ πππ₯ ~1 The injection fraction, x(t), decreases with time. Long-term evolution of x(t) can be studied with other methods such as hybrid simulations.
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Ion reflection at super-critical shocks:
Edmiston & Kennel (1984) High beta plasma (IPM or ICM) Low beta plasma (solar flare) For ICM shocks with b~50,
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