Electron Acceleration at a Low Mach Number Perpendicular Collisionless Shock

Umeda, Takayuki; Yamao, Masahiro; Yamazaki, Ryo

doi:10.1088/0004-637x/695/1/574

Cited by 38 publications

(53 citation statements)

References 42 publications

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“…The dependence of this mechanism on different shock parameters needs to be clarified, and in fact, in this work we find that this mechanism is not very efficient at realistic mass ratios. Also, using twodimensional PIC simulations Umeda et al (2009) showed that, for M A = 5 shocks, pre-acceleration in Buneman waves can be complemented by further energization due to scattering at the shock ripples, supporting the picture laid out by Burgess (2006). The energy spectrum of the electrons in this case, however, does not correspond to a power law tail, but is rather described by two Maxwellian distributions at different temperatures.…”

Section: Introductionsupporting

confidence: 65%

“…PIC simulations have been used by several authors to study the electron acceleration problem in nonrelativistic shocks (e.g., Amano & Hoshino 2007; Umeda et al 2009). For instance, using two-dimensional PIC simulations, Amano & Hoshino (2009) showed that efficient electron acceleration (with spectral index α = 2-2.5) can happen in a perpendicular, M A = 14 shock due to "shock surfing" of electrons on electrostatic waves from Buneman instability excited at the leading edge of the shock foot.…”

Section: Introductionmentioning

confidence: 99%

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Electron injection by whistler waves in non-relativistic shocks

Riquelme

Spitkovsky

2012

AIP Conference Proceedings

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Electron acceleration to non-thermal, ultra-relativistic energies (∼ 10−100 TeV) is revealed by radio and X-ray observations of shocks in young supernova remnants (SNRs). The diffusive shock acceleration (DSA) mechanism is usually invoked to explain this acceleration, but the way in which electrons are initially energized or 'injected' into this acceleration process starting from thermal energies is an unresolved problem. In this paper we study the initial acceleration of electrons in non-relativistic shocks from first principles, using two-and three-dimensional particle-in-cell (PIC) plasma simulations. We systematically explore the space of shock parameters (the Alfvénic Mach number, M A , the shock velocity, v sh , the angle between the upstream magnetic field and the shock normal, θ Bn , and the ion to electron mass ratio, m i /m e ). We find that significant non-thermal acceleration occurs due to the growth of oblique whistler waves in the foot of quasi-perpendicular shocks. This acceleration strongly depends on using fairly large numerical mass ratios, m i /m e , which may explain why it had not been observed in previous PIC simulations of this problem. The obtained electron energy distributions show power law tails with spectral indices up to α ∼ 3 − 4. The maximum energies of the accelerated particles are consistent with the electron Larmor radii being comparable to that of the ions, indicating potential injection into the subsequent DSA process. This injection mechanism, however, requires the shock waves to have fairly low Alfénic Mach numbers, M A 20, which is consistent with the theoretical conditions for the growth of whistler waves in the shock foot (M A (m i /m e ) 1/2 ). Thus, if the whistler mechanism is the only robust electron injection process at work in SNR shocks, then SNRs that display non-thermal emission must have significantly amplified upstream magnetic fields. Such field amplification is likely achieved by the escaping cosmic rays, so electron and proton acceleration in SNR shocks must be interconnected.

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Section: Introductionsupporting

confidence: 65%

Section: Introductionmentioning

confidence: 99%

Electron injection by whistler waves in non-relativistic shocks

Riquelme

Spitkovsky

2012

AIP Conference Proceedings

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“…These regions then provide stronger Buneman instability which should lead to more efficient localized electron heating and acceleration (see Umeda et al 2009 for similar effects in low-Mach-number shocks). Due to the poorly resolved foot structure at the reverse shock, a detailed analysis of this possible connection between shock ripples and electron energization must be deferred to future work.…”

Section: Structure Of the Reverse Shockmentioning

confidence: 99%

“…For panel (b), we compensated the average velocity of all ions (see Figure 10), whereas for panel (c) on the right we compensated only the average motion of incoming ions. Umeda et al 2009Umeda et al , 2010Umeda et al , 2014. The ion temperature anisotropy arising from ion reflection at the shock can drive the Alfvén ion cyclotron (AIC) or the mirror instability in the shock ramp, and the resulting unstable modes have wavelengths of a few ion skin depths and propagate along the regular magnetic field, significantly contributing to ion isotropization and thermalization at the shock and downstream.…”

Section: Structure Of the Reverse Shockmentioning

confidence: 99%

Nonrelativistic Perpendicular Shocks Modeling Young Supernova Remnants: Nonstationary Dynamics and Particle Acceleration at Forward and Reverse Shocks

Wieland

Pohl

Niemiec

et al. 2016

ApJ

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For parameters that are applicable to the conditions at young supernova remnants, we present results of twodimensional, three-vector (2D3V)particle-in-cell simulations of a non-relativistic plasma shock with a large-scale perpendicular magnetic field inclined at a  45 angle to the simulation plane to approximate three-dimensional (3D) physics. We developed an improved clean setup that uses the collision of two plasma slabs with different densities and velocities, leading to the development of two distinctive shocks and a contact discontinuity. The shock formation is mediated by Weibel-type filamentation instabilities that generate magnetic turbulence. Cyclic reformation is observed in both shocks with similar period, for which we note global variations due to shock rippling and local variations arising from turbulent current filaments. The shock rippling occurs on spatial and temporal scales produced by the gyro-motions of shock-reflected ions. The drift motion of electrons and ions is not a gradient drift, but is commensurate withÉ B drift. We observe a stable supra-thermal tail in the ion spectra, but no electron acceleration because the amplitude of the Buneman modes in the shock foot is insufficient for trapping relativistic electrons. We see no evidence of turbulent reconnection. A comparison with other twodimensional (2D) simulation results suggests that the plasma beta and the ion-to-electron mass ratio are not decisive for efficient electron acceleration, but the pre-acceleration efficacy might be reduced with respect to the 2D results once 3D effects are fully accounted for. Other microphysical factors may also play a part in limiting the amplitude of the Buneman waves or preventing the return of electrons to the foot region.

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“…A detailed initial setup for 2-D simulations of (quasi-)perpendicular shocks was described in our previous studies (Umeda et al, 2008(Umeda et al, , 2009(Umeda et al, , 2010(Umeda et al, , 2011(Umeda et al, , 2012a(Umeda et al, , b, 2014 and is not repeated here.…”

Section: Simulation Setupmentioning

confidence: 99%

Periodic self-reformation of rippled perpendicular collisionless shocks in two dimensions

Umeda

Daicho

2018

Ann. Geophys.

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Abstract. Large-scale two-dimensional (2-D) full particlein-cell (PIC) simulations are carried out for studying periodic self-reformation of a supercritical collisionless perpendicular shock with an Alfvén-Mach number M A ∼ 6.Previous self-consistent one-dimensional (1-D) hybrid and full PIC simulations have demonstrated that the periodic reflection of upstream ions at the shock front is responsible for the formation and vanishing of the shock-foot region on a timescale of the local ion cyclotron period, which was defined as the reformation of (quasi-)perpendicular shocks.The present 2-D full PIC simulations with different ionto-electron mass ratios show that the dynamics at the shock front is strongly modified by large-amplitude ion-scale fluctuations at the shock overshoot, which are known as ripples.In the run with a small mass ratio, the simultaneous enhancement of the shock magnetic field and the reflected ions take place quasi-periodically, which is identified as the reformation. In the runs with large mass ratios, the simultaneous enhancement of the shock magnetic field and the reflected ions occur randomly in time, and the shock magnetic field is enhanced on a timescale much shorter than the ion cyclotron period.These results indicate a coupling between the shock-front ripples and electromagnetic microinstabilities in the foot region in the runs with large mass ratios.

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Electron Acceleration at a Low Mach Number Perpendicular Collisionless Shock

Cited by 38 publications

References 42 publications

Electron injection by whistler waves in non-relativistic shocks

Electron injection by whistler waves in non-relativistic shocks

Nonrelativistic Perpendicular Shocks Modeling Young Supernova Remnants: Nonstationary Dynamics and Particle Acceleration at Forward and Reverse Shocks

Periodic self-reformation of rippled perpendicular collisionless shocks in two dimensions

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