Particle Acceleration In Collisionless Shocks Research Articles

Measurement of Ion Energy Distribution Function of Fast Plasma Flow Driven by Plasma Focus Device Using Retarding Field Energy Analyzer

To understand the mechanism of particle acceleration in collisionless shocks, generating a collisionless plasma flowing through dilute gas is required. We have proposed a compact plasma focus (CPF) device to generate collisionless plasma by using pulsed-power discharge. To discuss a particle acceleration process in collisionless shocks, it is necessary to evaluate an ion energy distribution function (IEDF) of the plasma. In this study, we measured the IEDF of the plasma flow using a retarding field energy analyzer (RFA). The experimental results measured by the RFA showed that ions with a few eV are dominant in the plasma flow generated by the CPF device. The IEDF could be fitted with the shifted Maxwellian distribution. The plasma parameters were estimated to be the ion number density ni = 4.6+−00..28 × 1019 m−3, the ion temperature Ti = 0.8+−00..64 eV, and the drift velocity vd = 11+−03..55 km/s. The estimated velocity approximately agreed with the result of a time-of-flight method calculated by the ion current waveform.

Plasma heating and particle acceleration in collisionless shocks through astrophysical observations

Supernova remnants (SNRs), the products of stellar explosions, are powerful astrophysical laboratories, which allow us to study the physics of collisionless shocks, thanks to their bright electromagnetic emission. Blast wave shocks generated by supernovae (SNe) provide us with an observational window to study extreme conditions, characterized by high Mach (and Alfvénic Mach) numbers, together with powerful nonthermal processes. In collisionless shocks, temperature equilibration between different species may not be reached at the shock front. In this framework, different particle species may be heated at different temperatures (depending on their mass) in the post-shock medium of SNRs. SNRs are also characterized by broadband nonthermal emission stemming from the shock front as a result of nonthermal populations of leptons and hadrons. These particles, known as cosmic rays, are accelerated up to ultrarelativistic energies via diffusive shock acceleration. If SNRs lose a significant fraction of their ram energy to accelerate cosmic rays, the shock dynamics should be altered with respect to the adiabatic case. This shock modification should result in an increase in the total shock compression ratio with respect to the Rankine–Hugoniot value of 4. Here, I show that the combination of x-ray high resolution spectroscopy (to measure ion temperatures) and moderate resolution spectroscopy (for a detailed diagnostic of the post-shock density) can be exploited to study both the heating mechanism and the particle acceleration in collisionless shocks. I report on new results on the temperatures measured for different ion species in the remnant of the SN observed in 1987 in the Large Magellanic Cloud (SN 1987A). I also discuss evidence of shock modification recently obtained in the remnant of SN 1006 a. D., where the shock compression ratio increases significantly as the angle between the shock velocity and the ambient magnetic field is reduced.

Microinstabilities in the Transition Region of Weak Quasi-perpendicular Intracluster Shocks

Abstract Microinstabilities play important roles in both entropy generation and particle acceleration in collisionless shocks. Recent studies have suggested that in the transition region of quasi-perpendicular (Q ⊥) shocks in the high-beta (β = P gas/P B) intracluster medium (ICM), the ion temperature anisotropy due to the reflected-gyrating ions could trigger the Alfvén ion cyclotron (AIC) instability and the ion-mirror instability, while the electron temperature anisotropy induced by magnetic field compression could excite the whistler instability and the electron-mirror instability. Adopting the numerical estimates for ion and electron temperature anisotropies found in the particle-in-cell (PIC) simulations of Q ⊥ shocks with sonic Mach numbers, M s = 2–3, we carry out a linear stability analysis for these microinstabilities. The kinetic properties of the microinstabilities and the ensuing plasma waves on both ion and electron scales are described for wide ranges of parameters, including β and the ion-to-electron mass ratio. In addition, the nonlinear evolution of the induced plasma waves are examined by performing 2D PIC simulations with periodic boundary conditions. We find that for β ≈ 20–100, the AIC instability could induce ion-scale waves and generate shock surface ripples in supercritical shocks above the AIC critical Mach number, M AIC * ≈ 2.3 . Also, electron-scale waves are generated primarily by the whistler instability in these high-β shocks. The resulting multiscale waves from electron to ion scales are thought to be essential in the electron injection to diffusive shock acceleration in Q ⊥ shocks in the ICM.

Ion acceleration at two collisionless shocks in a multicomponent plasma.

Intense laser-plasma interactions are an essential tool for the laboratory study of ion acceleration at a collisionless shock. With two-dimensional particle-in-cell calculations of a multicomponent plasma we observe two electrostatic collisionless shocks at two distinct longitudinal positions when driven with a linearly polarized laser at normalized laser vector potential a_{0} that exceeds 10. Moreover, these shocks, associated with protons and carbon ions, show a power-law dependence on a_{0} and accelerate ions to different velocities in an expanding upstream with higher flux than in a single-component hydrogen or carbon plasma. This results from an electrostatic ion two-stream instability caused by differences in the charge-to-mass ratio of different ions. Particle acceleration in collisionless shocks in multicomponent plasma are ubiquitous in space and astrophysics, and these calculations identify the possibility for studying these complex processes in the laboratory.

Monte Carlo modelling of particle acceleration in collisionless shocks with effective mean electric field

Relativistic particle acceleration in collisionless shocks of supernova remnants is accompanied with magnetic field amplification by cosmic ray (CR) driven plasma instabilities. Bell’s fast CR-current instability is predicted to produce turbulence with a non-zero mean electric field in the shock precursor. We present a Monte Carlo model of Fermi shock acceleration explicitly taking into account an effective mean upstream electric field. Our model is nonlinear and includes the backreaction effects of efficient Fermi acceleration on the shock structure.

Three criteria for particle acceleration in collisionless shocks

AbstractCollisionless shocks have been the subject on many studies in recent years, due to their ability to accelerate particles. In order to do so, a shock must fulfill three criteria. First, it must be strong enough to accelerate particles efficiently. Second, both the upstream and the downstream must be collisionless. Third, the shock front must be surrounded by electromagnetic turbulence capable of scattering particles back and forth. We here consider the encounter of two identical plasma shells with initial density, temperature, and velocity n0, T0, v0, respectively. We translate the three criteria to the corresponding requirements on these parameters. A non-trivial map of the allowed region for particle acceleration emerges in the (n0, T0, v0) phase space, especially at low velocities or high densities. We first assess the case of pair plasma shells, before we turn to electrons/protons.

Conditions for the onset of the current filamentation instability in the laboratory

The current filamentation instability (CFI) is capable of generating strong magnetic fields relevant to the explanation of radiation processes in astrophysical objects and leads to the onset of particle acceleration in collisionless shocks. Probing such extreme scenarios in the laboratory is still an open challenge. In this work, we investigate the possibility of using neutral$e^{-}~e^{+}$beams to explore the CFI with realistic parameters, by performing two-dimensional particle-in-cell simulations. We show that CFI can occur unless the rate at which the beam expands due to finite beam emittance is larger than the CFI growth rate and as long as the role of the competing electrostatic two-stream instability (TSI) is negligible. We also show that the longitudinal energy spread, typical of plasma-based accelerated electron–positron fireball beams, plays a minor role in the growth of CFI in these scenarios.

COLLISIONLESS SHOCKS IN A PARTIALLY IONIZED MEDIUM. III. EFFICIENT COSMIC RAY ACCELERATION

In this paper we present the first formulation of the theory of non-linear particle acceleration in collisionless shocks in the presence of neutral hydrogen in the acceleration region. The dynamical reaction of the accelerated particles, the magnetic field amplification and the magnetic dynamical effects on the shock are also included. The main new aspect consists however in accounting for charge exchange and ionization of neutral hydrogen, which profoundly change the structure of the shock, as discussed in our previous work. This important dynamical effect of neutrals is mainly associated to the so-called neutral return flux, namely the return of hot neutrals from the downstream region to the upstream, where they deposit energy and momentum through charge exchange and ionization. We also present the self-consistent calculation of Balmer line emission from the shock region and discuss how to use measurements of the anomalous width of the different components of the Balmer line to infer the cosmic ray acceleration efficiency in supernova remnants showing Balmer emission: the broad Balmer line, which is due to charge exchange of hydrogen atoms with hot ions downstream of the shock, is shown to become narrower as a result of the energy drainage into cosmic rays, while the narrow Balmer line, due to charge exchange in the cosmic-ray--induced precursor, is shown to become broader. In addition to these two well-known components, the neutral return flux leads to the formation of a third component with intermediate width: this too contains information on ongoing processes at the shock.

Evidence for mild deviation from power-law distribution of electrons in relativistic shocks: GRB 090902B

Many previous studies have determined that the long lasting emission at X-ray, optical and radio wavelengths from gamma-ray bursts (GRBs), called the afterglow, is likely produced by the external forward shock model. In this model, the GRB jet interacts with the circum-stellar medium and drives a shock that heats the medium, which radiates via synchrotron emission. In this work, we carried out a detailed analysis of the late time afterglow data of GRB 090902B using a very careful accounting of the Inverse Compton losses. We find that in the context of the external forward shock model, the only viable option to explain the X-ray and optical data of GRB 090920B is to have the electron energy distribution deviate from a power-law shape and exhibit some slight curvature immediately downstream of the shock front (we explored other models that rely on a single power-law assumption, but they all fail to explain the observations). We find the fraction of the energy of shocked plasma in magnetic field to be ~10^{-6} using late time afterglow data, which is consistent with the value obtained using early gamma-ray data. Studies like the present one might be able to provide a link between GRB afterglow modeling and numerical simulations of particle acceleration in collisionless shocks. We also provide detailed calculations for the early (< 10^3 s) high energy (> 100 MeV) emission and confirm that it is consistent with origin in the external forward shock. We investigated the possibility that the ~10 keV excess observed in the spectrum during the prompt phase also has its origin in the external shock and found the answer to be negative.

Cluster Merger Shock Constraints on Particle Acceleration and Nonthermal Pressure in the Intracluster Medium

X-ray observations of galaxy cluster merger shocks can be used to constrain nonthermal processes in the intracluster medium (ICM). The presence of nonthermal pressure components in the ICM, as well as the shock acceleration of particles and their escape, all affect shock jump conditions in distinct ways. Therefore, these processes can be constrained using X-ray surface brightness and temperature maps of merger shock fronts. Here we use these observations to place constraints on particle acceleration efficiency in intermediate Mach number (M ~ 2-3) shocks and explore the potential to constrain the contribution of nonthermal components (e.g., cosmic rays, magnetic field, and turbulence) to ICM pressure in cluster outskirts. We model the hydrodynamic jump conditions in merger shocks discovered in the galaxy clusters A520 (M ~ 2) and 1E 0657-56 (M ~ 3) using a multifluid model comprised of a thermal plasma, a nonthermal plasma, and a magnetic field. Based on the published X-ray spectroscopic data alone, we find that the fractional contribution of cosmic rays accelerated in these shocks is lower than about 10% of the shock downstream pressure. Current observations do not constrain the fractional contribution of nonthermal components to the pressure of the undisturbed shock upstream. Future X-ray observations, however, have the potential to either detect particle acceleration in these shocks through its effect on the shock dynamics, or to place a lower limit on the nonthermal pressure contributions in the undisturbed ICM. We briefly discuss implications for models of particle acceleration in collisionless shocks and the estimates of galaxy cluster masses derived from X-ray and Sunyaev-Zel'dovich effect observations.

Gamma-ray bursts and collisionless shocks**Invited review presented at the 33rd annual European Physical Society Conference, Rome, 2006.

Particle acceleration in collisionless shocks is believed to be responsible for the production of cosmic-rays over a wide range of energies, from a few GeV to > 1020 eV, as well as for the non-thermal emission of radiation from a wide variety of high energy astrophysical sources. A theory of collisionless shocks based on first principles does not, however, exist. Observations of γ-ray burst (GRB) ‘afterglows’ provide a unique opportunity for diagnosing the physics of relativistic collisionless shocks. Most GRBs are believed to be associated with explosions of massive stars. Their ‘afterglows’, delayed low energy emission following the prompt burst of γ-rays, are well accounted for by a model in which afterglow radiation is due to synchrotron emission of electrons accelerated in relativistic collisionless shock waves driven by the explosion into the surrounding plasma. Within the framework of this model, some striking characteristics of collisionless relativistic shocks are implied. These include the generation of downstream magnetic fields with energy density exceeding that of the upstream field by ∼8 orders of magnitude, the survival of this strong field at distances ∼1010 skin-depths downstream of the shock and the acceleration of particles to a power-law energy spectrum, d log n/d logε ≈ −2, possibly extending to 1020 eV. I review in this talk the phenomenological considerations, based on which these characteristics are inferred, and the challenges posed to our current models of particle acceleration and magnetic field generation in collisionless shocks. Some recent theoretical results derived based on the assumption of a self-similar shock structure are briefly discussed.

The Efficiency of Electron Acceleration in Collisionless Shocks and Gamma‐Ray Burst Energetics

Afterglow observations are commonly used to determine the parameters of GRB explosions, the energy E, surrounding density n, postshock magnetic field equipartition fraction B, and electron equipartition fraction e, under the frequently made assumption that the efficiency of electron injection into relativistic shock acceleration is high, i.e., that the fraction f of electrons that undergo acceleration is f ≈ 1. We show that the value of f cannot be determined by current observations, since currently testable model predictions for a parameter choice {E' = E/f, n' = n/f, = fB, = fe} are independent of the value of f for me/mp ≤ f ≤ 1. Current observations imply that the efficiency f is similar for highly relativistic and subrelativistic shocks and plausibly suggest that f ~ 1, quite unlike the situation in the Crab Nebula. However, values me/mp ≤ f 1 cannot be ruled out, implying a factor me/mp uncertainty in determination of model parameters. We show that early, ≤10 hr, radio afterglow observations, which will be far more accessible in the Swift era, may provide constraints on f. Such observations will therefore provide a powerful diagnostic of GRB explosions and of the physics of particle acceleration in collisionless shocks.

Particle acceleration in collisionless shocks - Regulated injection and high efficiency

Particle acceleration in supernova remnants and radio galaxies proceeds with high efficiency, and at the same time puts out most of the cosmic ray power at moderately relativistic energies. This suggests that there is a regulated injection mechanism that selects only a very small fraction of particles out of the thermal pool, but enough that, when accelerated to moderately relativistic energies, these suprathermal particles contain a significant fraction of the total available energy. It is shown that particle acceleration in shocks has precisely this property because the high-energy particles, by mediating the shock, regulate the acceleration rate of the low-energy ones. A bifurcation in the energy distribution among the particles results; a distinct population of cosmic rays is consistently created out of an initially cold plasma in such a way that the cosmic rays contain more than half the energy in the shocked fluid. Given this model for injection, it becomes possible to study preferential acceleration in detail, and it is shown that partially ionized heavy elements are preferentially accelerated.