Collisionless Shock Waves Research Articles

Laser plasma simulations of the generation processes of Alfven and collisionless shock waves in space plasma

Generation of Alfven waves propagating along external magnetic field B0 and Collisionless Shock Waves propagating across B0 are studied in experiments with laser- produced plasma and magnetized background plasma. The collisionless interaction of interpenetrating plasma flows takes place through a so-called Magnetic Laminar Mechanism (MLM) or Larmor Coupling. At the edge of diamagnetic cavity LP-ions produce induction electric field Eφ which accelerates BP-ions while LP-ions rotate in opposite direction. The ions movement generates sheared azimuthal magnetic field Bφ which could launches torsional Alfven wave. In previous experiments at KI-1 large scale facility a generation of strong perturbations propagating across B0 with magnetosonic speed has been studied at a moderate value of interaction parameter δ∼0.3. In the present work we report on experiments at conditions of 5∼R2 and large Alfven-Mach number MA∼10 in which strong transverse perturbations traveling at a scale of ∼1 m in background plasma at a density of ∼3*1013 cm-3 is observed. At the same conditions but smaller MA ∼ 2 a generation, the structure and dynamic of Alfven wave with wavelength ∼0.5 m propagating along fields B0∼100÷500 G for a distance of ∼2.5 m is studied.

Critical pitch angle for electron acceleration in a collisionless shock layer

Abstract. Collisionless shock waves in space and astrophysical plasmas can accelerate electrons along the shock layer by an electrostatic potential, and scatter or reflect electrons back to the upstream region by the amplified magnetic field or turbulent fluctuations. The notion of the critical pitch angle is introduced for non-adiabatic electron acceleration by balancing the two timescales under a quasi-perpendicular shock wave geometry in which the upstream magnetic field is nearly perpendicular to the shock layer normal direction. An analytic expression of the critical pitch angle is obtained as a function of the electron velocity parallel to the magnetic field, the ratio of the electron gyro- to plasma frequency, the cross-shock potential, the width of the shock transition layer, and the shock angle (which is the angle between the upstream magnetic field and the shock normal direction). For typical non-relativistic solar system applications, the critical pitch angle is predicted to be about 10°. An efficient acceleration is expected below the critical pitch angle.

Generation of laser plasma bunches with a high efficiency of energy concentration for laboratory simulation of collisionless shock waves in magnetised cosmic plasma

We present the results of first experiments on the formation of collisionless shock waves (CSWs) in background plasma by injecting laser plasma bunches transverse to the magnetic field (as a piston) with a maximum energy up to per unit of solid angle and with a high enough degree of ion magnetisation. With this aim in view, on a unique KI-1 facility at the Institute of Laser Physics, Siberian Branch of the Russian Academy of Sciences (ILP), a plastic (polyethylene) target irradiated by a laser in the most energy-efficient regime (near the plasma formation threshold) and a highly ionised hydrogen plasma with a high concentration in a large volume (not less than ) have been employed. As a result of model experiments performed on the basis of a model of collisionless interaction of plasma flows, developed at the VNIIEF and being adequate to the problem under consideration, not only an intensive, background-induced, deceleration of a super-Alfven laser plasma flow, but also the formation in that flow of a strong perturbation having the properties of a subcritical CSW and propagating transverse to the magnetic field, have been first registered in the laboratory conditions.

Relaxation of heavy ions in collisionless shock waves in cosmic plasma

We report on the results of hybrid particle-in-cell simulation of shock waves (SWs) in the cosmic plasma with admixture of heavy weakly charged ions. The dependence of ion relaxation and the SW structure on the angle between the magnetic field and the normal to the wavefront is analyzed. The conditions for invariability of the anisotropic ion velocity distribution behind the front of quasi-transverse SWs are indicated on scales substantially exceeding the width of the collisionless SW front (up to the Coulomb relaxation length). The obtained results are essential for determining the effectiveness of heating of heavy ions and observation diagnostic of collisionless SWs in the cosmic plasma.

Magnetized Collisionless Shock Waves Measured in the Lab

Scientists create collisionless shock waves to better understand the phenomenon in nature.

Cosmic-ray acceleration at collisionless astrophysical shocks using Monte-Carlo simulations

Context. The di usive shock acceleration mechanism has been widely accepted as the acceleration mechanism for galactic cosmic rays. While self-consistent hybrid simulations have shown how power-law spectra are produced, detailed information on the interplay of di usive particle motion and the turbulent electromagnetic fields responsible for repeated shock crossings are still elusive. Aims. The framework of test-particle theory is applied to investigate the e ect of di usive shock acceleration by inspecting the obtained cosmic-ray energy spectra. The resulting energy spectra can be obtained this way from the particle motion and, depending on the prescribed turbulence model, the influence of stochastic acceleration through plasma waves can be studied. Methods. A numerical Monte-Carlo simulation code is extended to include collisionless shock waves. This allows one to trace the trajectories of test particle while they are being accelerated. In addition, the di usion coe cients can be obtained directly from the particle motion, which allows for a detailed understanding of the acceleration process. Results. The classic result of an energy spectrum with E 2 is only reproduced for parallel shocks, while, for all other cases, the energy spectral index is reduced depending on the shock obliqueness. Qualitatively, this can be explained in terms of the di usion coe cients in the directions that are parallel and perpendicular to the shock front.

Ion-acoustic shocks with reflected ions: modelling and particle-in-cell simulations

Non-relativistic collisionless shock waves are widespread in space and astrophysical plasmas and are known as efficient particle accelerators. However, our understanding of collisionless shocks, including their structure and the mechanisms whereby they accelerate particles, remains incomplete. We present here the results of numerical modelling of an ion-acoustic collisionless shock based on the one-dimensional kinetic approximation for both electrons and ions with a real mass ratio. Special emphasis is paid to the shock-reflected ions as the main driver of shock dissipation. The reflection efficiency, the velocity distribution of reflected particles and the shock electrostatic structure are studied in terms of the shock parameters. Applications to particle acceleration in geophysical and astrophysical shocks are discussed.

Adaptation of the de Hoffmann–Teller frame for quasi-perpendicular collisionless shocks

Abstract. The concept of the de Hoffmann–Teller frame is revisited for a high Mach-number quasi-perpendicular collisionless shock wave. Particle-in-cell simulation shows that the local magnetic field oscillations in the shock layer introduce a residual motional electric field in the de Hoffmann–Teller frame, which is misleading in that one may interpret that electrons were not accelerated but decelerated in the shock layer. We propose the concept of the adaptive de Hoffmann–Teller (AHT) frame in which the residual convective field is canceled by modulating the sliding velocity of the de Hoffmann–Teller frame. The electrostatic potential evaluated by Liouville mapping supports the potential profile obtained by electric field in this adaptive frame, offering a wide variety of applications in shock wave studies.

Dynamics of colliding ultracold plasmas

Formation of a secondary plasma of ${\mathrm{NO}}^{+}$ ions and electrons within an ultracold plasma produces an observable change in the hydrodynamics of the system. Direct photoionization adds energetic electrons, which increases the rate of expansion. The introduction of a secondary Rydberg gas has the opposite effect. In both cases, the added ions create an inertial drag that acts initially to retard the expansion of the electron gas. A cold-ion hydrodynamic shell model, which accounts well for the effect of energy added by photoionization electrons, predicts the formation of collisionless shock waves.

Hybrid simulation of collisionless shock waves in space plasma

A new version of a multiprocessor 3D hybrid code is described, which makes it possible to simulate the formation and evolution of collisionless shock waves in a multicomponent space plasma with ions of different masses and charge states. The algorithm ensures exact conservation of zero magnetic field divergence and self-consistent electric field dynamics and is of the second order of accuracy in time. The peculiarities of the effect of the boundary conditions at the reflecting wall on the formation of quasi-parallel and quasi-perpendicular shock waves are analyzed. It is shown that the rates of collisionless relaxation of ions to quasi-equilibrium distributions for quasi-longitudinal and quasi-transverse shock waves are substantially different.

Collisionless shocks driven by 800 nm laser pulses generate high-energy carbon ions

We present experimental studies on ion acceleration from diamond-like carbon (DLC) foils irradiated by 800 nm, linearly polarized laser pulses with peak intensity of 1.7 × 1019 W/cm2 to 3.5 × 1019 W/cm2 at oblique incidence. Diamond-like carbon foils are heated by the prepulse of a high-contrast laser pulse and expand to form plasmas of near-critical density caused by thermal effect before the arrival of the main pulse. It is demonstrated that carbon ions are accelerated by a collisionless shock wave in slightly overdense plasma excited by forward-moving hot electrons generated by the main pulse.

On the problems of relativistic laboratory astrophysics and fundamental physics with super powerful lasers

The ways toward modeling of astrophysical processes and extreme field regimes with super-power lasers are discussed. The main attention is paid to the problem of limited similarity in using the dimensionless parameters characterizing the processes in the laser and astrophysical plasmas. As the most typical examples, we address the magnetic reconnection and collisionless shock waves relevant to the problem of ultrarelativistic particle acceleration. In the extreme field limits we consider the regimes of dominant radiation reaction, changing the electromagnetic wave-matter interaction. In these regimes it, in particular, results in a new powerful source of ultra high-brightness gamma-rays and will make possible electron-positron pair creation in vacuum in a multiphoton processes. This will allow modeling under terrestrial laboratory conditions the processes in astrophysical objects and paves the way to experimental verifications using ultra intense lasers as they are currently developed within the ELI project.

Nonlinear collisionless damping of Weibel turbulence in relativistic blast waves

The Weibel/filamentation instability is known to play a key role in the physics of weakly magnetized collisionless shock waves. From the point of view of high energy astrophysics, this instability also plays a crucial role because its development in the shock precursor populates the downstream with a small-scale magneto-static turbulence which shapes the acceleration and radiative processes of suprathermal particles. The present work discusses the physics of the dissipation of this Weibel-generated turbulence downstream of relativistic collisionless shock waves. It calculates explicitly the first-order nonlinear terms associated to the diffusive nature of the particle trajectories. These corrections are found to systematically increase the damping rate, assuming that the scattering length remains larger than the coherence length of the magnetic fluctuations. The relevance of such corrections is discussed in a broader astrophysical perspective, in particular regarding the physics of the external relativistic shock wave of a gamma-ray burst.

Quantified energy dissipation rates in the terrestrial bow shock: 1. Analysis techniques and methodology

AbstractWe present a detailed outline and discussion of the analysis techniques used to compare the relevance of different energy dissipation mechanisms at collisionless shock waves. We show that the low‐frequency, quasi‐static fields contribute less to ohmic energy dissipation, (−j·E), than their high‐frequency counterparts. In fact, we found that high‐frequency, large‐amplitude (>100 mV/m and/or >1 nT) waves are ubiquitous in the transition region of collisionless shocks. We quantitatively show that their fields, through wave‐particle interactions, cause enough energy dissipation to regulate the global structure of collisionless shocks. The purpose of this paper, part one of two, is to outline and describe in detail the background, analysis techniques, and theoretical motivation for our new results presented in the companion paper. The companion paper presents the results of our quantitative energy dissipation rate estimates and discusses the implications. Together, the two manuscripts present the first study quantifying the contribution that high‐frequency waves provide, through wave‐particle interactions, to the total energy dissipation budget of collisionless shock waves.

Evolution of a density disturbance in a collisionless plasma

The evolution of a hump-like density disturbance in a collisionless plasma is considered. The effect caused by the initial shape of the disturbance is demonstrated. In the case of cold ions the development of a collisionless shock wave is observed over a long period of time. The formation of separate ion-acoustic solitons, their reflection from a wall and passage through each other are described. Also the ion temperature effect on the flow is studied. The ion reflection and the formation of a two-stream structure are illustrated.

In situ observations of high-Mach number collisionless shocks in space plasmas

Shock waves are widespread in collisionless space plasmas throughout the Universe. How particles are accelerated at these shocks has been the subject of much research attention. The dominant source of the high-energy particles that pervade our Galaxy (cosmic rays) is thought to be the high-Mach number collisionless shocks that form around young supernova remnants, but it is unclear how much the lower Mach number collisionless shock waves frequently encountered by spacecraft in Solar System space plasmas can tell us about particle acceleration in the higher Mach number regime. Here we review recent studies of the shock wave that stands in the solar wind in front of the planet Saturn (Saturn's bow shock), based on Cassini spacecraft observations. This review represents a new direction of shock physics research, with the potential to bridge the gap between Solar System and astrophysical shocks. These studies have confirmed that Saturn's bow shock is one of the strongest shocks in the Solar System, and a recent discovery indicates that electron acceleration at high-Mach numbers may occur irrespective of the upstream magnetic field geometry. This is important because astrophysical shocks can often only be studied remotely via emissions associated with accelerated electrons. We discuss possible future directions of this emerging sub-field of collisionless space plasma shock physics.

Non-adiabatic electron behaviour due to short-scale electric field structures at collisionless shock waves

Abstract. Under sufficiently high electric field gradients, electron behaviour within exactly perpendicular shocks is unstable to the so-called trajectory instability. We extend previous work paying special attention to short-scale, high-amplitude structures as observed within the electric field profile. Via test particle simulations, we show that such structures can cause the electron distribution to heat in a manner that violates conservation of the first adiabatic invariant. This is the case even if the overall shock width is larger than the upstream electron gyroradius. The spatial distance over which these structures occur therefore constitutes a new scale length relevant to the shock heating problem. Furthermore, we find that the spatial location of the short-scale structure is important in determining the total effect of non-adiabatic behaviour – a result that has not been previously noted.

Electron acceleration to relativistic energies at a strong quasi-parallel shock wave

Data from the Cassini spacecraft identify strong electron acceleration as the solar wind approaches the magnetosphere of Saturn. This so-called bow shock unexpectedly occurs even when the magnetic field is roughly parallel to the shock-surface normal. Knowledge of the magnetic dependence of electron acceleration will aid understanding of supernova remnants. Electrons can be accelerated to ultrarelativistic energies at strong (high Mach number) collisionless shock waves that form when stellar debris rapidly expands after a supernova1,2,3. Collisionless shock waves also form in the flow of particles from the Sun (the solar wind), and extensive spacecraft observations have established that electron acceleration at these shocks is effectively absent whenever the upstream magnetic field is roughly parallel to the shock-surface normal (quasi-parallel conditions)4,5,6,7,8. However, it is unclear whether this magnetic dependence of electron acceleration also applies to the far stronger shocks around young supernova remnants, where local magnetic conditions are poorly understood. Here we present Cassini spacecraft observations of an unusually strong solar system shock wave (Saturn’s bow shock) where significant local electron acceleration has been confirmed under quasi-parallel magnetic conditions for the first time, contradicting the established magnetic dependence of electron acceleration at solar system shocks4,5,6,7,8. Furthermore, the acceleration led to electrons at relativistic energies (about megaelectronvolt), comparable to the highest energies ever attributed to shock acceleration in the solar wind4. These observations suggest that at high Mach numbers, such as those of young supernova remnant shocks, quasi-parallel shocks become considerably more effective electron accelerators.

Particle transport and heating in the microturbulent precursor of relativistic shocks

Collisionless relativistic shocks have been the focus of intense theoretical and numerical investigations in recent years. The acceleration of particles, the generation of electromagnetic microturbulence and the building up of a shock front are three interrelated essential ingredients of a relativistic collisionless shock wave. In this paper we investigate two issues of importance in this context: (1) the transport of suprathermal particles in the excited microturbulence upstream of the shock and its consequences regarding particle acceleration; (2) the preheating of incoming background electrons as they cross the shock precursor and experience relativistic oscillations in the microturbulent electric fields. We place emphasis on the importance of the motion of the electromagnetic disturbances relatively to the background plasma and to the shock front. This investigation is carried out for the two major instabilities involved in the precursor of relativistic shocks, the filamentation instability and the oblique two stream instability. Finally, we use our results to discuss the maximal acceleration at the external shock of a gamma-ray burst; we find in particular a maximal synchrotron photon energy of the order of a few GeV.

Synchrotron signature of a relativistic blast wave with decaying microturbulence

Microphysics of weakly magnetized relativistic collisionless shock waves, corroborated by recent high performance numerical simulations, indicate the presence of a microturbulent layer of large magnetic field strength behind the shock front, which must decay beyond some hundreds of skin depths. The present paper discusses the dynamics of such microturbulence, borrowing from these same numerical simulations, and calculates the synchrotron signature of a powerlaw of shock accelerated particles. The decaying microturbulent layer is found to leave distinct signatures in the spectro-temporal evolution of the spectrum $F_\nu \propto t^{-\alpha}\nu^{-\beta}$ of a decelerating blast wave, which are potentially visible in early multi-wavelength follow-up observations of gamma-ray bursts. This paper also discusses the influence of the evolving microturbulence on the acceleration process, with particular emphasis on the maximal energy of synchrotron afterglow photons, which falls in the GeV range for standard gamma-ray burst parameters. Finally, this paper argues that the evolving microturbulence plays a key role in shaping the spectra of recently observed gamma-ray bursts with extended GeV emission, such as GRB090510.