Shock Precursor Research Articles

Polycyclic Aromatic Hydrocarbon Emission in the Central Regions of Three Seyferts and the Implication for Underlying Feedback Mechanisms

We analyze JWST Mid-Infrared Instrument/Medium Resolution Spectrograph integral field unit observations of three Seyferts from the Galactic Activity, Torus, and Outflow Survey (GATOS) and showcase the intriguing polycyclic aromatic hydrocarbon (PAH) and emission-line characteristics in regions of ∼500 pc scales over or around their active galactic nuclei (AGN). Combing the measurements and model predictions, we find that the central regions containing a high fraction of neutral PAHs with small sizes, e.g., those in ESO137-G034, are in highly heated environments, due to collisional shock heating, with hard and moderately intense radiation fields. Such environments are proposed to result in inhibited growth or preferential erosion of PAHs, decreasing their average size and overall abundance. We additionally find that the central regions containing a high fraction of ionized PAHs with large sizes, e.g., those in MCG-05-23-016, are likely experiencing severe photoionization because of the radiative effects from the radiative shock precursor besides the AGN. The severe photoionization can contribute to the ionization and further destruction of PAHs. Overall, different Seyferts, even different regions in the same galaxy, e.g., those in NGC 3081, can contain PAH populations of different properties. Specifically, Seyferts that exhibit similar PAH characteristics to ESO137-G034 and MCG-05-23-016 also tend to have similar emission-line properties to them, suggesting that the explanations for PAH characteristics of ESO137-G034 and MCG-05-23-016 may also apply generally. These results have promising application in the era of JWST, especially in diagnosing different (i.e., radiative and kinetic) AGN feedback modes.

Electrostatic Waves and Electron Holes in Simulations of Low-Mach Quasi-perpendicular Shocks

Collisionless low-Mach-number shocks are abundant in astrophysical and space plasma environments, exhibiting complex wave activity and wave–particle interactions. In this paper, we present 2D Particle-in-Cell (PIC) simulations of quasi-perpendicular nonrelativistic (v sh ≈ (5500–22000) km s−1) low-Mach-number shocks, with a specific focus on studying electrostatic waves in the shock ramp and precursor regions. In these shocks, an ion-scale oblique whistler wave creates a configuration with two hot counterstreaming electron beams, which drive unstable electron acoustic waves (EAWs) that can turn into electrostatic solitary waves (ESWs) at the late stage of their evolution. By conducting simulations with periodic boundaries, we show that the EAW properties agree with linear dispersion analysis. The characteristics of ESWs in shock simulations, including their wavelength and amplitude, depend on the shock velocity. When extrapolated to shocks with realistic velocities (v sh ≈ 300 km s−1), the ESW wavelength is reduced to one-tenth of the electron skin depth and the ESW amplitude is anticipated to surpass that of the quasi-static electric field by more than a factor of 100. These theoretical predictions may explain a discrepancy, between PIC and satellite measurements, in the relative amplitude of high- and low-frequency electric field fluctuations.

Ion–Electron Instabilities in the Precursors of Weakly Magnetized, Transrelativistic Shocks

Collisionless shocks are known to induce turbulence via upstream ion reflection within the precursor region. This study elucidates the properties of electrostatic and electromagnetic instabilities, exploring their role over the transrelativistic range. Notably, the growth of oblique Buneman waves and previously overlooked, secondary, small-scale ion–electron instabilities is observed to be particularly promoted in the relativistic regime. Furthermore, the growth of large-scale electromagnetic modes encounters severe limitations in any cold, baryon-loaded precursor. It is argued that electron preheating in electrostatic modes may alleviate these constraints, facilitating the subsequent growth of large-scale filamentation modes in the precursors of transrelativistic shocks.

Weak shock compaction on granular salt

This study conducted integrated experiments and computational modeling to investigate the speeds of a developing shock within granular salt and analyzed the effect of various impact velocities up to 245 m/s. Experiments were conducted on table salt utilizing a novel setup with a considerable bore length for the sample, enabling visualization of a moving shock wave. Experimental analysis using particle image velocimetry enabled the characterization of shock velocity and particle velocity histories. Mesoscale simulations further enabled advanced analysis of the shock wave’s substructure. In simulations, the shock front’s precursor was shown to have a heterogeneous nature, which is usually modeled as uniform in continuum analyses. The presence of force chains results in a spread out of the shock precursor over a greater ramp distance. With increasing impact velocity, the shock front thickness reduces, and the precursor of the shock front becomes less heterogeneous. Furthermore, mesoscale modeling suggests the formation of force chains behind the shock front, even under the conditions of weak shock. This study presents novel mesoscale simulation results on salt corroborated with data from experiments, thereby characterizing the compaction front speeds in the weak shock regime.

Return Currents in Collisionless Shocks

Collisionless shocks tend to send charged particles into the upstream, driving electric currents through the plasma. Using kinetic particle-in-cell simulations, we investigate how the background thermal plasma neutralizes such currents in the upstream of quasi-parallel non-relativistic electron–proton shocks. We observe distinct processes in different regions: the far upstream, the shock precursor, and the shock foot. In the far upstream, the current is carried by nonthermal protons, which drive electrostatic modes and produce suprathermal electrons that move toward upstream infinity. Closer to the shock (in the precursor), both the current density and the momentum flux of the beam increase, which leads to electromagnetic streaming instabilities that contribute to the thermalization of suprathermal electrons. At the shock foot, these electrons are exposed to shock-reflected protons, resulting in a two-stream type instability. We analyze these processes and the resulting heating through particle tracking and controlled simulations. In particular, we show that the instability at the shock foot can make the effective thermal speed of electrons comparable to the drift speed of the reflected protons. These findings are important for understanding both the magnetic field amplification and the processes that may lead to the injection of suprathermal electrons into diffusive shock acceleration.

Dynamical Coupling between Anomalous Cosmic Rays and Solar Wind in Outer Heliosphere

Voyager 2 (V2) observed that pickup ions (PUIs) and anomalous cosmic rays (ACRs) have a significant influence on the solar wind structures in the outer heliosphere. In particular, the maxima in energetic particle intensities often lagged behind the shock front, while the flow velocity in some cases featured a precursor in front of the shock. These two effects are believed to be caused by the cease of ACR injection from PUIs at large heliocentric distances, and the backward diffusion of ACRs, respectively. This paper investigates the dynamical coupling between the ACRs and the solar wind in the outer heliosphere by means of a time-dependent numerical MHD simulation, in which the ACRs are treated as a massless fluid that only contributes its pressure to the system. Two types of inner boundary conditions were used, namely a synthetic short-term shock event and an extended period of data-driven solar wind variability based on the OMNI database. The lag of the ACR pressure maximum relative to the shock front, and the extended shock precursor were reproduced by the numerical results. The increase rate of the lag is related to the corresponding diffusion coefficient and the injection efficiency from PUIs to ACRs. The model was also applied to the termination shock, where simulations likewise showed that the peak in the ACR distribution can be located a short distance downstream of the shock front, indicating that the time-dependent diffusive shock acceleration mechanism is a candidate to interpret the lag between the ACR pressure peak and the shock front observed by V2.

Magnetic Field Amplification by a Plasma Cavitation Instability in Relativistic Shock Precursors

Abstract Plasma streaming instabilities play an important role in magnetic field amplification and particle acceleration in relativistic shocks and their environments. However, in the far shock precursor region where accelerated particles constitute a highly relativistic and dilute beam, streaming instabilities typically become inefficient and operate at very small scales when compared to the gyroradii of the beam particles. We report on a plasma cavitation instability that is driven by dilute relativistic beams and can increase both the magnetic field strength and coherence scale by orders of magnitude to reach near-equipartition values with the beam energy density. This instability grows after the development of the Weibel instability and is associated with the asymmetric response of background leptons and ions to the beam current. The resulting net inductive electric field drives a strong energy asymmetry between positively and negatively charged beam species. Large-scale particle-in-cell simulations are used to verify analytical predictions for the growth and saturation level of the instability and indicate that it is robust over a wide range of conditions, including those associated with pair-loaded plasmas. These results can have important implications for the magnetization and structure of shocks in gamma-ray bursts, and more generally for magnetic field amplification and asymmetric scattering of relativistic charged particles in plasma astrophysical environments.

Shock Acceleration with Oblique and Turbulent Magnetic Fields

Abstract We investigate shock acceleration in a realistic astrophysical environment with density inhomogeneities. The turbulence induced by the interaction of the shock precursor with upstream density fluctuations amplifies both upstream and downstream magnetic fields via the turbulent dynamo. The dynamo-amplified turbulent magnetic fields (a) introduce variations of shock obliquities along the shock face, (b) enable energy gain through a combination of shock drift and diffusive processes, (c) give rise to various spectral indices of accelerated particles, (d) regulate the diffusion of particles both parallel and perpendicular to the magnetic field, and (e) increase the shock acceleration efficiency. Our results demonstrate that upstream density inhomogeneities and dynamo amplification of magnetic fields play an important role in shock acceleration, and thus shock acceleration depends on the condition of the ambient interstellar environment. The implications on understanding radio spectra of supernova remnants are also discussed.

Forbidden Line Emission from Type Ia Supernova Remnants Containing Balmer-dominated Shells

Abstract Balmer-dominated shells in supernova remnants (SNRs) are produced by collisionless shocks advancing into a partially neutral medium and are most frequently associated with Type Ia supernovae. We have analyzed Hubble Space Telescope (HST) images and Very Large Telescope (VLT)/Multi-Unit Spectroscopic Explorer (MUSE) or AAT/Wide Field Integral Spectrograph observations of five Type Ia SNRs containing Balmer-dominated shells in the LMC: 0509–67.5, 0519–69.0, N103B, DEM L71, and 0548–70.4. Contrary to expectations, we find bright forbidden-line emission from small dense knots embedded in four of these SNRs. The electron densities in some knots are higher than 104 cm−3. The size and density of these knots are not characteristic for interstellar medium—they most likely originate from a circumstellar medium ejected by the SN progenitor. Physical property variations of dense knots in the SNRs appear to reflect an evolutionary effect. The recombination timescales for high densities are short, and HST images of N103B taken 3.5 yr apart already show brightness changes in some knots. VLT/MUSE observations detect [Fe xiv] line emission from reverse shocks into SN ejecta as well as forward shocks into the dense knots. Faint [O iii] line emission is also detected from the Balmer shell in 0519–69.0, N103B, and DEM L71. We exclude the postshock origin because the [O iii] line is narrow. For the preshock origin, we considered three possibilities: photoionization precursor, cosmic-ray precursor, and neutral precursor. We conclude that the [O iii] emission arises from oxygen that has been photoionized by [He ii] λ304 photons and is then collisionally excited in a shock precursor heated mainly by cosmic rays.

Cosmic ray electrons released by supernova remnants

ABSTRACT The process that allows cosmic rays (CRs) to escape from their sources and be released into the Galaxy is still largely unknown. The comparison between CR electron and proton spectra measured at Earth suggests that electrons are released with a spectrum steeper than protons by Δsep ∼ 0.3 for energies above ∼10 GeV and by Δsep ∼ 1.2 above ∼1 TeV. Assuming that both species are accelerated at supernova remnant shocks, we here explore two possible scenarios that can in principle justify steeper electron spectra: (i) energy losses due to synchrotron radiation in an amplified magnetic field and (ii) time-dependent acceleration efficiency. We account for magnetic field amplification produced by either CR-induced instabilities or by magnetohydrodynamics instabilities my means of a parametric description. We show that both mechanisms are required to explain the electron spectrum. In particular, synchrotron losses can only produce a significant electron steepening above ∼1 TeV, while a time-dependent acceleration can explain the spectrum at lower energies if the electron injection into diffusive shock acceleration is inversely proportional to the shock speed. We discuss observational and theoretical evidences supporting such a behaviour. Furthermore, we predict two additional spectral features: a spectral break below ∼few GeV (as required by existing observations) due to the acceleration efficiency drop during the adiabatic phase, and a spectral hardening above ∼20 TeV (where no data are available yet) resulting from electrons escaping from the shock precursor.

Solar Wind Discontinuity Transformation at the Bow Shock

Abstract Solar wind plasma at the Earth’s orbit carries transient magnetic field structures including discontinuities. Their interaction with the Earth’s bow shock can significantly alter discontinuity configuration and stability. We investigate such an interaction for the most widespread type of solar wind discontinuities—rotational discontinuities (RDs). We use a set of in situ multispacecraft observations and perform kinetic hybrid simulations. We focus on the RD current density amplification that may lead to magnetic reconnection. We show that the amplification can be as high as two orders of magnitude and is mainly governed by three processes: the transverse magnetic field compression, global thinning of RD, and interaction of RD with low-frequency electromagnetic waves in the magnetosheath, downstream of the bow shock. The first factor is found to substantially exceed simple hydrodynamic predictions in most observed cases, the second effect has a rather moderate impact, while the third causes strong oscillations of the current density. We show that the presence of accelerated particles in the bow shock precursor highly boosts the current density amplification, making the postshock magnetic reconnection more probable. The pool of accelerated particles strongly affects the interaction of RDs with the Earth’s bow shock, as it is demonstrated by observational data analysis and hybrid code simulations. Thus, shocks should be distinguished not by the inclination angle, but rather by the presence of foreshocks populated with shock reflected particles. Plasma processes in the RD–shock interaction affect magnetic structures and turbulence in the Earth’s magnetosphere and may have implications for the processes in astrophysics.

Emission Spectroscopy of Ionizing Superorbital Expanding Flow

This paper reports on the use of spectral techniques for measuring excitation levels of a radiating hypersonic flow undergoing a rapid expansion. The X2 expansion tube, located at The University of Queensland was used to generate a test case physically similar to superorbital flight conditions, where a argon test flow was first processed by an oblique shock and then by an expansion fan. Emission spectroscopy was conducted to determine the populations of electronic energy levels of the argon by measuring electromagnetic radiation intensity at visible and infrared wavelengths. Excitation temperatures were resolved along spatial lines above the model, from upstream of the shock to downstream of the expansion fan. These temperatures were found to increase from 9000 to 10,000 K between the shock and the expansion fan, with the temperature frozen in the expansion fan. Numerical simulations of the flow showed a similar trend, with the computed excitation temperature in the expansion fan also freezing. Shock precursor radiation was also detected, indicating that the excitation temperature upstream of the shock and through the expansion fan is likely to be influenced by radiation from the immediate postshock region.

Radiation properties of radiative shock in xenon

<sec>Radiative shock is an important phenomenon both in astrophysics and in inertial confinement fusion. In this paper, the radiation properties of X-ray heated radiatve shock in xenon is studied with the simulation method. The radiative shock is described by a one-dimensional, multi-group radiation hydrodynamics model proposed by Zinn [Zinn J 1973 <i> J. Comput. Phys.</i> <b>13</b> 569]. To conduct computation, the opacity and equation-of-state data of xenon are calculated and put into the model. The reliabilities of the model and the physical parameters of xenon are verified by comparing the temperature and velocity of the radiative shock calculated by the model with those measured experimentally. </sec><sec>The evolution of the radiative shock involves abundant physical processes. The core of the xenon can be heated up to 100 eV, resulting in a thermal wave and forming an expanding high-temperature-core. Shortly, the hydrodynamic disturbances reach the thermal wave front, generating a shock. As the thermal wave slows down, the shock gradually exceeds the high-temperature-core, forming a double-step distribution in the temperature profile. </sec><sec>The time evolution of the effective temperature of the radiative shock shows two maximum values and one minimum value, and the radiation spectra often deviate from blackbody spectrum. By analyzing the radiation and absorption properties at different positions of the shock, it can be found that the optical property of the shock is highly dynamic and can generate the above-mentioned radiation characteristics. When the radiative shock is just formed, the radiation comes from the shock surface and the shock precursor has a significant absorption of the radiation. As the shock temperature falls during expansion, the shock precursor disappears and the radiation inside the shock can come out owing to absorption coefficient decreases. When the shock becomes transparent, the radiation surface reaches the outside edge of the high-temperature-core. Then, the temperature of the high-temperature-core decreases further, making this region also optically thin, and the radiation from the inner region can come out. Finally, the radiation strength falls because of temperature decreasing. </sec>

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.

Physics of relativistic collisionless shocks. II. Dynamics of the background plasma.

In this second paper of a series, we discuss the dynamics of a plasma entering the precursor of an unmagnetized, relativistic collisionless pair shock. We discuss how this background plasma is decelerated and heated through its interaction with a microturbulence that results from the growth of a current filamentation instability in the shock precursor. We make use, in particular, of the reference frame R_{w} in which the turbulence is mostly magnetic. This frame moves at relativistic velocities towards the shock front at rest, decelerating gradually from the far to the near precursor. In a first part, we construct a fluid model to derive the deceleration law of the background plasma expected from the scattering of suprathermal particles off the microturbulence. This law leads to the relationship γ_{p}∼ξ_{b}^{-1/2} between the background plasma Lorentz factor γ_{p} and the normalized pressure of the beam ξ_{b}; it is found to match nicely the spatial profiles observed in large-scale 2D3V particle-in-cell simulations. In a second part, we model the dynamics of the background plasma at the kinetic level, incorporating the inertial effects associated with the deceleration of R_{w} into a Vlasov-Fokker-Planck equation for pitch-angle diffusion. We show how the effective gravity in R_{w} drives the background plasma particles through friction on the microturbulence, leading to efficient plasma heating. Finally, we compare a Monte Carlo simulation of our model with dedicated PIC simulations and conclude that it can satisfactorily reproduce both the heating and the deceleration of the background plasma in the shock precursor, thereby providing a successful one-dimensional description of the shock transition at the microscopic level.

Physics of relativistic collisionless shocks: The scattering-center frame.

In this first paper of a series dedicated to the microphysics of unmagnetized, relativistic collisionless pair shocks, we discuss the physics of the Weibel-type transverse current filamentation instability that develops in the shock precursor, through the interaction of an ultrarelativistic suprathermal particle beam with the background plasma. We introduce in particular the notion of the "Weibel frame," or scattering center frame, in which the microturbulence is of mostly magnetic nature. We calculate the properties of this frame, using first a kinetic formulation of the linear phase of the instability, relying on Maxwell-Jüttner distribution functions, then using a quasistatic model of the nonlinear stage of the instability. Both methods show that (i) the Weibel frame moves at subrelativistic velocities relative to the background plasma, therefore at relativistic velocities relative to the shock front; (ii) the velocity of the Weibel frame relative to the background plasma scales with ξ_{b}, i.e., the pressure of the suprathermal particle beam in units of the momentum flux density incoming into the shock; and (iii) the Weibel frame moves slightly less fast than the background plasma relative to the shock front. Our theoretical results are found to be in satisfactory agreement with the measurements carried out in dedicated large-scale 2D3V particle-in-cell simulations.

HPC simulations of shock front evolution for a study of the shock precursor decay in a submicron thick nanocrystalline aluminum

The Hugoniot elastic limit (HEL, or the shock precursor) decay phenomenon was investigated under an uniaxial strain condition, in a plate-on-plate impact configuration, using large-scale molecular dynamics (MD) high performance computing (HPC) simulations on a multi-billion 5000 Å thick nanocrystalline aluminum (nc-Al) system with an average grain size of 1000 Å and at five impact velocities ranging from 0.7 to 1.5 km s−1. The averaged stress and strain distributions were obtained in the shock fronts’ travel direction using a material conserving atom slicing method. The loading paths in terms of the Rayleigh lines experienced by the atom system in the evolving shock fronts exhibited a strong dependency on the shock stress levels. This dependency decreased as the impact velocity increased from 0.7 to 1.5 km s−1. By combining the HELs from MD results with plate impact experimental data, the precursor decay for the nc-Al was predicted from nano-to-macro scale thickness range. The evolving shock fronts were characterized in terms of parameters such as the shock front thickness, shock rise time and strain rate. The MD results were further analyzed using a crystal analysis algorithm and a twin dislocation identification method to obtain the densities of the atomistic defects evolving behind the evolving shock fronts. High-fidelity large-scale HPC simulation results showed that certain dislocation partials strongly influenced the elastic–plastic transition response across the HELs. The twinning dislocations increased by more than a factor of 10 during the transition and remained constant under further shock compression.

Nonlinear Monte Carlo model of superdiffusive shock acceleration with magnetic field amplification.

Fast collisionless shocks in cosmic plasmas convert their kinetic energy flow into the hot downstream thermal plasma with a substantial fraction of energy going into a broad spectrum of superthermal charged particles and magnetic fluctuations. The superthermal particles can penetrate into the shock upstream region producing an extended shock precursor. The cold upstream plasma flow is decelerated by the force provided by the superthermal particle pressure gradient. In high Mach number collisionless shocks, efficient particle acceleration is likely coupled with turbulent magnetic field amplification (MFA) generated by the anisotropic distribution of accelerated particles. This anisotropy is determined by fast particle transport, making the problem strongly nonlinear and multiscale. Here, we present a nonlinear Monte Carlo model of collisionless shock structure with superdiffusive propagation of high-energy Fermi accelerated particles coupled to particle acceleration and MFA, which affords a consistent description of strong shocks. A distinctive feature of the Monte Carlo technique is that it includes the full angular anisotropy of the particle distribution at all precursor positions. The model reveals that the superdiffusive transport of energetic particles (i.e., Lévy-walk propagation) generates a strong quadruple anisotropy in the precursor particle distribution. The resultant pressure anisotropy of the high-energy particles produces a nonresonant mirror-type instability that amplifies compressible wave modes with wavelengths longer than the gyroradii of the highest-energy protons produced by the shock.

Abnormal geophysical and seismotectonic processes observed during the period of preparation and development of the earthquake with a magnetude of 8,8 Maule 2010 (Chile)

Focal zone, basic shock, jerking of strongest aftershocks and progress of seismic energy release during the aftershock process of the earthquake Maule 27.02.2010 (M W =8,8) in Chile have been analyzed. Macroseismic manifestations, tectonic position of the focus and reconstruction of acting plane position in the focus have been considered. Geophysical treatment of anomalous gravimagnetic disturbances and seismic precursors induced in variations of magnetic field and geoacoustic fields of the Earth during preparation of the basic shock and of three strongest iterative shocks on March 11 —16 2010 has been done for the first time. It has been shown that application of present-day cosmic observations, which allow to analyze gravity disturbances (so far only coseismic), reflected on the Earth’s surface makes possible to consider the precursors of seismic shocks of the arthquake Maule and other seismic events in a new way. Long-, middle- and short-term approaches to prognostication of strong seismic events both on land and sea have been stated

The formation and evolution of reconnection-driven, slow-mode shocks in a partially ionised plasma

The role of slow-mode MHD shocks in magnetic reconnection is one of great importance for energy conversion and transport, but in many astrophysical plasmas the plasma is not fully ionised. In this paper, we investigate, using numerical simulations, the role of collisional coupling between a proton-electron charge-neutral fluid and a neutral hydrogen fluid for the 1D Riemann problem initiated in a constant pressure and density background state by a discontinuity in the magnetic field. This system, in the MHD limit, is characterised by two waves: a fast-mode rarefaction wave that drives a flow towards a slow-mode MHD shock. The system evolves through four stage: initiation, weak coupling, intermediate coupling and a quasi steady state. The initial stages are characterised by an over-pressured neutral region that expands with characteristics of a blast wave. In the later stages, the system tends towards a self-similar solution where the main drift velocity is concentrated in the thin region of the shock front. Due to the nature of the system, the neutral fluid is overpressured by the shock when compared to a purely hydrodynamic shock which results in the neutral fluid expanding to form the shock precursor. The thickness of the shockfront once it has formed proportional to the ionisation fraction to the power -1.2, which is a smaller exponent than would be naively expected from simple scaling arguments. One interesting result is that the shock front is a continuous transition of the physical variables for sub-sonic velocity upstream of the shock front (a c-shock) to a sharp jump in the physical variables followed by a relaxation to the downstream values for supersonic upstream velocity (a j-shock). The frictional heating that results from the velocity drift across the shock front can amount to approximately two per cent of the reference magnetic energy.