Explosive phenomena such as supernova remnant shocks and solar flares have demonstrated evidence for the production of relativistic particles. Interest has therefore been renewed in collisionless shock waves and magnetic reconnection as a means to achieve such energies. Although ions can be energized during such phenomena, the relativistic energy of the electrons remains a puzzle for theory. We present supercomputer simulations showing that efficient electron energization can occur during turbulent magnetic reconnection arising from a strong collisionless shock. Upstream electrons undergo first-order Fermi acceleration by colliding with reconnection jets and magnetic islands, giving rise to a nonthermal relativistic population downstream. These results shed new light on magnetic reconnection as an agent of energy dissipation and particle acceleration in strong shock waves.

Electron accelerations at high Mach number collision-less shocks are investigated by means of two-dimensional electromagnetic Particle-in-Cell simulations with various Alfvén Mach numbers, ion-to-electron mass ratios, and the upstream electron β e (the ratio of the thermal pressure to the magnetic pressure). We found electrons are effectively accelerated at a super-high Mach number shock (M A ∼ 30) with a mass ratio of M/m = 100 and β e = 0.5. The electron shock surfing acceleration is an effective mechanism for accelerating the particles toward the relativistic regime even in two dimensions with the large mass ratio.Buneman instability excited at the leading edge of the foot in the super-high Mach number shock results in a coherent electrostatic potential structure. While multi-dimensionality allows the electrons to escape from the trapping region, they can interact with the strong electrostatic field several times. Simulation runs in various parameter regimes indicate that the electron shock surfing acceleration is an effective mechanism for producing relativistic particles in extremely-high Mach number shocks in supernova remnants, provided that the upstream electron temperature is reasonably low.

A two‐dimensional MHD simulation of the Kelvin‐Helmholtz (K‐H) instability in a non‐uniform density medium shows a strong development of turbulence through non‐linear instabilities. The difference in density between the two media plays a crucial role on the fast turbulent mixing and transport. The onset of the turbulence is triggered not only by secondary K‐H instability but also by Rayleigh‐Taylor (R‐T) instability at the density interface inside the normal K‐H vortex, whose onset mechanisms are attributed to the centrifugal force of the rotating motion. The secondary R‐T instability alters the macroscopic structure by transporting dense fluids to tenuous region, while the secondary K‐H instability is just a seed for the turbulence. The onset mechanism and the formation of the broad mixing layer give a new understanding of the mixing process in a variety of geo‐ and astrophysical phenomena.

We investigated the efficiency of coherent upstream large-amplitude electromagnetic wave emission via synchrotron maser instability at relativistic magnetized shocks by using two-dimensional particle-in-cell simulations. We considered the purely perpendicular shock in an electron-positron plasma. The coherent wave emission efficiency was measured as a function of the magnetization parameter σ, which is defined by the ratio of the Poynting flux to the kinetic energy flux. The wave amplitude was systematically smaller than that observed in one-dimensional simulations. However, it continued to persist, even at a considerably low magnetization rate, where the Weibel instability dominated the shock transition. The emitted electromagnetic waves were sufficiently strong to disturb the upstream medium, and transverse filamentary density structures of substantial amplitude were produced. Based on this result, we discuss the possibility of the wakefield acceleration model for the production of non-thermal electrons in a relativistic magnetized ion-electron shock.

How electrons get accelerated to relativistic energies in a high-Mach-number quasi-perpendicular shock is presented by means of ab initio particle-in-cell simulations in three dimensions. We found that coherent electrostatic Buneman waves and ion-Weibel magnetic turbulence coexist in a strongshock structure whereby particles gain energy during shock-surfing and subsequent stochastic drift accelerations. Energetic electrons that initially experienced the surfing acceleration undergo pitchangle diffusion by interacting with magnetic turbulence and continuous acceleration during confinement in the shock transition region. The ion-Weibel turbulence is the key to the efficient nonthermal electron acceleration.PACS numbers: 52.35. Tc, 52.65.Rr, 96.50.Pw, 98.70.Sa Elucidating the acceleration mechanisms of charged particles have been of great interests in laboratory, space, and astrophysical plasma physics. Among other mechanisms, a collisionless shock is thought to be an efficient particle accelerator. This idea has been strengthened by radio, X-ray, and gamma-ray observations of astrophysical objects such as supernova remnant shocks, indicating that protons and electrons are efficiently accelerated to TeV energies at such very strong shock waves [1][2][3][4]. Efficient electron acceleration at high-Mach-number shocks was also recently suggested by in-situ measurements at Saturn's bow shock [5]. Motivated by these circumstances, laboratory experiments using high-power laser facilities have emerged to provide a new platform for tackling such problems [6][7][8][9][10][11].The diffusive shock acceleration (DSA) theory [12, 13] has provided a solution to observational evidences for efficient accelerations at collisionless shocks, as it predicts a power-law energy spectrum of particles having a spectral index that is close to the values suggested by multiwavelength observations. As the DSA theory presumes pre-existing mildly energetic particles, pre-acceleration mechanisms are required to provide a seed population for DSA, particularly for electrons [14,15]. The connection between pre-acceleration and DSA remains a critical issue in shock acceleration theory.One possible pre-acceleration mechanism is the socalled shock drift acceleration (SDA), in which a particle gains energy in the shock transition region during its gradient-|B| drift. For an electron with a Larmor radius much smaller than the shock thickness, the interaction time with the shock (and hence the energy gain) is * ymatumot@chiba-u.jp † Also at Institute for Global Prominent Research, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan determined by the adiabatic theory [16][17][18]. Subsequent acceleration can be realized by self-generated electromagnetic waves excited by accelerated electrons [19][20][21][22][23]. Alternatively, the shock surfing acceleration (SSA) becomes particularly important for electrons in highMach-number perpendicular shocks [24][25][26][27]. This process uses large-amplitude electrostatic waves generated by the Bun...

[1] Two-dimensional simulations of the Kelvin-Helmholtz (K-H) instability in a nonuniform density medium and with the transverse magnetic field show the strong development of turbulence through nonlinear instabilities. Ideal MHD simulation results have indicated that the difference in density between two media plays a crucial role on the fast turbulent mixing and transport. The onset of the turbulence is triggered not only by the secondary K-H instability but also by the Rayleigh-Taylor (R-T) instability at the density interface inside the normal K-H vortex. The secondary R-T instability alters macroscopic structure by transporting dense fluids to tenuous region, while the secondary K-H instability is just a seed for the turbulence. Full particle simulations are also conducted and reproduces the similar result of the ideal MHDs, except that the strong electrostatic field caused by the secondary R-T instability scatters ions and deforms the electron density interface. As a result, the mixing area increases anomalously fast and extends spatially as compared to the result in the uniform density case.Citation: Matsumoto, Y., and M. Hoshino (2006), Turbulent mixing and transport of collisionless plasmas across a stratified velocity shear layer,

The first-order Fermi acceleration of electrons requires an injection of electrons into a mildly relativistic energy range. However, the mechanism of injection has remained a puzzle both in theory and observation. We present direct evidence for a novel stochastic shock drift acceleration theory for the injection obtained with Magnetospheric Multiscale (MMS) observations at Earth's bow shock. The theoretical model can explain electron acceleration to mildly relativistic energies at high-speed astrophysical shocks, which may provide a solution to the long-standing issue of electron injection.

Electron acceleration associated with various plasma kinetic instabilities in a nonrelativistic, very-high-Alfvén Mach-number (M A ∼ 45) shock is revealed by means of a two-dimensional fully kinetic PIC simulation. Electromagnetic (ion Weibel) and electrostatic (ion-acoustic and Buneman) instabilities are strongly activated at the same time in different regions of the two-dimensional shock structure. Relativistic electrons are quickly produced predominantly by the shock surfing mechanism with the Buneman instability at the leading edge of the foot. The energy spectrum has a high-energy tail exceeding the upstream ion kinetic energy accompanying the main thermal population. This gives a favorable condition for the ion acoustic instability at the shock front, which in turn results in additional energization. The large-amplitude ion Weibel instability generates current sheets in the foot, implying another dissipation mechanism via magnetic reconnection in a three-dimensional shock structure in the very-high-M A regime.