Mach Number Dependence of Electron Heating in High Mach Number Quasiperpendicular Shocks

Efficiency of electron heating through microinstabilities generated in the transition region of a quasi-perpendicular shock for wide ange of Mach numbers is investigated by utilizing PIC (Particle-In-Cell) simulation and model analyses. In the model analyses saturation levels of effective electron temperature as a result of microinstabilities are estimated from an extended quasilinear (trapping) analysis for relatively low (high) Mach number shocks. Here, MTSI (modified two-stream instability) is assumed to become dominant in low Mach number regime, while BI (Buneman instability) to become dominant in high Mach number regime, respectively. It is revealed that Mach number dependence of the effective electron temperature in the MTSI dominant case is essentially different from that in the BI dominant case. The effective electron temperature through the MTSI does not depend much on the Mach number, although that through the BI increases with the Mach number as in the past studies. The results are confirmed to be consistent with the PIC simulations both in qualitative and quantitative levels. The model analyses predict that a critical Mach number above which steep rise of electron heating rate occurs may arise at the Mach number of a few tens.Comment: 9 pages, 5 figures, Phys. Plasmas in pres

Microinstabilities at perpendicular collisionless shocks: A comparison of full particle simulations with different ion to electron mass ratio

A full particle simulation study is carried out for studying microinstabilities generated at the shock front of perpendicular collisionless shocks. The structure and dynamics of shock waves are determined by Alfven Mach number and plasma beta, while microinstabilities are controlled by the ratio of the upstream bulk velocity to the electron thermal velocity and the plasma-to-cyclotron frequency. Thus, growth rates of microinstabilities are changed by the ion-to-electron mass ratio, even with the same Mach number and plasma beta. The present two-dimensional simulations show that the electron cyclotron drift instability is dominant for a lower mass ratio, and electrostatic electron cyclotron harmonic waves are excited. For a higher mass ratio, the modified two-stream instability is dominant and oblique electromagnetic whistler waves are excited, which can affect the structure and dynamics of collisionless shocks by modifying shock magnetic fields.Comment: 13 pages, 7 figures, Physics of Plasmas, in press; the paper with full resolution images is http://www.phys.aoyama.ac.jp/~ryo/papers/microinsta_PoP.pd

Effect of Upstream ULF Waves on the Energetic Ion Diffusion at the Earthʼs Foreshock. I. Theory and Simulation

Field-aligned diffusion of energetic ions in the Earth’s foreshock is investigated by using the quasi-linear theory (QLT)and test particle simulation

Electron Acceleration at Rippled Low-Mach-number Shocks in High-beta Collisionless Cosmic Plasmas

Using large-scale fully-kinetic two-dimensional particle-in-cell simulations, we investigate the effects of shock rippling on electron acceleration at low-Mach-number shocks propagating in high-$\beta$ plasmas, in application to merger shocks in galaxy clusters. We find that the electron acceleration rate increases considerably when the rippling modes appear. The main acceleration mechanism is stochastic shock-drift acceleration, in which electrons are confined at the shock by pitch-angle scattering off turbulence and gain energy from the motional electric field. The presence of multi-scale magnetic turbulence at the shock transition and the region immediately behind the main shock overshoot is essential for electron energization. Wide-energy non-thermal electron distributions are formed both upstream and downstream of the shock. The maximum energy of the electrons is sufficient for their injection into diffusive shock acceleration. We show for the first time that the downstream electron spectrum has a~power-law form with index $p\approx 2.5$, in agreement with observations.Comment: 15 pages, 14 figures, to be published in Ap

Model experiment of magnetic field amplification in laser-produced plasmas via the Richtmyer-Meshkov instability

A model experiment of magnetic field amplification (MFA) via the Richtmyer-Meshkov instability (RMI) in supernova remnants (SNRs) was performed using a high-power laser. In order to account for very-fast acceleration of cosmic rays observed in SNRs, it is considered that the magnetic field has to be amplified by orders of magnitude from its background level. A possible mechanism for the MFA in SNRs is stretching and mixing of the magnetic field via the RMI when shock waves pass through dense molecular clouds in interstellar media. In order to model the astrophysical phenomenon in laboratories, there are three necessary factors for the RMI to be operative: a shock wave, an external magnetic field, and density inhomogeneity. By irradiating a double-foil target with several laser beams with focal spot displacement under influence of an external magnetic field, shock waves were excited and passed through the density inhomogeneity. Radiative hydrodynamic simulations show that the RMI evolves as the density inhomogeneity is shocked, resulting in higher MFA

Modified two-stream instability in the foot of high Mach number quasi-perpendicular shocks

The modified two-stream instability in the foot of supercritical quasi-perpendicular shock wave is investigated. A linear analysis shows that the instability can sufficiently grow during the shock reformation cycle for the case of a realistic ion to electron mass ratio. The wave-particle interactions of the Landau type of both the electrons and ions are essential in a finite beta plasma and lead to a reduction of the growth rate with increasing beta. The magnetic polarization in terms of wavevector is also analyzed. Additional onedimensional full-particle electromagnetic simulations in a periodic system reveal some important nonlinear wave properties. The wave spectra indicate a lower cascade due to wave-wave interactions. The parallel phase space distribution of electrons correlates well with the wave profile of the magnetic field component perpendicular to both the ambient magnetic field and the wavevector. The nonlinearly generated waves lead to electron heating parallel to the magnetic field