Dynamics and microinstabilities at perpendicular collisionless shock: A comparison of large-scale two-dimensional full particle simulations with different ion to electron mass ratio

Abstract

Large-scale two-dimensional (2D) full particle-in-cell simulations are carried out for studying the relationship between the dynamics of a perpendicular shock and microinstabilities generated at the shock foot. The structure and dynamics of collisionless shocks are generally determined by Alfven Mach number and plasma beta, while microinstabilities at the shock foot are controlled by the ratio of the upstream bulk velocity to the electron thermal velocity and the ratio of the plasma-to-cyclotron frequency. With a fixed Alfven Mach number and plasma beta, the ratio of the upstream bulk velocity to the electron thermal velocity is given as a function of the ion-to-electron mass ratio. The present 2D full PIC simulations with a relatively low Alfven Mach number (M A ∼ 6) show that the modified two-stream instability is dominant with higher ion-to-electron mass ratios. It is also confirmed that waves propagating downstream are more enhanced at the shock foot near the shock ramp as the mass ratio becomes higher. The result suggests that these waves play a role in the modification of the dynamics of collisionless shocks through the interaction with shock front ripples.

Nonrelativistic Perpendicular Shocks Modeling Young Supernova Remnants: Nonstationary Dynamics and Particle Acceleration at Forward and Reverse Shocks

“…The process is called a cyclic self-reformation of the shock and is caused by the dynamics of the shock-reflected ions (see, e.g., Treumann 2009; Umeda et al 2010Umeda et al , 2014. Figure 18 shows the time evolution of the average density profile in the vicinity of the forward shock.…”

“…For panel (b), we compensated the average velocity of all ions (see Figure 10), whereas for panel (c) on the right we compensated only the average motion of incoming ions. Umeda et al 2009Umeda et al , 2010Umeda et al , 2014. The ion temperature anisotropy arising from ion reflection at the shock can drive the Alfvén ion cyclotron (AIC) or the mirror instability in the shock ramp, and the resulting unstable modes have wavelengths of a few ion skin depths and propagate along the regular magnetic field, significantly contributing to ion isotropization and thermalization at the shock and downstream.…”

Periodic self-reformation of rippled perpendicular collisionless shocks in two dimensions

“…The MTSI was dominant in the foot region in the runs with m i /m e = 100, 256, and 625 (Umeda et al, 2012b(Umeda et al, , 2014. The excitation of the whistler mode in the foot region was also indicated by the enhancement of the shock magnetic field at the timescale much shorter than the local ion cyclotron period.…”

“…The excitation of the whistler mode in the foot region was also indicated by the enhancement of the shock magnetic field at the timescale much shorter than the local ion cyclotron period. It was also shown that the wavelength in the shock tangential direction of the whistler mode excited by the MTSI is close to the wavelength of the ripples (Umeda et al, 2014). It is suggested that wave-wave coupling between the whistler mode and the ripples is possible, which disturbs the periodic reformation of (quasi-)perpendicular shocks.…”

“…The bulk flow velocity of the upstream plasma is u x1 /v te1 = 3.0, 1.5, 0.9375, and 0.6 for Runs A, B, C, and D, respectively. The previous studies showed that the electron cyclotron drift instability (ECDI) is driven with the parameters of Run A (Umeda et al, 2012b(Umeda et al, , 2014, which excites electrostatic waves at multiple electron cyclotron harmonic frequencies Lembege, 2006, 2013). On the other hand, the modified two-stream instability (MTSI) is driven with the parameters of Runs B-D (Umeda et al, 2012a(Umeda et al, , b, 2014, which excites obliquely propagating electromagnetic whistler mode waves at a frequency between the electron cyclotron frequency and the lower hybrid resonance frequency Scholer, 2003, 2006).…”