Microinstabilities in the Transition Region of Weak Quasi-perpendicular Intracluster Shocks

Abstract

Microinstabilities play important roles in both entropy generation and particle acceleration in collisionless shocks. Recent studies have suggested that in the transition region of quasi-perpendicular (Q ⊥ ) shocks in the high-beta (β = P gas /P B ) intracluster medium (ICM), the ion temperature anisotropy due to the reflected-gyrating ions could trigger the Alfvén ion cyclotron (AIC) instability and the ion-mirror instability, while the electron temperature anisotropy induced by magnetic field compression could excite the whistler instability and the electron-mirror instability. Adopting the numerical estimates for ion and electron temperature anisotropies found in the particle-incell (PIC) simulations of Q ⊥ shocks with sonic Mach numbers, M s = 2-3, we carry out a linear stability analysis for these microinstabilities. The kinetic properties of the microinstabilities and the ensuing plasma waves on both ion and electron scales are described for wide ranges of parameters, including β and the ion-to-electron mass ratio. In addition, the nonlinear evolution of the induced plasma waves are examined by performing 2D PIC simulations with periodic boundary conditions. We find that for β ≈ 20-100, the AIC instability could induce ion-scale waves and generate shock surface ripples in supercritical shocks above the AIC critical Mach number,. Also, electron-scale waves are generated primarily by the whistler instability in these high-β shocks. The resulting multiscale waves from electron to ion scales are thought to be essential in the electron injection to diffusive shock acceleration in Q ⊥ shocks in the ICM.

In-situ Measurement of the Energy Fraction in Supra-thermal and Energetic Particles at ACE, Wind, and PSP Interplanetary Shocks

“…Berezhko et al 2009;Morlino et al 2009;Yuan et al 2012). PIC simulations of SNR shocks suggest that, at high obliquity, electrons drive upstream turbulence prior to ions significant acceleration: again, fully kinetic simulations are needed to determine ion energy spectra (Bohdan et al 2020;Kim et al 2021). We note that low Mach number (∼ 2 − 4) shocks in galaxy clusters (Vazza et al 2015) are also consistent with the near-equipartitioning of (relativistic) electrons and protons or electron acceleration efficiency exceeding protons (K down e/p ∼ 1 − 100).…”

“…High-amplitude and high-frequency whistler waves excited by electrons in the shock layer (Katou & Amano 2019;Amano et al 2020) and traveling upstream might also drive upstream turbulence. A number of electron-scale instabilities in the high plasma beta regime of intra-cluster shocks were studied in 2D PIC (Kim et al 2021), suggesting that ion-scale waves are generated by distinct instabilities on longer time scales, partially overlapping with electron time scales. Figure 6 shows the ratio of the downstream enthalpy flux of energized particles to the total downstream enthalpy flux as a function of M f .…”

Numerical Modeling of Spectral Hardening at a Finite-width Shock

“…The numerical scheme considered in this work can be applicable to study, e.g., electron acceleration at Earth's bow shock and ion acceleration at a propagating CME-driven shock as well. Note that PIC simulations of low Mach number shocks (e.g., Kim et al 2021) have shown that instabilities can generate small-scale structures that also lead to electron energization. In our work, however, we focus only on DSA.…”

Cosmic Ray Acceleration and Nonthermal Radiation at Accretion Shocks in the Outer Regions of Galaxy Clusters

“…Mainly, two types of electron preacceleration mechanisms have been proposed: (1) the socalled Fermi-like SDA due to multiple cycles of SDA and diffusive scattering of electrons between the shock ramp and upstream self-generated waves (Guo et al 2014a;Kang et al 2019), and (2) the SSDA due to the extended gradient-drift of electrons, while being confined in the shock overshoot (Niemiec et al 2019;Ha et al 2021;Kobzar et al 2021). Reflected particles result in the ion and electron temperature anisotropies, which in turn excite multi-scale plasma waves via various microinstabilities: e.g., the electron firehose instability in the upstream region (Guo et al 2014b;Kim et al 2020), the Alfvén ion cyclotron and ion-mirror instabilities, and the electron whistler and electron-mirror instabilities in the shock overshoot (e.g., Katou & Amano 2019;Kim et al 2021).…”