Abstract
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-in-cell (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, M AIC * ≈ 2.3 . 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.
Highlights
Major mergers of galaxy clusters are known to drive weak shocks with sonic Mach numbers, Ms 3, in the hot intracluster medium (ICM) of high β (e.g., Ryu et al 2003; Skillman et al 2008; Vazza et al 2009; Hong et al 2014; Ha et al 2018)
In a separate paper (Ha et al 2021, HKRK2021 hereafter), we report a similar study of β ≈ 20–100 shocks, which is design to explore through 2D PIC simulations how the multiscale waves excited mainly by the Alfvén ion cyclotron (AIC) and whistler instabilities in the shock transition can assist the electron injection to diffusive shock acceleration (DSA) in ICM shocks
The multiscale plasma waves generated by these microinstabilities are thought to be crucial for electron preacceleration via the stochastic shock drift acceleration (SSDA) (e.g., Katou & Amano 2019; Niemiec et al 2019; Trotta & Burgess 2019)
Summary
Major mergers of galaxy clusters are known to drive weak shocks with sonic Mach numbers, Ms 3, in the hot intracluster medium (ICM) of high β (e.g., Ryu et al 2003; Skillman et al 2008; Vazza et al 2009; Hong et al 2014; Ha et al 2018). In the transition region behind the shock ramp, on the other hand, the AIC and i-mirror instabilities can be triggered by the ion perpendicular anisotropy (Ti⊥ > TiP) mainly due to the shock-reflected ions advected downstream, while the whistler and e-mirror instabilities can be excited by the electron perpendicular anisotropy (Te⊥ > TeP) mainly due to magnetic field compression at the shock ramp (Guo et al 2017; Katou & Amano 2019) Such multiscale waves from electron to ion scales are essential in the electron preacceleration via the SSDA (Matsukiyo & Matsumoto 2015; Niemiec et al 2019; Trotta & Burgess 2019).
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