The cosmic-ray staircase: the outcome of the cosmic-ray acoustic instability

Recently, cosmic rays (CRs) have emerged as a leading candidate for driving galactic winds. Small-scale processes can dramatically affect global wind properties. We run two-moment simulations of CR streaming to study how sound waves are driven unstable by phase-shifted forces and heating. verify linear theory growth rates. As the grow non-linear, they steepen into quasi-periodic series propagating shocks; density jumps at shocks create bottlenecks. The depth bottleneck depends on both jump its velocity; {\Delta}P_c is smaller rapidly moving A bottlenecks creates staircase structure, which be understood from convex hull construction. system reaches steady state between new perturbations, stair mergers. CRs decoupled plateaus, but exert intense heating jumps. absence plateaus leads cooling, strong gas pressure gradients further shocks. If stationary, drastically modify flows; if their propagation times comparable dynamical times, effects momentum energy transfer modest. acoustic instability likely relevant in thermal interfaces cold hot gas, well Similar increased opacity radiative flows, build-up due significantly increase mass outflow rates, up an order magnitude. It seeds unusual forms instability, could distinct observational signatures.