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A case of two interpenetrating, cold and quasi-neutral ion-electron plasmasis investigated with the multi-fluid approach. We consider that one plasmaflows quasi-parallel to the lines of a background magnetic field embedded inanother static plasma. If the flow turns super-Alfv\'enic, we show thatparallel R/L-modes and perpendicular X/O-modes become unstable and grow inamplitude. Within the linear theory, we find that the growth rate curve of anunstable mode has a maximum at some wavenumber specific to each mode. If weconsider a shock-like plasma configuration, we find that the fastest growingmode is the resonant one (with $k \sim r_{gi}^{-1}$) which strongly interactswith ions. In Particle-In-Cell (PIC) simulations, we observe that a resonantwave with the same properties is excited during the early phases of shockformation. Once the wave becomes non-linear, it efficiently scatters ions andtriggers the initial shock formation. This implies that the typical compressionratio of $\sim 4$ could naturally arise as a consequence of a highly resonantmicro-physical process. We model the interaction of ions reflected from thereforming shock barrier in a weak-beam case, and we show that the upstream wavenow matches the instability we expect from the equations. By using PICsimulations, we explain how the strong-beam resonant instability triggers shockformation in the non-linear stage, and how the weak-beam instability reformsand transmits the shock afterwards. The micro-instabilities that we study herecould largely contribute to shock triggering as well as to the reformation andtransmission of the shock itself.

Resonant micro-instabilities at quasi-parallel collisionless shocks: cause or consequence of shock (re)formation