Role of Alfvénic Mach number and ambient magnetic field orientation on electron-to-proton temperature ratio in collisionless shocks.

Collisionless shocks are common in space and astrophysical plasmas, and are critical for magnetic field amplification and particle acceleration. Understanding electron heating and particle injection into the non-thermal tail remains a challenge. We present the results of a comprehensive survey of large-scale, one-dimensional particle-in-cell simulations of non relativistic, magnetized shocks to investigate how energy partition and injection efficiency vary with Alfvénic Mach number and magnetic field orientation. We analyze both quasi-parallel and quasi-perpendicular shocks, focusing on the interplay between ion reflection, the cross-shock potential, and upstream modes in mediating electron heating. Our results show that reflected particles exchange energy through wave-particle interactions, triggering instabilities in the upstream that play an important role in energy partition. We find that at low Mach numbers, the magnetic field orientation plays a critical role with efficient electron heating observed only in quasi-parallel shocks. For Alfvénic Mach numbers above ~40, both quasi-parallel and quasi-perpendicular shocks converge in terms of electron heating with Te / Ti ~ 0.3-0.4.