  Shocks waves are a ubiquitous feature of many astrophysical plasma systems,
and an important process for energy dissipation and transfer. The physics of
these shock waves are frequently treated/modeled as a collisional, fluid MHD
discontinuity, despite the fact that many shocks occur in the collisionless
regime. In light of this, using fully kinetic, 3D simulations of
non-relativistic, parallel propagating collisionless shocks comprised of
electron-positron plasma, we detail the deviation of collisionless shocks form
MHD predictions for varying magnetization/Alfv\'enic Mach numbers, with
particular focus on systems with Alf\'enic Mach numbers much smaller than sonic
Mach numbers. We show that the shock compression ratio decreases for
sufficiently large upstream magnetic fields, in agreement with the predictions
of Bret & Narayan (2018). Additionally, we examine the role of magnetic field
strength on the shock front width. This work reinforces a growing body of work
that suggest that modeling many astrophysical systems with only a fluid plasma
description omits potentially important physics.
