Authors: S. A. Conley (Princeton University), G. G. Howes (University of Iowa), J. Juno (Princeton Plasma Physics Laboratory), J. M. TenBarge (Princeton University)

Increased internal energy density is often discussed as plasma heating in weakly collisional systems, though it is recognized that energy transport in the absence of collisions is reversible and that particle distributions commonly exhibit non-Maxwellian features. A potential driver of large increases in internal energy density is the co-location of particle subpopulations. We highlight the importance of employing phase-space diagnostics to distinguish true plasma heating from other sources of free energy in the distribution function such as co-located populations through (i) a simple example of counter-streaming ion beams and (ii) a self-consistent perpendicular shock simulation. We emphasize that local increases in temperature or measurements of increased internal energy density should not be interpreted as markers of local particle heating without corroboration from phase-space.