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We describe a systematic development of kinetic entropy as a diagnostic infully kinetic particle-in-cell (PIC) simulations and use it to interpret plasmaphysics processes in heliospheric, planetary, and astrophysical systems. First,we calculate kinetic entropy in two forms -- the ``combinatorial'' form relatedto the logarithm of the number of microstates per macrostate and the``continuous'' form related to $f \ln f$, where $f$ is the particledistribution function. We discuss the advantages and disadvantages of each anddiscuss subtleties about implementing them in PIC codes. Using collisionlessPIC simulations that are two-dimensional in position space andthree-dimensional in velocity space, we verify the implementation of thekinetic entropy diagnostics and discuss how to optimize numerical parameters toensure accurate results. We show the total kinetic entropy is conserved tothree percent in an optimized simulation of anti-parallel magneticreconnection. Kinetic entropy can be decomposed into a sum of a position spaceentropy and a velocity space entropy, and we use this to investigate the natureof kinetic entropy transport during collisionless reconnection. We find thevelocity space entropy of both electrons and ions increases in time due toplasma heating during magnetic reconnection, while the position space entropydecreases due to plasma compression. This project uses collisionlesssimulations, so it cannot address physical dissipation mechanisms; nonetheless,the infrastructure developed here should be useful for studies of collisionalor weakly collisional heliospheric, planetary, and astrophysical systems.Beyond reconnection, the diagnostic is expected to be applicable to plasmaturbulence and collisionless shocks.

Decomposition of Plasma Kinetic Entropy into Position and Velocity Space and the Use of Kinetic Entropy in Particle-in-Cell Simulations