Author ORCID Identifier



Date of Graduation


Document Type


Degree Type



Eberly College of Arts and Sciences


Physics and Astronomy

Committee Chair

Paul Cassak

Committee Member

Earl Scime

Committee Member

Weichao Tu

Committee Member

Michael Shay


The study of energy conversion in collisionless plasmas that are not in local thermodynamic equilibrium (LTE) is at the leading edge of plasma physics research. Plasma constituents in such systems can exhibit highly structured phase space densities that deviate significantly from that of a Maxwellian. A standard approach has emerged in recent years for investigating energy conversion between bulk flow and thermal energy in collisionless plasmas using the non-LTE generalization of the first law of thermodynamics. The primary focus is placed on pressure-strain interaction (PS) term, with a particular emphasis on its non-LTE piece called Pi − D. Recent studies have found that Pi − D can be negative, which makes its identification as collisionless viscous heating counterintuitive. A kinetic understanding of Pi − D has been limited. We argue that the non-LTE generalization of the first law of thermodynamics and subsequent attempts to extend thermodynamics overlooks the kinetic aspects associated with phase space densities having arbitrary shapes that can deviate significantly from a Maxwellian. Only changes in work due to compression that changes the zeroth moment of the phase space density, i.e., the number density, and Pi − D and heat flux which change the second moment, i.e., effective temperature are considered by the non-LTE generalization of the first law of thermodynamics. However, it remains agnostic to energy conversion associated with changes to any higher moment of the phase space density. We address these limitations by first developing a kinetic understanding of Pi − D and introducing an alternative decomposition of the PS term in Cartesian coordinates which separates the physics of converging/ diverging flows from shear deformation. We further find that in magnetic field-aligned coordinates, the PS term can be decomposed into eight groups of terms, each corresponding to a different physical mechanism. Lastly, we develop a first-principles theory of the energy conversion associated with all higher moments of the phase space density. Using particle-in-cell simulations of a well understood non-LTE system, i.e., two-dimensional antiparallel magnetic reconnection, we first examine the decompositions of PS term in both Cartesian and magnetic field-aligned coordinates. This enables us to identify the predominant mechanisms contributing to positive and negative PS terms during reconnection, thereby facilitating the interpretation of numerical and observational data. Additionally, simulation results reveal that energy conversion associated with higher-order moments can be locally significant by being a substantial fraction of the internal energy and even surpassing it in regions characterized by strongly non-LTE phase space densities. These results may be useful in numerous plasma settings, such as heliospheric, planetary, and astrophysical plasmas, and for other non-LTE phenomenon such as turbulence, shocks and wave-particle interactions.