Authors: M. Hasan Barbhuiya (West Virginia University), Paul Cassak (West Virginia University), Subash Adhikari (University of Delaware), Tulasi Parashar (Victoria University of Wellington), Haoming Liang (University of Maryland College Park), Matthew Argall (University of New Hampshire)

Kinetic processes in collisionless space plasmas generate phase-space densities that can be arbitrarily different than Maxwellian phase space densities, i.e., arbitrarily far from local thermodynamic equilibrium (LTE). In phenomena like reconnection and turbulence—where non-Maxwellian phase space densities are routinely seen in both spacecraft data and simulations—the fluid-scale energy evolution measures of pressure–strain interaction and the divergence of the vector heat flux density quantify the how internal energy density evolves. Here, we introduce the “higher-order nonequilibrium term” (HORNET) power density: an effective power density that quantifies the rate of change of the departure of a phase space density from LTE. HORNET has units of power density, enabling direct, quantitative comparison with established terms, such as pressure-strain interaction and the divergence of the vector heat flux density. We compute HORNET using two-dimensional particle-in-cell simulations of anti-parallel reconnection and decaying kinetic turbulence in collisionless, magnetized plasmas. We map the spatial distribution of HORNET and other power densities and track the time evolution of their volume-averaged value. By comparing HORNET with power densities describing changes to the internal energy (pressure dilatation, Pi − D, and divergence of the vector heat flux density), we find that HORNET can be a significant fraction of these other measures (8% and 35% for electrons and ions, respectively, for reconnection; up to 67% for both electrons and ions for turbulence), meaning evolution of the system towards or away from LTE can be dynamically important.

 

 

[1]: M.H. Barbhuiya, P.A. Cassak, S. Adhikari, T.N. Parashar, H. Liang, and M.R. Argall, “Higher-order nonequilibrium term: Effective power density quantifying evolution towards or away from local thermodynamic equilibrium,” Phys. Rev. E 109(1), 015205 (2024).