Theoretical Developments on Energy Conversion via the Pressure-Strain Interaction

Energy conversion between bulk kinetic and thermal energy in weakly collisional and collisionless plasma processes such as magnetic reconnection and plasma turbulence has recently been the subject of intense scrutiny. This channel of energy conversion is described by the pressure-strain interaction. In a closed system, this quantity accounts for all the net change of the thermal energy. It is common to decompose it into pressure dilatation and «Pi-D», which isolates energy conversion via compressible and incompressible physics, respectively. Here, we propose an alternative decomposition of pressure-strain interaction that instead isolates flow convergence/divergence and bulk flow shear. We furnish a simple example to illustrate how Pi-D can be counterintuitive and the new decomposition is intuitive. Moreover, for applications to magnetized plasmas, we derive the pressure-strain interaction in a magnetic field-aligned coordinate system. This results in its decomposition into eight terms, each with a different physical mechanism that changes the thermal energy. Results from particle-in-cell simulations of two-dimensional magnetic reconnection plotting the decompositions in both Cartesian and magnetic field-aligned coordinates are shown. We identify the mechanisms contributing to heating and cooling during reconnection. The results of this study are readily applicable for interpreting numerical and observational data of pressure-strain interaction in both Cartesian and field-aligned coordinates in fundamental plasma processes such as reconnection, turbulence and collisionless shocks.