Magnetic Energy Conversion in Magnetically Dominated Systems and Implications for Particle Energization Processes*

The mechanisms and pathways for magnetic energy dissipation are an important subject for many space and astrophysical systems, such as the solar corona and astrophysical jets. Here, we present a new perspective on describing the magnetic energy conversion through the relaxation of magnetic curves and the perpendicular expansion of magnetic lines. By analyzing the evolution of three nonlinear systems, 3D reconnection, kink unstable jets, and magnetized turbulence, we quantify the relative importance of the curvature relaxation vs. the expansion terms. Compressible MHD simulations of various physical systems show that the expansion term often has a more important, sometimes dominant, effect on the energization. In contrast, the curvature relaxation process, despite being crucial during the early evolution, may not be as important in converting magnetic energy overall. We discuss the physical interpretation of these processes and compare 3D MHD and 3D PIC results. Implications for particle energization processes are also explored. In particular, Parker Solar Probe (PSP) provides measurements of transonic velocity fluctuations that can drive significantly more compressible turbulence. We also discuss the dependence of density fluctuations on plasma beta, cross helicity, and polytropic index in the solar wind.