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The effect of compressibility in charged particle energization bymagnetohydrodynamic (MHD) fields is studiedin the context of test particlesimulations. This problem is relevant to the solar wind and the solar coronadue to the compressible nature of the flow in those astrophysical scenarios. Weconsider turbulent electromagnetic fields obtained from direct numericalsimulations of the MHD equations with a strong background magnetic field. Inorder to explore the flow compressibilty effect over the particle dynamics weperformed different numerical experiments: an incompressible case, and two weakcompressible cases with Mach number M = 0.1 and M = 0.25. We analyze thebehavior of protons and electrons in those turbulent fields, which are wellknown to form aligned current sheets in the direction of the guide magneticfield. What we call protons and electrons are test particles with scalescomparable to (for protons) and much smaller than (for electrons) thedissipative scale of MHD turbulence, maintaining the correct mass ratio me /mi.For these test particles we show that compressibility enhances the efficiencyof proton acceleration, and that the energization is caused by perpendicularelectric fields generated between currents sheets. On the other hand, electronsremain magnetized and display an almost adiabatic motion, with no effect ofcompressibility observed. Another set of numerical experiments takes intoaccount two fluid modifications, namely electric field due to Hall effect andelectron pressure gradient. We show that the electron pressure has an importantcontribution to electron acceleration allowing highly parallel energization. Incontrast, no significant effect of these additional terms is observed for theprotons.

On the compressibility effect in test particle acceleration by magnetohydrodynamic turbulence