  The effect of compressibility in charged particle energization by
magnetohydrodynamic (MHD) fields is studiedin the context of test particle
simulations. This problem is relevant to the solar wind and the solar corona
due to the compressible nature of the flow in those astrophysical scenarios. We
consider turbulent electromagnetic fields obtained from direct numerical
simulations of the MHD equations with a strong background magnetic field. In
order to explore the flow compressibilty effect over the particle dynamics we
performed different numerical experiments: an incompressible case, and two weak
compressible cases with Mach number M = 0.1 and M = 0.25. We analyze the
behavior of protons and electrons in those turbulent fields, which are well
known to form aligned current sheets in the direction of the guide magnetic
field. What we call protons and electrons are test particles with scales
comparable to (for protons) and much smaller than (for electrons) the
dissipative scale of MHD turbulence, maintaining the correct mass ratio me /mi.
For these test particles we show that compressibility enhances the efficiency
of proton acceleration, and that the energization is caused by perpendicular
electric fields generated between currents sheets. On the other hand, electrons
remain magnetized and display an almost adiabatic motion, with no effect of
compressibility observed. Another set of numerical experiments takes into
account two fluid modifications, namely electric field due to Hall effect and
electron pressure gradient. We show that the electron pressure has an important
contribution to electron acceleration allowing highly parallel energization. In
contrast, no significant effect of these additional terms is observed for the
protons.
