  Estimation of the energy cascade rate in the inertial range of solar wind
turbulence has been done so far mostly within the incompressible
magnetohydrodynamics (MHD) theory. Here, we go beyond that approximation to
include plasma compressibility using a reduced form of a recently derived exact
law for compressible, isothermal MHD turbulence. Using in-situ data from the
THEMIS/ARTEMIS spacecraft in the fast and slow solar wind, we investigate in
detail the role of the compressible fluctuations in modifying the energy
cascade rate with respect to the prediction of the incompressible MHD model. In
particular, we found that the energy cascade rate: i) is amplified particularly
in the slow solar wind; ii) exhibits weaker fluctuations in spatial scales,
which leads to a broader inertial range than the previous reported ones; iii)
has a power law scaling with the turbulent Mach number; iv) has a lower level
of spatial anisotropy. Other features of solar wind turbulence are discussed
along with their comparison with previous studies that used incompressible or
heuristic (non exact) compressible MHD models.
