  The role of compressible fluctuations in the energy cascade of fast solar
wind turbulence is studied using a reduced form of an exact law derived
recently (Banerjee and Galtier, PRE, 2013) for compressible isothermal
magnetohydrodynamics and in-situ observations from the THEMIS B/ARTEMIS P1
spacecraft. A statistical survey of the data revealed a turbulent energy
cascade over two decades of scales, which is broader than the previous
estimates made from an exact incompressible law. A term-by-term analysis of the
compressible model reveals new insight into the role played by the compressible
fluctuations in the energy cascade. The compressible fluctuations are shown to
amplify (2 to 4 times) the turbulent cascade rate with respect to the
incompressible model in 10 % of the analyzed samples. This new estimated
cascade rate is shown to provide the adequate energy dissipation required.
