  The linear scaling of the mixed third-order moment of the magnetohydrodynamic
fluctuations is used to estimate the energy transfer rate of the turbulent
cascade in the expanding solar wind. In 1976 the Helios 2 spacecraft measured
three samples of fast solar wind originating from the same coronal hole, at
different distance from the sun. Along with the adjacent slow solar wind
streams, these represent a unique database for studying the radial evolution of
turbulence in samples of undisturbed solar wind. A set of direct numerical
simulations of the MHD equations performed with the Lattice-Boltzmann code
FLAME is also used for interpretation. We show that the turbulence energy
transfer rate decays approximately as a power law of the distance, and that
both the amplitude and decay law correspond to the observed radial temperature
profile in the fast wind case. Results from magnetohydrodynamic numerical
simulations of decaying magnetohydrodynamic turbulence show a similar trend for
the total dissipation, suggesting an interpretation of the observed dynamics in
terms of decaying turbulence, and that multi-spacecraft studies of the solar
wind radial evolution may help clarifying the nature of the evolution of the
turbulent fluctuations in the ecliptic solar wind.
