  Whether the phenomenology governing MHD turbulence is Kolmogorov or
Iroshnikov-Kraichnan (IK) remains an open question, theoretically as well as
observationally. The ion heating profile observed in the solar wind provides a
quantitative, if indirect, observational constraint on the relevant
phenomenology. Recently, a solar wind heating model based on Kolmogorov
spectral scaling has produced reasonably good agreement with observations,
provided the effect of turbulence generation due to pickup ions is included in
the model. Without including the pickup ion contributions, the Kolmogorov
scaling predicts a proton temperature profile that decays too rapidly beyond a
radial distance of 15 AU. In the present study, we alter the heating model by
applying an energy cascade rate based on IK scaling, and show that the model
yields higher proton temperatures, within the range of observations, with or
without the inclusion of the effect due to pickup ions. Furthermore, the
turbulence correlation length based on IK scaling seems to follow the trend of
observations better.
