Heating of the Solar Wind Beyond 1 AUby

The deposition of energy into the solar wind beyond 1 AU is thought to result from the dissipation of low frequency magnetohydrodynamic (MHD) turbulence via kinetic processes at spatial scales comparable to the ion gyroradius. Beyond 1 AU, solar wind turbulence is comprised of both a decaying component generated in the corona and turbulence generated dynamically in situ by processes such as stream shear, interplanetary shocks, and, beyond the ionization cavity, the pickup of interstellar neutral atoms. A turbulence-theoretic model describing the radial evolution of the power in magnetic uctuations in the solar wind has been developed recently and the predictions were compared successfully with Voyager data. Using the dissipation rate predicted by this model, we evaluate the expected heating of the solar wind by MHD turbulence. The eeective adiabatic index of the solar wind is reduced from 5/3 and the theory accounts for the apparent heating of ions in the outer heliosphere.