TURBULENT HEATING AND TEMPERATURE EVOLUTION IN THE SOLAR-WIND PLASMA

We calculate the turbulent dissipation rate of incompressive hydromagn etic fluctuations and the resulting radial evolution of temperature in the solar wind using generalizations of Kolmogorov and Kraichnan MHD turbulence phenomenologies that include the suppression of dissipation by high cross helicity. The results for the temperature evolution are compared to a variety of data sets to test the phenomenologies over a wide parameter range. Motivated by the observations, we use different power laws in radius for the amplitudes of Alfvenic and non-Alfvenic fluctuations to determine the cascade rates. To explain the observatio ns using Kolmogorov-like models, we found it necessary to suppress the dissipation rates for the high cross helicity streams even further th an predicted by simple models; this may be due to the nonequilibrium n ature of the spectrum or to other causes as yet unknown. The Kolmogoro v-like model gives rise to a significant amount of turbulent heating, implying that turbulent heating, while likely dominant only in the inn er heliosphere, may be competitive with heating by shocks and the assi milation of interstellar pickup ions in the outer heliosphere. In cont rast, the generalized Kraichnan phenomenology yields less turbulent he ating than the Kolmogorov-like model and seems inadequate to explain t he observations. We conclude that while no existing turbulence model a dequately explains the observed radial dependence of temperature in th e solar wind, there appears to be sufficient energy available for turb ulent heating to contribute significantly, even in the outer heliosphe re.