On the evolution of outward and inward Alfvenic fluctuations in the polar wind

Plasma and magnetic field measurements by Ulysses are used to investigate t he radial evolution of hourly-scale Alfvenic fluctuations in the polar wind . The data span from 1.4 to 4.3 AU in heliocentric distance. Different radi al regimes at different distances emerge. Inside about 2.5 AU the large out ward traveling fluctuations decrease faster, in terms of energy per unit ma ss, than the small inward ones. This is in agreement with previous low-lati tude observations inside 1 AU within the trailing edge of fast streams. As a result of this different gradient the ratio of inward to outward fluctuat ion energy rises to about 0.5 near 2.5 AU. Beyond this distance the radial gradient of the inward fluctuations becomes increasingly steeper, while tha t of the outward ones does not vary appreciably. A state is quickly reached where both populations decline at almost the same rate. These results on t he behavior of outward and inward Alfvenic fluctuations are new and represe nt a constraint for models of turbulence evolution in steadily expanding fl ows like the polar wind. Finally, an extrapolation to regions near the Sun would suggest that Alfvenic fluctuations at hourly scale should not play a relevant role in solar wind heating and acceleration. Obviously, this last conclusion may be invalidated by non-WKB effects and by compressive and dis sipative processess close to the Sun.