On the radial decrease of Alfvenic fluctuations in the polar wind

Plasma and magnetic field measurements by Ulysses are used to investigate the radial variation of Alfenic fluctuations in the polar wind. The data span from 1.4 to 4.3 AU in heliocentric distance. Different radial regimes at different distances emerge. Inside about 2.5 AU the large outward travelling fluctuations decrease faster, in terms of energy per unit mass, than the small inward ones. This is in agreement with previous low-latitude observations inside 1 AU within the trailing edge of fast streams. As a result of this different gradient the ratio of the inward to outward fluctuation energy rises to about 0.5 near 2.5 AU. Beyond this distance the radial gradient of the inward fluctuations becomes increasingly steeper, while that of the outward fluctuations does not vary appreciably. A state is quickly reached where both components decline at almost the same rate. These observations represent a constraint for models of Alfvenic turbulence evolution in steadily expanding hows like the polar wind.