Radial evolution of outward and inward Alfvenic fluctuations in the solar wind: A comparison between equatorial and polar observations by Ulysses

Ulysses measurements done during the ecliptic phase of the mission are used to investigate the radial evolution of outward and inward Alfvenic fluctuations at hourly scale in near-equatorial solar wind. This analysis has been stimulated by a recent study on Alfvenic turbulence in polar wind, showing that at hourly scale different radial regimes develop at different distances. In the present analysis a total of 30 time intervals, characterized by highly Alfvenic fluctuations, are examined, for a total duration of 2558 hours. They are quite uniformly distributed from 1.2 to 5.2 AU along the ecliptic trajectory of Ulysses. The results clearly indicate that in the investigated radial range the energy per unit mass of the dominant outward propagating fluctuations declines, for increasing distance, with approximately the same rate observed for inward fluctuations. In other words: the ratio of inward to outward fluctuation energy roughly remains the same in the examined region. Moreover, the gradient does not vary appreciably with radial distance. These features indicate that between 1 and 5 AU the Alfvenic fluctuations have a quite different behavior in polar and in near-equatorial solar wind. All this should imply a different role, in the two kinds of wind, of the mechanisms expected to be active in driving the Alfvenic turbulence evolution at hourly scale. Our results, combined with previous observations by Hellos spacecraft, also suggest the likely presence of solar cycle effects.