CROSS-HELICITY AND RESIDUAL ENERGY IN SOLAR-WIND TURBULENCE - RADIAL EVOLUTION AND LATITUDINAL DEPENDENCE IN THE REGION FROM 1 TO 5 AU

Solar wind plasma and magnetic field measurements by Ulysses have been used to study magnetohydrodynamic turbulence in different heliospheri c regions. Four intervals of six solar rotations have been analyzed. T wo of them are on the ecliptic around 2 and 5 AU, respectively, one is at midlatitude near 5 AU, and the last one is at high latitude around 3 AU. Conditions on the ecliptic are those typical of high solar acti vity periods. The midlatitude interval is characterized by very strong gradients in the wind speed, due to an intermittent appearance of the wind coming from the polar coronal hole. In the high-latitude interva l, fully inside the polar wind, the speed is steadily high. We investi gated at three different scales (1, 4, and 12 hours) the level of corr elation between velocity and magnetic field fluctuations, as given by the normalized cross-helicity, and the sharing of the fluctuation ener gy between its kinetic and magnetic component, as measured by the norm alized residual energy. The observations on the ecliptic, while confir ming previous findings based on Voyagers data, clearly indicate that t he normalized cross-helicity is well different from zero also at dista nces as large as 5 AU. The midlatitude turbulence, when compared to th at at low and high heliographic latitudes, appears much more evolved, with a remarkably lower normalized cross-helicity (in absolute value). This unambiguously highlights that processes at velocity gradients ar e an important factor in the turbulence evolution. For all the analyze d intervals the residual energy values indicate an imbalance in favor of magnetic fluctuations, in agreement with previous results. The stro ngest imbalance is observed for the high-latitude sample, where the tu rbulence is comparatively the least evolved. This is a quite unexpecte d result, probably related to the presence of interstellar pickup ion populations. In conclusion, our analysis indicates that (1) velocity g radients play a dominant role in driving the turbulence evolution in t he solar wind and (2) pickup ion effects might be significant.