B. Bavassano et al., Radial evolution of outward and inward Alfvenic fluctuations in the solar wind: A comparison between equatorial and polar observations by Ulysses, J GEO R-S P, 106(A6), 2001, pp. 10659-10668
Ulysses measurements done during the ecliptic phase of the mission are used
to investigate the radial evolution of outward and inward Alfvenic fluctua
tions 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 distanc
es. In the present analysis a total of 30 time intervals, characterized by
highly Alfvenic fluctuations, are examined, for a total duration of 2558 ho
urs. They are quite uniformly distributed from 1.2 to 5.2 AU along the ecli
ptic trajectory of Ulysses. The results clearly indicate that in the invest
igated radial range the energy per unit mass of the dominant outward propag
ating fluctuations declines, for increasing distance, with approximately th
e same rate observed for inward fluctuations. In other words: the ratio of
inward to outward fluctuation energy roughly remains the same in the examin
ed region. Moreover, the gradient does not vary appreciably with radial dis
tance. These features indicate that between 1 and 5 AU the Alfvenic fluctua
tions 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 ev
olution at hourly scale. Our results, combined with previous observations b
y Hellos spacecraft, also suggest the likely presence of solar cycle effect
s.