Radial Variations of Outward and Inward Alfvénic Fluctuations Based on Ulysses Observations

Ulysses magnetic and plasma data are used to study hourly scale Alfvenic fluctuations in the solar polar wind. The calculated energy ratio R-vA(2) (cal) of inward to outward Alfven waves is obtained from the observed Walen slope through an analytical expression, and the observed R-vA(2) (obs) is based on a direct decomposition of original Alfvenic fluctuations into outward-and inward-propagating Alfven waves. The radial variation of R-vA(2) (cal) shows a monotonically increasing trend with heliocentric distance r, implying the increasing local generation or contribution of inward Alfven waves. The contribution is also shown by the radial increase in the occurrence of dominant inward fluctuations. We further pointed out a higher occurrence (similar to 83% of a day in average) of dominant outward Alfvenic fluctuations in the solar wind than previously estimated. Since R-vA(2) (cal) is more accurate than R-vA(2) (obs) in the measurement of the energy ratio for dominant outward fluctuations, the values of R-vA(2) (cal) in our results are likely more realistic in the solar wind than those previously estimated as well as R-vA(2) (obs) in our results. The duration ratio R-T of dominant inward to all Alfvenic fluctuations increases monotonically with r, and is about two or more times that from Voyager 2 observations at r >= 4 au. These results reveal new qualitative and quantitative features of Alfvenic fluctuations therein compared with previous studies and put constraints on modeling the variation of solar wind fluctuations.