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

Ulysses magnetic and plasma data are used to study hourly scale Alfvénic fluctuations in the solar polar wind. The calculated energy ratio R²_VA(cal) of inward to outward Alfvén waves is obtained from the observed Walén slope through an analytical expression, and the observed R²_VA(obs) is based on a direct decomposition of original Alfvénic fluctuations into outward- and inward-propagating Alfvén waves. The radial variation of R²_VA(cal) shows a monotonically increasing trend with heliocentric distance r, implying the increasing local generation or contribution of inward Alfvén waves. The contribution is also shown by the radial increase in the occurrence of dominant inward fluctuations. We further pointed out a higher occurrence (∼80% of a day in average) of dominant outward Alfvénic fluctuations in the solar wind than previously estimated. Since R²_VA(cal) is more accurate than R²_VA(obs) in the measurement of the energy ratio for dominant outward fluctuations, the values of R²_VA(cal) in our results are likely more realistic in the solar wind than those previously estimated as well as R²_VA(obs) in our results. The duration ratio R_T of dominant inward to all Alfvénic 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 Alfvénic fluctuations therein compared with previous studies and put constraints on modeling the variation of solar wind fluctuations.