Alfvénic fluctuations in fast and slow solar winds

Abstract

We compute properties of Alfvénic fluctuations within a wide frequency range (i.e., 10^-6s^-1<~f<~10^-2s^-1) in both high- and low-speed magnetized radial solar winds. In particular, the radial and frequency variations of the normalized cross helicity σ_c, the Alfvén ratio <script>R</script>_A, and other relevant quantities associated with Alfvénic fluctuations are presented and analyzed. Because of the existence of the Alfvén critical radius r_A and a characteristic frequency f_c for a given magnetized solar wind profile, continuous reflection effects are clearly manifest at frequencies lower than several times f_c. Since outward propagating fluctuations which many authors consider Alfvénic fluctuations have been detected in the high-latitude solar wind (which is fast and steady), such continuous reflection effects in the low-frequency end of the fluctuation spectrum may be found in the Ulysses data. In order to strengthen our case, comparisons are carried out between our theoretical results and previous data analyses of observations from the Helios 1 and 2 spacecraft in the radial range of 0.29AU<~r<~1.0AU. Stronger evidence for continuous reflection effects of low-frequency Alfvénic fluctuations is found in the high-speed solar wind close to the Sun than in the low-speed solar wind in general.