A new approach to identify interplanetary Alfvén waves and to obtain their frequency properties

Conventional diagnosis of interplanetary Alfven waves requires an accurately determined de Hoffmann-Teller (HT) frame or background magnetic field. For simplicity, the averaged HT frame and the mean value of the magnetic field are often used in the literature. However, HT frame can change quite fast in high-speed solar wind streams, and it is not always appropriate to take the average value of the magnetic field to be the background state. In order to reduce the uncertainty introduced by determining HT frame and background magnetic field, we propose a new approach for identifying large-amplitude interplanetary Alfven waves. This new approach is independent of HT frame and of background magnetic field. Instead of the original data sets, the band-pass filtered signals of plasma velocity and magnetic field observations are used to check the Walen relation. The robustness of this technique is verified by applying to simulated pure Alfven waves with two separate frequencies and contaminated by pink colored noises in a varying solar wind stream. Furthermore, in our approach, more properties of Alfven waves in frequency domain can be obtained, which have been rarely discussed before. Our analysis technique is applied to two intervals of solar wind high-speed streams, and it is shown that large-amplitude Alfven waves near 1AU are frequently found during these two intervals.