A New Way for Walén Test of Alfvénic Fluctuations in Solar Wind Streams via EEMD

Abstract

Abstract Pure Alfvén waves at 1 au can be effectively identified by a Walén test of plasma and magnetic field observations. In this study, a new method, based on the Ensemble Empirical Mode Decomposition (EEMD) technique, is proposed for analyzing Alfvén fluctuations in two ways: (1) the Walén test relied on a time-dependent frame, which is extracted from the trend of differences between plasma and Alfvén velocity; and (2) a modified Walén test of the reconstructed Alfvénic and plasma velocity fluctuations by EEMD, after removing the high/low-frequency components irrelevant to Alfvénicity. Referencing the three validated methods mentioned in Chao et al., the same four cases in high-speed solar wind steams are tested by the EEMD method, considering Alfvénic parameters such as the Walén slope, the standard deviation ratio, the Alfvén ratio, the normalized cross helicity, the normalized residual energy, and the mean deviation relative to 1. The test results indicate that the first EEMD method can improve most Alfvénic parameters, since a time-varying velocity of the De Hoffmann–Teller frame is obtained from the process of self-adaptive data analysis, especially for complex cases with nonstationary and multiple background frames. The second EEMD method provides a kind of flexible testing to reconstruct the optimal fluctuations satisfied for the Walén relation, which achieves similar or even better results than normal prior methods based on bandpass filtering. We suggest that the EEMD method can be an alternative way to identify large-amplitude Alfvén waves in solar wind streams with single-satellite data.