A New Way for Walen Test of Alfvenic Fluctuations in Solar Wind Streams via EEMD

Pure Alfven waves at 1 au can be effectively identified by a Walen 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 Alfven fluctuations in two ways: (1) the Walen test relied on a time-dependent frame, which is extracted from the trend of differences between plasma and Alfven velocity; and (2) a modified Walen test of the reconstructed Alfvenic and plasma velocity fluctuations by EEMD, after removing the high/low-frequency components irrelevant to Alfvenicity. 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 Alfvenic parameters such as the Walen slope, the standard deviation ratio, the Alfven 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 Alfvenic 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 Walen 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 Alfven waves in solar wind streams with single-satellite data.