Possible Noise Nature of Elsässer Variable z− in Highly Alfvénic Solar Wind Fluctuations

• Published in: Journal of geophysical research. Space physics 2018-01, Vol.123 (1), p.57-67
• Main Authors: Wang, X., Tu, C.‐Y., He, J.‐S., Wang, L.‐H., Yao, S., Zhang, L.
• Format: Article
• Language: English
• Subjects: Alfven waves; Autocorrelation function; Autocorrelation functions; Correlation; Data analysis; Fluctuations; magnetic field; Magnetohydrodynamics; plasma; Solar wind; Spacecraft; Superposition (mathematics); Time lag; Wave propagation; White noise; Wind fluctuations; Wind observation
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1002/2017JA024743

It has been a long‐standing debate on the nature of Elsässer variable z− observed in the solar wind fluctuations. It is widely believed that z− represents inward propagating Alfvén waves and interacts nonlinearly with z+ (outward propagating Alfvén waves) to generate energy cascade. However, z− variations sometimes show a feature of convective structures. Here we present a new data analysis on autocorrelation functions of z− in order to get some definite information on its nature. We find that there is usually a large drop on the z− autocorrelation function when the solar wind fluctuations are highly Alfvénic. The large drop observed by Helios 2 spacecraft near 0.3 AU appears at the first nonzero time lag τ = 81 s, where the value of the autocorrelation coefficient drops to 25%–65% of that at τ = 0 s. Beyond the first nonzero time lag, the autocorrelation coefficient decreases gradually to zero. The drop of z− correlation function also appears in the Wind observations near 1 AU. These features of the z− correlation function may suggest that z− fluctuations consist of two components: high‐frequency white noise and low‐frequency pseudo structures, which correspond to flat and steep parts of z− power spectrum, respectively. This explanation is confirmed by doing a simple test on an artificial time series, which is obtained from the superposition of a random data series on its smoothed sequence. Our results suggest that in highly Alfvénic fluctuations, z− may not contribute importantly to the interactions with z+ to produce energy cascade. Key Points A large drop is observed on the correlation function of z− in highly Alfvenic fluctuations by Helios and Wind The autocorrelation function of an artificial random data series is shown to be similar to the observed ones z− is suggested to be composed of high‐frequency white noise and low‐frequency pseudostructures in the studied cases