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

E-ISSN： 2169-9402|123|1|57-67

ISSN： 2169-9380

Source： JOURNAL OF GEOPHYSICAL RESEARCH: SPACE PHYSICS, Vol.123, Iss.1, 2018-01, pp. : 57-67

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Abstract

AbstractIt 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.

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