Possible Noise Nature of Elsasser Variable z- in Highly Alfvenic Solar Wind Fluctuations

It has been a long-standing debate on the nature of Elsasser variable z(-) observed in the solar wind fluctuations. It is widely believed that z(-) represents inward propagating Alfven waves and interacts nonlinearly with z(+) (outward propagating Alfven 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 Alfvenic. The large drop observed by Helios 2 spacecraft near 0.3AU appears at the first nonzero time lag r = 81s, where the value of the autocorrelation coefficient drops to 25%-65% of that at r = 0s. 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 1AU. 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 Alfvenic fluctuations, z(-) may not contribute importantly to the interactions with z(+) to produce energy cascade.