Symbolic analysis of slow solar wind data using rank order statistics

We analyze time series data of the fluctuations of slow solar wind velocity using rank order statistics. We selected a total of 18 data sets measured by the Helios spacecraft at a distance of 0.32 AU from the sun in the inner heliosphere. The data sets correspond to the years 1975–1982 and cover the end of the solar activity cycle 20 to the middle of the activity cycle 21. We first apply rank order statistics to time series from known nonlinear systems and then extend the analysis to the solar wind data. We find that the underlying dynamics governing the solar wind velocity remains almost unchanged during an activity cycle. However, during a solar activity cycle the fluctuations in the slow solar wind time series increase just before the maximum of the activity cycle.