Inherentness of Non-stationarity in Solar Wind

Most studies of turbulence in the solar wind invoke stationarity as a working hypothesis. Unfortunately, this concept is difficult to verify in practice. To investigate the validity of the weak stationarity assumption we consider magnetic field measurements made by the WIND satellite and study the properties of the autocorrelation function (ACF), which is a classical gauge for characteristic times or scales. We find that the ACF suffers from a high variance, which precludes the routine interpretation of correlation times and scales. In addition, the ACF fails to converge toward a constant function, even when considering the longest available intervals of either fast or slow solar wind. The reasons behind this lack of convergence are better understood by considering the power spectral density (PSD) of the magnetic field and analyzing synthetic data that exhibit the same PSD. Interestingly, we find evidence for an f −1 scaling at low frequencies in both fast and slow solar winds. These results, together with the theoretical properties of processes with f −γ scaling all point to the non-stationary behavior of the solar wind, in particular for scales that correspond to the inertial range. They also impose strong constraints on the applicability of ACF analysis as a tool for characterizing statistical properties of solar wind turbulence.