Wavelet determination of magnetohydrodynamic range power spectral exponents in solar wind turbulence seen by Parker Solar Probe

Abstract

The high Reynolds number solar wind flow provides a natural laboratory for the study of turbulence in-situ. Parker Solar Probe samples the solar wind between 0.2 AU to 1 AU, providing an opportunity to study how turbulence evolves in the expanding solar wind. We obtain the scaling exponents andscale breaks of wavelet power spectra of magnetic field fluctuations sampled by PSP/FIELDS. We identified multiple, long-duration intervals of uniformsolar wind turbulence, selected to exclude coherent structures such as pressure pulses and current sheets, and in which the primary proton populationvelocity varies by less than 20% of its mean value. All selected events span the spectral scales from the approximately 1/f range at low frequencies, through the magnetohydrodynamic (MHD) inertial range of turbulence and into the kinetic range, below the ion gyrofrequency. We estimate the power spectral density (PSD) using a Haar wavelet decomposition which provides accurate estimates of the exponents. There is a clear transition between Kolmogorov −5/3 and Iroshnikov-Kraichnan −3/2 scaling inwards of 0.5 − 0.6AU. In some cases, we find two ranges of scaling within the inertial range, with scaling exponents that can be discriminated within uncertainties in the wavelet PSD. These correspond to relatively small plasma beta. Since the PSD estimated scaling exponents are a central prediction of turbulence theories, these results provide new insights into our understanding of the evolution of turbulence in the solar wind.