The Evolution of the 1/<i>f</i> Range within a Single Fast-solar-wind Stream between 17.4 and 45.7 Solar Radii

Abstract

The power spectrum of magnetic field fluctuations in the fast solar wind (VSW > 500 km s−1) at magnetohydrodynamic scales is characterized by two different power laws on either side of a break frequency fb. The low-frequency range at frequencies f smaller than fb is often viewed as the energy reservoir that feeds the turbulent cascade at f > fb. At heliocentric distances r exceeding 60 solar radii (Rs), the power spectrum often has a 1/f scaling at f < fb, i.e., the spectral index is close to −1. In this study, measurements from the Parker Solar Probe's Encounter 10 with the Sun are used to investigate the evolution of the magnetic field power spectrum at f < fb at r < 60 Rs during a fast radial scan of a single fast-solar-wind stream. We find that the spectral index in the low-frequency part of the spectrum decreases from approximately −0.61 to −0.94 as r increases from 17.4 to 45.7 Rs. Our results suggest that the 1/f spectrum that is often seen at large r in the fast solar wind is not produced at the Sun, but instead develops dynamically as the wind expands outward from the corona into the interplanetary medium.