The Evolution of the 1/f 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 ($V_{\rm SW}> 500 \mbox{ km} \mbox{ s}^{-1}$) at magnetohydrodynamic (MHD) scales is characterized by two different power laws on either side of a break frequency $f_{\mathrm{b}}$. The low-frequency range at frequencies $f$ smaller than $f_{\mathrm{b}}$ is often viewed as the energy reservoir that feeds the turbulent cascade at $f>f_{\mathrm{b}}$. At heliocentric distances $r$ exceeding $60$ solar radii ($R_{\mathrm{s}}$), the power spectrum often has a $1/f$ scaling at $f<f_{\mathrm{b}}$; i.e., the spectral index is close to $-1$. In this study, measurements from the encounter $10$ of $ParkerSolarProbe$ (PSP) with the Sun are used to investigate the evolution of the magnetic-field power spectrum at $f<f_{\mathrm{b}}$ at $r<60R_{\mathrm{s}}$ 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 $45.7$ solar radii. 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.

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Cited by in corpus (6)

• Prediction and Verification of Parker Solar Probe Solar Wind Sources at 13.3 R${}_{\odot }$
• Dominance of 2-Minute Oscillations near the Alfvén Surface
• Formation of Magnetic Switchbacks via expanding Alfvén Waves
• Solar Wind Structures from the Gaussianity of Magnetic Magnitude
• Two Types of $1/f$ Range in Solar Wind Turbulence
• Interplanetary magnetic correlation and low-frequency spectrum over many solar rotations