New Observations of Solar Wind 1/f Turbulence Spectrum from Parker Solar Probe

TL;DR

This study analyzes Parker Solar Probe data to understand the origin and evolution of the solar wind's 1/f turbulence spectrum, revealing its dynamic formation and coexistence with other mechanisms.

Contribution

It provides new observational evidence on the radial evolution of the 1/f spectrum and supports the dynamic origin hypothesis through spectra analysis and comparison with models.

Findings

Spectra shallower than 1/f near the sun evolve towards 1/f with distance.

Evidence of triple power law spectra consistent with parametric decay models.

Supports the dynamic formation of the 1/f spectrum in the solar wind.

Abstract

The trace magnetic power spectrum in the solar wind is known to be characterized by a double power law at scales much larger than the proton gyro-radius, with flatter spectral exponents close to -1 found at the lower frequencies below an inertial range with indices closer to $[-1.5,-1.6]$. The origin of the $1/f$ range is still under debate. In this study, we selected 109 magnetically incompressible solar wind intervals ($\delta |\boldsymbol B|/|\boldsymbol B| \ll 1$) from Parker Solar Probe encounters 1 to 13 which display such double power laws, with the aim of understanding the statistics and radial evolution of the low frequency power spectral exponents from Alfv\'en point up to 0.3 AU. New observations from closer to the sun show that in the low frequency range solar wind turbulence can display spectra much shallower than $1/f$, evolving asymptotically to $1/f$ as advection time increases, indicating a dynamic origin for the $1/f$ range formation. We discuss the implications of this result on the Matteini et al. (2018) conjecture for the $1/f$ origin as well as example spectra displaying a triple power law consistent with the model proposed by Chandran et al. (2018), supporting the dynamic role of parametric decay in the young solar wind. Our results provide new constraints on the origin of the $1/f$ spectrum and further show the possibility of the coexistence of multiple formation mechanisms.