Two Types of $1/f$ Range in Solar Wind Turbulence

TL;DR

This paper identifies two distinct types of 1/f noise in solar wind turbulence, linking their properties to different solar wind conditions and revealing new insights into their origins and evolution.

Contribution

It systematically characterizes two types of 1/f ranges in solar wind turbulence and explores their distinct properties and origins, especially in relation to Alfvénic and slow/mixed wind types.

Findings

Fast/Alfvénic wind exhibits WKB evolution of fluctuations.

Slow/mixed wind shows solar cycle dependence of 1/f noise.

Autocorrelation reveals relationship between 1/f range and magnetic field coherence.

Abstract

The $1/f$ noise is a ubiquitous phenomenon in natural systems. Since the advent of space exploration, the $1/f$ range has been consistently observed in \textit{in situ} solar wind measurements throughout the heliosphere, sparking decades of debate regarding its origin. Recent Parker Solar Probe (PSP) observations near the Alfv\'en surface have revealed a systematic absence of the $1/f$ range in pristine solar wind, providing a unique opportunity to investigate its origin in solar wind turbulence. Despite numerous observations of the $1/f$ range at varying frequencies, no study has systematically examined its properties across different solar wind conditions. Here, we identify two distinct types of $1/f$ ranges in solar wind turbulence: the fast/Alfv\'enic wind type and the slow/mixed wind type. The fast/Alfv\'enic type appears to be an intrinsic feature of Alfv\'enic turbulence, while the slow/mixed type resembles classical flicker noise. For the fast/Alfv\'enic type, we find a near-perfect WKB evolution of the frequency-averaged fluctuation amplitude and an intriguing migration pattern in frequency space. For the slow/mixed type, we examine the solar cycle dependence of the $1/f$ noise using the OMNI-LRO dataset spanning solar cycles 22 to 25. We also analyze the autocorrelation function of the magnetic field vectors and identify a clear relationship between the $1/f$ range and the decline in correlation, as well as unexpected resonance peaks in the autocorrelation function.