Two Types of 1/<i>f</i> Range in Solar Wind Turbulence

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 in situ solar wind measurements throughout the heliosphere, sparking decades of debate regarding its origin. Recent Parker Solar Probe observations near the Alfvén surface have revealed a systematic absence of the 1/ f range above 10 −4 Hz 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/Alfvénic wind type and the slow/mixed wind type. The fast/Alfvénic type appears to be an intrinsic feature of Alfvénic turbulence, while the slow/mixed type resembles classical flicker noise. For the fast/Alfvénic 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) data set spanning solar cycles 22–25. 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.