A Study on the Transition-Range Spectral Index of the Solar Wind Turbulence in the Inner Heliosphere

Abstract

Based on observations from Parker Solar Probe, this paper studies the dependence of the correlations between the transition-range spectral index of the magnetic energy spectrum and four parameters (inertial-range cross-helicity and magnetic energy density, transition-range magnetic helicity, and bulk proton temperature) on solar wind speed and heliocentric distance. Results show significant correlations between the spectral index and both cross-helicity and magnetic energy density. Notably, at lower speeds or closer distances, the correlation coefficient (CC) between the cross-helicity and spectral index is smaller than that between the magnetic energy density and spectral index. Conversely, at higher speeds or greater distances, the correlation with cross-helicity becomes comparable to or even exceeds that with magnetic energy density. Additionally, as the speed increases or the heliocentric distance decreases, cross-helicity, magnetic energy density, proton temperature, and the absolute values of the spectral index show a mostly upward trend. Moreover, cross-helicity, absolute values of the spectral index, and CC between the magnetic energy density and spectral index exhibit a similar trend with the speed, initially rising and then declining at the highest speed bin. We discuss the results using the recently proposed “helicity barrier” effect.