Cross-scale Correlations in Imbalanced Solar Wind Turbulence: Parker Solar Probe Observations

Abstract

Based on Parker Solar Probe observations, this paper investigates the proton temperature, bulk speed, proton-scale magnetic helicity, and spectral index in the parameter space of (P I , σ c ), where P I is the magnetic energy density and σ c is the normalized cross helicity at proton inertial scales. Various correlations between these parameters are discussed and their relations are explored. These correlations indicate the following: (1) the turbulence with a larger P I is characterized by a higher σ c ; (2) a larger P I corresponds to a higher proton temperature, faster bulk speed, and steeper proton-scale magnetic spectrum; (3) a higher σ c accounts for higher proton-scale magnetic helicity. In particular, the P I has the largest correlation coefficient (CC) of 0.85 with proton temperature and has a considerably large CC of 0.70 with a proton-scale spectral index. The σ c has a moderate CC of 0.52 with magnetic helicity in the low β p case (β p < 0.6), where β p is the ratio of plasma to magnetic pressure. The correlation between σ c and P I is considerable with a CC exceeding 0.6 in the low β p case and tends to be negligible when β p approaches 1.5. These findings could be understood by the recently discovered “helicity barrier” effect and underline the importance of the generalized helicity invariant in low β p collisionless plasma for understanding imbalanced solar wind turbulence.