Scale and Time Dependence of Alfvénicity in the Solar Wind as Observed by {\it Parker Solar Probe}

Abstract

Alfvénicity is a well-known property, common in the solar wind, characterized by a high correlation between magnetic and velocity fluctuations. Data from the Parker Solar Probe (PSP) enable the study of this property closer to the Sun than ever before, as well as in sub-Alfvénic solar wind. We consider scale-dependent measures of Alfvénicity based on second-order functions of the magnetic and velocity increments as a function of time lag, including the normalized cross-helicity $σ_c$ and residual energy $σ_r$. Scale-dependent Alfvénicity is strongest for lags near the correlation scale and increases when moving closer to the Sun. We find that $σ_r$ typically remains close to the maximally negative value compatible with $σ_c$. We did not observe significant changes in measures of Alfvénicity between sub-Alfvénic and super-Alfvénic wind. During most times, the solar wind was highly Alfvénic; however, lower Alfvénicity was observed when PSP approached the heliospheric current sheet or other magnetic structures with sudden changes in the radial magnetic field, non-unidirectional strahl electron pitch angle distributions, and strong electron density contrasts. These results are consistent with a picture in which Alfvénic fluctuations generated near the photosphere transport outward forming highly Alfvénic states in the young solar wind and subsequent interactions with large scale structures and gradients leads to weaker Alfvénicity, as commonly observed at larger heliocentric distances.