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

Abstract

Alfv\'enicity 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-Alfv\'enic solar wind. We consider scale-dependent measures of Alfv\'enicity based on second-order functions of the magnetic and velocity increments as a function of time lag, including the normalized cross-helicity $\sigma_c$ and residual energy $\sigma_r$. Scale-dependent Alfv\'enicity is strongest for lags near the correlation scale and increases when moving closer to the Sun. We find that $\sigma_r$ typically remains close to the maximally negative value compatible with $\sigma_c$. We did not observe significant changes in measures of Alfv\'enicity between sub-Alfv\'enic and super-Alfv\'enic wind. During most times, the solar wind was highly Alfv\'enic; however, lower Alfv\'enicity 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 Alfv\'enic fluctuations generated near the photosphere transport outward forming highly Alfv\'enic states in the young solar wind and subsequent interactions with large scale structures and gradients leads to weaker Alfv\'enicity, as commonly observed at larger heliocentric distances.