Scale and Time Dependence of Alfvénicity in the Solar Wind as Observed by the Parker Solar Probe

Alfvenicity 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 the sub-Alfvenic solar wind. We consider scale-dependent measures of Alfvenicity 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 Alfvenicity 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 Alfvenicity between sub-Alfvenic and super-Alfvenic wind. During most times, the solar wind was highly Alfvenic; however, lower Alfvenicity 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 Alfvenic fluctuations generated near the photosphere transport outward, forming highly Alfvenic states in the young solar wind, and subsequent interactions with large-scale structures and gradients lead to weaker Alfvenicity, as commonly observed at larger heliocentric distances.