Measures of Scale-dependent Alfvénicity in the First PSP Solar Encounter

Abstract

The solar wind shows periods of highly Alfvénic activity, where velocity fluctuations and magnetic fluctuations are aligned or antialigned with each other. It is generally agreed that solar wind plasma velocity and magnetic field fluctuations observed by the Parker Solar Probe (PSP) during the first encounter are mostly highly Alfvénic. However, quantitative measures of Alfvénicity are needed to understand how the characterization of these fluctuations compares with standard measures from prior missions in the inner and outer heliosphere, in fast wind and slow wind, and at high and low latitudes. To investigate this issue, we employ several measures to quantify the extent of Alfvénicity—the Alfvén ratio rA, the normalized cross helicity σc, the normalized residual energy σr, and the cosine of angle between velocity and magnetic fluctuations $\cos {\theta }_{{vb}}$. We show that despite the overall impression that the Alfvénicity is large in the solar wind sampled by PSP during the first encounter, during some intervals the cross helicity starts decreasing at very large scales. These length scales (often >1000di) are well inside inertial range, and therefore, the suppression of cross helicity at these scales cannot be attributed to kinetic physics. This drop at large scales could potentially be explained by large scale shears present in the inner heliosphere sampled by PSP. In some cases, despite the cross helicity being constant down to the noise floor, the residual energy decreases with scale in the inertial range. These results suggest that it is important to consider all these measures to quantify Alfvénicity.