Alfvénicity of velocity and magnetic field Increments Observed by Parker Solar Probe

Alfvénicity is a well-known property of the solar wind. The magnetic and velocity fluctuations are highly correlated over much of the region between the Sun and the Earth. The Parker Solar Probe (PSP) spacecraft enables us to probe Alfvénicity closer to the Sun than before. One previous finding is that the Alfvén ratio increases as the scales become smaller, as shown in some works by using Fourier analysis. This work measures Alfvénicity from PSP observations using 2nd-order structure functions and other quantities derived from magnetic and velocity increments as functions of time lag. We introduce a method to subtract noise from the velocity structure function. We also provide the relation between a time lag in our work and the frequency that contributes most to the 2nd-order structure function for such a time lag. This relation allows a direct comparison between functions of time lag and Fourier spectra. This research has been supported in part by grant RGNS 63-045 from Office of the Permanent Secretary, Ministry of Higher Education, Science, Research and Innovation, and by grant RTA6280002 from Thailand Science Research and Innovation.