In situ measurements from PSP during Encounter 15. The gray dashed line indicates the selected data point for the left panel of Figure 3. (a) The magnetic field magnitude. (b) The magnetic field magnitude and three components in RTN coordinates. (c) Solar wind velocity components in RTN coordinates. The blue dashed line is the background velocity v₀ defined by 10% lower data average. (d), (e), and (f) Three components of magnetic and velocity fluctuations. Magnetic field perturbations are normalized by ﹩\delta b=\frac{\delta B}{\sqrt{{\mu }_{0}\rho }}﹩. (g) The electron density data from quasi-thermal noise spectroscopy. (h) The propagation speed of Alfvén waves calculated by two different methods.