Panels (a)–(e): proton PADs in the magnetic-field frame at five selected times on 2023 March 13 and seven EPI-Lo energy ranges, spanning the preshock and foreshock evolution (time intervals labeled at the top). Positive (negative) μ_B corresponds to particles moving purely antisunward (sunward) along the local magnetic field (μ_B = 0 is perpendicular to B). Dashed curves show the best-fit Legendre expansion for each energy. Panels (f) and (g): the 10 minute time evolution of the proton pitch-angle anisotropy from 03:00 to 12:00 UT on 2023 March 13. Panel (f) shows the first-order anisotropy A₁ (net streaming, with A₁ > 0 antisunward and A₁ < 0 sunward), and panel (g) shows the second-order anisotropy A₂ (A₂ > 0 bidirectional along ±B, A₂ < 0 pancake perpendicular to B). Panel (h): PSP/FIELDS magnetic-field components in RTN coordinates as in Figure 1. The five vertical lines correspond to the five PAD snapshot times.