Constraints on electron strahl parameters in the near-Sun environment. The two panels show 2-d frequency distributions of normalized strahl velocity and fractional strahl density from heliocentric distances < 0.25 AU from the first two PSP orbits (top panel) and from the fourth and fifth orbits (bottom panel). The dashed red line indicates a theoretical threshold for an oblique fast magnetosonic instability (Verscharen et al. 2019).