Beta-dependent electron heat flux constraints. The two panels show 2-d frequency distributions of normalized electron heat flux and β_|| from heliocentric distances <0.25 AU from the first two PSP orbits (top panel) and from the fourth and fifth orbits (bottom panel). Orange lines indicate theoretical thresholds for the whistler fan instability (Vasko et al. 2019) and green lines indicate an empirical threshold (solid) and a theoretical threshold (dashed) for the whistler heat flux instability (Gary et al. 1999).