Statistical evolution of the electron density n_e (in cm⁻³) measured by MEA1 with respect to the distance to the Sun. The colormap represents a 2D distribution, with 25 bins for the distance and 100 bins for the density. n_e is estimated from MEA by removing the contribution of the secondary electrons. For each distance bin, the density distribution is normalized by its maximum value. The solid black line links all the density maxima at each distance bin. Three power-law fits are represented: the black dashed line, the blue stars, and the purple diamonds for the Sun et al. (2022) fit, n_e without the secondary electrons removed, and n_e with the secondary electrons removed, respectively.