(a) Two-dimensional histogram of the spacecraft floating potential as a function of radial distance (in solar radii, R_S) from the Sun. Each radial distance bin of the histogram is normalized by the maximum of its distribution. (b) Voltage corresponding to the peak counts in each radial bin are plotted against radial distance (black). Spacecraft charging model predictions (green, red, blue star) are overplotted for comparison. (c) Number of data samples per radial distance bin of the two-dimensional histogram.