Error estimates for distributions with non-zero temperatures and for distributions subjected to heating. (a) Proton distribution functions: Upstream 10 eV distribution, drifting at 380 km s⁻¹ (solid black line); the same population downstream at U = −500 V (dashed black line); the upstream population heated by 100 eV (solid red line); and the same population downstream of the potential drop (dashed red line). (b) solar wind speed error due to non-zero upstream temperatures. (c) potential difference error due to non-zero upstream temperatures. (d) solar wind speed error due to heating. (e) potential difference error due to heating. In panels b–d, the absolute error is colour-coded, and the relative error, expressed as a percentage, is shown by overlaid contours.