Error bars on the estimated atmospheric hydrogen escape rate and stellar proton density for HD 189733b. The ionizing flux is fixed to 10  F_⊙. Observations are best fitted inside the striped zones, with a χ² of 62.8  ±   1 for 87 degrees of freedom. Proton densities above ~ 3 × 10⁵ cm^-3 (orange striped zone) correspond to an “escape-limited” saturation regime in which all escaping hydrogen atoms interact with the stellar wind. Outside of this regime, observations are well reproduced for decreasing proton densities with higher escape rates.