Top: velocity (black) and density (dashed orange, read from the right y-axis) structure for the Parker wind profile of HD 209458 b with T₀ = 8000 K, g s−1. Middle: simulated temperature structure (black) and optical depth (dashed orange, read from the right y-axis) in the major component of the metastable helium line. Bottom: relevant heating and cooling rates shown in red and blue, respectively. We observe that at lower altitudes (≲2R_p), radiative heating and cooling dominate, while at higher altitudes (≳3R_p), advection heating and expansion cooling are the dominant thermal processes.