Upper limit on the neutral hydrogen escape rate as a function of the mean temperature in the HD 97658 b isotropic atmosphere. Lines in the lower part of the plot correspond to the 1σ confidence level, lines in the upper part to the 3σ level. Solid lines correspond to R_trans = 4 R_pl and outflow bulk velocities v_trans = 1 km s^-1 (blue), 10 km s^-1 (green), and 20 km s^-1 (red). Dashed lines correspond to v_trans = 10 km s^-1 and transitions between the isotropic atmosphere and the exosphere R_trans = 2 R_pl (blue), 4 R_pl (green), and 6 R_pl (red). The temperature is limited to the equilibrium temperature of the planet (T_eq ~ 725 K, Knutson et al. 2014).