Contributions to the absorption by the different regions of GJ 3470b upper atmosphere at mid-transit, from one of the best-fit simulations. Top panel: transit view of the exosphere (left panel) and of the ellipsoidal atmosphere (right panel). In the left subpanel we show in projection in the plane of sky the planetary orbit (green curve) and the boundary of the ellipsoidal atmosphere (dashed blue curve). The planet as seen at optical wavelengths is shown as a black and a blue disk in the left and right subpanels, respectively. Middle panel: total absorption profile from GJ 3470b (magenta), and independent contributions from its exosphere (blue) and ellipsoidal atmosphere (dark red), with neither ISM absorption nor instrumental convolution. Within the shaded region, the stellar flux is entirely absorbed by the ISM. Bottom panel: theoretical Lyman-α line profiles as they would be observed with STIS in Visit C, outside of the transit (black), and during the third orbit at mid-transit with absorption by GJ 3470b (magenta), its exosphere alone (blue), and its ellipsoidal atmosphere alone (red). Thereis flux in the full-absorption ISM region because it has been spread by the broad LSF of STIS.