Analysis of the spectral aspects of the Lyα signals: (a) Typical Lyα spectral fluxes from the Sun (solid) and from the IPH (dash-dotted) at the heliocentric distance of Jupiter. The IPH flux is shown as a Gaussian profile with T = 15 000 K, at an offset to the solar line because of the IPH movement relative to the Sun. The scatterable flux by the H in Europa’s exosphere varies with the relative line-of-sight velocity between the moon and the source (i.e., the Sun or IPH in a certain direction, as shown for three example velocities). The vertical dashed lines (purple, grey, orange) show how the contributions from the Sun and the IPH can vary due to the respective Doppler shift. (b) Lyα transmission of the Earth H exosphere in the rest frame of Earth for an assumed column density of 1 × 10¹³ cm⁻² and temperature of 1000 K (solid). Lyα emission profiles from Europa’s H exosphere (T = 1000 K) from resonant scattering before (dotted) and after (dashed) attenuation in the Earth exosphere from the shown transmission. Three example profiles (blue, grey, red) are shown for three different relative line-of-sight velocities of Europa with respect to Earth. The strongly shifted emission (blue) is not affected by the Earth at all.