Overview of the analyzed HST/STIS observations. Left: orbital positions of Europa from the start of the first exposures to end of the last exposures for each of the 23 HST/STIS visits. Note: the gaps between exposures of a visit are not shown. Visit 21 combined exposures before and after transit. Right: complete STIS detector spectral image from visit 22. The Lyα signal from the geocorona and the interplanetary hydrogen fills the complete slit (yellow-green region). Reflected continuum sunlight from Europa’s surface forms the trace around row y=200. Atmospheric oxygen emissions are present at 1304 Å and 1356 Å, exceeding the surface reflection.