Observations of the ion-neutral collisionopause crossings that took place in September and October 2015 during the excursion to 1500 km. The bar at the top of the figure indicates where the spacecraft crosses from the inner (black) to the outer (red) region and back again. The inner region is where the ion energies (panel a) are observed at the spacecraft potential (panel c), which varied between 0 and −20 V. Ions with energies greater than 50 eV are observed in the outer region. Some crossings appear to show a sharp boundary, such as the one on 15 October, while other crossings appear to represent a broad boundary. The boundary crossings that appear sharp likely do so because the solar wind dynamic pressure increased and caused the boundary to cross the spacecraft and not because the boundary itself is a sharp transition (see Mandt et al. 2016, for details).