Measurements of O+ composition in the F-region from a Langmuir Probe aboard the International Space Station

The Floating Potential Measurement Unit (FPMU) on board the International Space Station (ISS) includes a spherical wide-sweeping Langmuir Probe (WLP) that has been operated intermittently for over 14 years. With over 80% of these observations spanning the most recent solar cycle 24 from 2011 to 2019, FPMU has been in a unique position to monitor the variability of the F-region ionosphere at ~400 km altitude. Traditional Langmuir Probe estimates include electron density and temperature. However, we have extracted ion composition information from the ion saturation region of the Langmuir probe, assuming O+ and H+ to be the only constituents. The in-situ plasma measurements reveal striking findings such as deep O+ dropouts in the post-midnight sector during solar minimum, which are often much severe compared to IRI predictions. The O+ abundance can drop to less than 80% of background total quasi-neutral plasma density for many orbits for pre-dawn local times. In recent years, with the launch of the ICON and COSMIC-2 missions, it is possible to obtain a multi-layer observation based understanding of the topside ionosphere during these depletions, instances of which are presented in the poster.