Climatology of Deep O+ Dropouts in the Night-Time F-Region in Solar Minimum Measured by a Langmuir Probe Onboard the International Space Station

The Floating Potential Measurement Unit (FPMU) onboard the International Space Station includes a Wide sweeping Langmuir Probe (WLP) that has been operating in the F-region of the ionosphere at ∼400 km since 2006. While traditional Langmuir probe estimates include critical plasma parameters like electron density and temperature, we have also extracted the O+ percentage from the total ion constituents. This O+ composition dataset from the recent minimum in the Solar Cycle 24 reveals orbits with dropouts in O+ to below 80% of the total background ion density at ISS orbital altitudes. The observed O+ percentages during these dropouts are much lower than the values predicted by the International Reference Ionosphere 2016 (IRI2016) empirical model. In this paper, we present the climatology of these O+ dropouts with their dependency on season, local time and geographical location. The results show that the lowered O+ percentages are more significant in the winter hemispheres and are routinely observed for orbits in the pre-sunrise periods. The patterns in O+ dropouts can be explained in part from the lowering of the O+/H+ transition height during solar minimum along with patterns in neutral wind variation.