Observations and Validation of Plasma Density, Temperature, and O+ Abundance From a Langmuir Probe Onboard the International Space Station

The Floating Potential Measurement Unit (FPMU) has been operational on board the International Space Station (ISS) since 2006. One of the instruments in the FPMU suite is a spherical wide-sweeping Langmuir probe, referred to as the WLP, which is sampled at a temporal cadence of 1 s giving in-situ measurements of the plasma density and electron temperature. In this study we present our refinements to the Langmuir probe analysis algorithm that address the uncertainties associated with photoelectron emission current from the metal probe. We also derive the fraction of O ⁺ ions as a secondary data product, which shows decrease in O ⁺ abundance in the post-midnight sector during solar minimum. The derived plasma parameters are compared and validated with an independent in-situ measurement technique, overlapping ground-based incoherent scatter radar measurements, as well as International Reference Ionosphere model output. The reduced data set spans the entire solar cycle 24 and shows the F-region ionosphere variance at ISS altitudes.