Comparison of Plasma Parameters at Solar Minima between Langmuir Probe Observations onboard International Space Station and IRI Empirical Model

Abstract:

The Floating Potential Measurement Unit (FPMU) aboard the International Space Station (ISS) consists of two sweeping Langmuir probes that have been operational since 2006 making measurements of the plasma environment at ~400 km over mid-and-low latitudes. One of the two probes is a spherical wide-sweep Langmuir Probe (WLP) while the other is a cylindrical narrow-sweep probe (NLP). With careful analysis, the current-voltage relationship recorded by the in-situ Langmuir probes upon careful analysis reveals the background electron-temperature and plasma density at 1-second cadence (spatial scales of approximately 7.5 km). In addition, the WLP ion-saturation region has been used to extract composition information of the ion population. Thus, the observations made by the FPMU offer a unique perspective of the changing plasma conditions over different local times and seasons ranging from the minimum at the end of Solar Cycle 23 to the minimum of Solar Cycle 24 in recent years. In this study, we compare these measurements with model results from the empirical International Reference Ionosphere (IRI) as well as other coincident measurements from ground-based Incoherent Scatter Radars (Millstone-Hill, Jicamarca). The preliminary results provide evidence of a deep solar minimum in Solar Cycle 24 with low plasma densities and frequent large dropouts in the O+/H+ ratio at ISS altitudes.