Observations of Night‐Time Equatorial Ionosphere Structure With the FPMU on Board the International Space Station

Abstract

The equatorial ionization anomaly (EIA) is a prominent global-scale structuring of the terrestrial low-latitude ionosphere in the form of a trough in the F-region electron density centered on the geomagnetic (dip) equator with two peaks (crests) in the electron density on each side offset in latitude by ±15°. Originally discovered from analysis of ionosonde data (Appleton, 1946;Bailey, 1948), the EIA has been observed extensively with various ground-based and spacecraft instrumentation over many decades. It is now well-understood that EIA is formed by the combined effects of upward (vertical at the equator) E × B plasma drift and diffusion along magnetic field lines. These two forces work together to form the so-called equatorial fountain effect (e.g., Anderson, 1973;Balan et al., 2018;Hanson & Moffett, 1966;Kelley, 2009). The low-latitude E × B drift velocity is dominated by the neutral wind dynamo during geomagnetically quiet times but during active times, includes highly variable transient contributions from prompt penetration effects and longer lived effects from the disturbance dynamo (e.g., Fejer & Maute, 2021). The sensitivity of the EIA to the low-latitude E × B drift velocity (e.g., Anderson, 1973) makes it an excellent diagnostic for studying equatorial electrodynamics (e.g., Balan et al., 2009).The observed north-south asymmetry of the ionization anomaly crests with respect to the geomagnetic equator is caused by the season-dependent asymmetry in the meridional neutral winds (Anderson & Roble, 1981). Effects from the neutral atmosphere can also cause a longitudinal variation in the anomaly's occurrence and size, presumably due to the longitudinal variation of the quiet-time dynamo electric field (e.g., Fejer et al., 2008). In addition, the magnitude of the anomaly crests exhibit a 4-peak (sometimes also identified as a 3-wave) pattern in longitude for a given local time (LT), as observed by remote Far Ultraviolet (FUV) nightglow data in the evening LT sector (

Ion Density Climatology Based on FPMU Measurements on Board the International Space Station

“…In addition, Newheart et al. (2022) used a few days of FPMU and SWARM spacecraft ionospheric densities along with Total Electron Content (TEC) observations to examine well‐defined EIAs around local midnight during geomagnetically quiet periods.…”

“…Recently, FPMU data has also been used for studying morning electron temperature overshoots (Yang et al, 2020), and nighttime O + dropouts (Debchoudhury et al, 2022). In addition, Newheart et al (2022) used a few days of FPMU and SWARM spacecraft ionospheric densities along with Total Electron Content (TEC) observations to examine well-defined EIAs around local midnight during geomagnetically quiet periods.…”

Ion Density Climatology Based on FPMU Measurements on Board the International Space Station

“…Recently, FPMU data has also been used for studying morning electron temperature overshoots (Yang et al, 2020), and nighttime O + dropouts (Debchoudhury et al, 2022). In addition, Newheart et al (2022) We use FPMU measurements from 2008 to 2019 for the first comprehensive study of the seasonal and solar cycle-dependent quiet time mid-and low-latitude F region climatology and large-scale structures at ∼400 km. We will focus on the evolution of evening and nighttime low-latitude ionospheric structures during moderate solar activity and magnetically quiet conditions.…”