O⁺ density distribution in the nightside ionosphere reconstructed from ISS-IMAP/EUVI data

The O+ density distribution in the nightside ionosphere has been reconstructed from extreme ultraviolet (EUV) images taken by the EUVI-B imager of the International Space Station Ionosphere, Mesosphere, upper Atmosphere, and Plasmasphere mapping (ISS-IMAP) cameras. The EUVI-B imager covers the wavelength range from about 70 nm to 110 nm and mainly observes the 91.1 nm emission from the recombination of O+ ions and electrons. Assuming that the electron density is equal to the O+ density in the F-region where the imager observes, the EUV intensity observed by EUVI-B is approximately proportional to the line-of-sight integral of the square of the O+ density. This enables us to estimate the O+ density distribution in the F-region from a sequence of EUVI-B data in each International Space Station (ISS) orbit with a Bayesian method. We demonstrate the reconstruction of the O+ distribution. In particular, the O+ density structure of the equatorial ionization anomaly (EIA) in the vicinity of an ISS orbit is obtained.