\(\textrm{O}^+\) density distribution in the nightside ionosphere reconstructed from ISS-IMAP/EUVI data

The \(\textrm{O}^+\) ${\text{O}}^{+}$ 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 \(\textrm{O}^+\) ${\text{O}}^{+}$ ions and electrons. Assuming that the electron density is equal to the \(\textrm{O}^+\) ${\text{O}}^{+}$ 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 \(\textrm{O}^+\) ${\text{O}}^{+}$ density. This enables us to estimate the \(\textrm{O}^+\) ${\text{O}}^{+}$ 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 \(\textrm{O}^+\) ${\text{O}}^{+}$ distribution. In particular, the \(\textrm{O}^+\) ${\text{O}}^{+}$ density structure of the equatorial ionization anomaly (EIA) in the vicinity of an ISS orbit is obtained.