EUV signals associated with O+ ions observed from ISS-IMAP/EUVI in the nightside ionosphere

Abstract

The extreme ultraviolet (EUV) imager, EUVI-B, on board the International Space Station (ISS) under the International Space Station–ionosphere-mesosphere-atmosphere plasmasphere cameras (ISS-IMAP) mission was originally intended to observe EUV emissions at 83.4 nm scattered by {mathrm O}^+ ions. During the mission, EUVI-B occasionally detected evident EUV signals in the umbra of the Earth. However, the source of the signals has not been verified. To evaluate the effect of the 83.4 nm EUV, we conduct a Monte Carlo simulation which considers multiple scattering of the 83.4 nm EUV by {mathrm O}^+ ions. In addition, we modeled the contribution of the 91.1 nm emission, which is due to recombination of {mathrm O}^{+} ions and electrons, because the 91.1 nm EUV might affect the measurement from EUVI-B due to the wavelength range covered. The results suggest that the effect of the 83.4 nm EUV is likely to be negligible while the 91.1 nm EUV explains the observations from EUVI-B morphologically and quantitatively. We therefore conclude that the EUV signals observed by EUVI-B in the umbra of the Earth can largely be attributed to 91.1 nm emission due to recombination. This conclusion would facilitate the use of the EUVI-B data for reconstructing the {mathrm O}^+ density.