Low-latitude midnight brightness in 630.0 nm limb observations by FORMOSAT-2/ISUAL

This paper investigates the intense airglow brightness often observed in the 630.0 nm limb images taken using Imager of Sprites and Upper Atmospheric Lightnings (ISUAL), onboard FORMOSAT-2 satellite, where the tangent plane of the measurement falls in the local midnight sector. Most of the images show only single brightness, but in some cases there could be multiple peaks, which sometimes appears to be centered on geographic equator and in some cases falls on either sides of the magnetic equator. In order to understand such intense emission in the near-midnight hours, the observations are simulated using SAMI2 (SAMI2 is Another Model of the Ionosphere) model parameters based on the ISUAL viewing geometry. The simulations reproduced the measured airglow intensity pattern quite remarkably and suggested that the meridional neutral wind and the resulting plasma distribution are closely related with the observed brightness. The intensity and locations of the airglow brightness peaks could potentially be utilized to infer the strength of meridional neutral wind.