The FUV Spectrographic Imager (FUV/SI) onboard the IMAGE satellite takes images at two wavelengths (121.8 nm: SI-12 and 135.6 nm: SI-13). The SI-13 images of the low latitude region at night clearly show the intertropical FUV arcs corresponding to the equatorial anomaly (EA) in the ionosphere. The two-minute cadence of the instrument provides a unique data set of the nighttime low-latitude ionosphere. We compare the IMAGE observations of the OI 135.6 nm nightglow with the model emission intensities calculated by using the SAMI2 ionosphere model. The comparison shows that although both the FUV observation and the model give comparable emission intensities near the EA peak, the two differ in terms of their latitudinal and local time dependences. Large-scale (~1000 km) longitudinal wavy structures are often seen in OI 135.6 nm images. These structures drift eastward with a phase velocity of ~100 m/s. The structure may be due to the plasma density perturbations generated by acoustic gravity waves through amplification by the spatial resonance and the Rayleigh-Taylor (R-T) instability. This is a common structure seen in the pre-midnight sector of FUV/SI images regardless of longitude.