This study presents the O I 135.6 nm airglow observation of the middle-latitude electron density enhancement during local summer nighttime by Global Ultraviolet Imager (GUVI) on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft. The nighttime density enhancement at magnetic middle latitudes in the Southern Hemisphere, known as the Weddell Sea Anomaly (WSA), had been studied by using multipoint observations, and a similar anomalous nighttime enhancement was also found in the Northern Hemisphere recently. The resemblance of both anomalies at magnetic middle latitudes in the Northern and Southern Hemispheres suggests that they should be categorized as the midlatitude summer nighttime anomaly (MSNA). To further explore the three-dimensional structure of the MSNA and its day-to-day variation, the two-dimensional global radiance maps and the vertical electron density profiles derived from disk and limb scans of the TIMED/GUVI 135.6 nm airglow observations are utilized in this study. These global observations show that the northern MSNA mainly occurs in Asia, Europe, and the North Atlantic Ocean regions, while the southern MSNA occurs in the South America-Antarctica region, near the WSA region. The GUVI day-to-day observations in 2006 further illustrate that the southern MSNA appears nightly in January-February and November-December, while the northern MSNA appears in 36 out of 41 total observation nights in May-June.
