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The nightglow brightness at 630nm from the thermospheric O(<sup>1</sup>D) layerwas monitored nightly at Asmara, Eritrea (15.4° N, 39.9° E, 7° Ndip) with an all-sky imager. Averages of north-south strips of the imagesenabled contour plots of brightness on a latitude vs. local time grid. Thecontours show the movement of the intertropical arc southward beforemidnight, staying just north of Asmara after midnight, and graduallybrightening to a maximum at 02:00h local civil time, 02:00 LT, after which itdisappears before dawn. It is argued that all features of the plots can beexplained by known mechanisms capable of driving ions along magnetic fieldlines, including the fountain effect, summer to winter transequatorial winds,and the midnight pressure bulge.<P  style="line-height: 20px;">The 02:00 LT brightness maximum is the most striking and the most persistentfeature in the data. The persistence of the location of the 02:00 LTbrightening is attributed to a pressure bulge centered on the geographicequator at midnight and extending to higher latitudes with increasing localtime in both the winter and the summer hemispheres. The bulge is shown to bestronger near solstice than near equinox, confirming earlier work

Brightness variations of the northern 630nm intertropical arc and the midnight pressure bulge over Eritrea