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Ann. Geophys., 22, 3251-3259, 2004
www.ann-geophys.net/22/3251/2004/
© European Geosciences Union 2004
Brightness variations of the northern 630nm intertropical arc and the midnight pressure bulge over Eritrea
R. H. Wiens1,*, S. Habtemichael1, F. Andemariam1, K. Welday1, J. Criswick2, S. Brown2, and S. Sargoytchev2
1Department of Physics, University of Asmara, P.O. Box 1220, Asmara, Eritrea
2CRESS, York University, 4700 Keele St., Toronto, M3J 1P3, Canada
*now: 123 Ninth Street, Toronto, M8V 3E5, Canada
Abstract. The nightglow brightness at 630nm from the thermospheric O(1D) layer
was monitored nightly at Asmara, Eritrea (15.4° N, 39.9° E, 7° N
dip) with an all-sky imager. Averages of north-south strips of the images
enabled contour plots of brightness on a latitude vs. local time grid. The
contours show the movement of the intertropical arc southward before
midnight, staying just north of Asmara after midnight, and gradually
brightening to a maximum at 02:00h local civil time, 02:00 LT, after which it
disappears before dawn. It is argued that all features of the plots can be
explained by known mechanisms capable of driving ions along magnetic field
lines, including the fountain effect, summer to winter transequatorial winds,
and the midnight pressure bulge.
The 02:00 LT brightness maximum is the most striking and the most persistent
feature in the data. The persistence of the location of the 02:00 LT
brightening is attributed to a pressure bulge centered on the geographic
equator at midnight and extending to higher latitudes with increasing local
time in both the winter and the summer hemispheres. The bulge is shown to be
stronger near solstice than near equinox, confirming earlier work.
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