Longitudinal dependence of the equatorial ionization anomaly observed by IMAGE/FUV

E. Sagawa (1), T. J. Immel (2), H. U. Frey (2), S. B. Mende (2)

(1) National Institute of Information and Communications Technology, Japan
(2) Space Science Laboratory, University of California Berkeley


The Equatorial Anomaly (EA) of the ionosphere has been studied by ground-based
radio and optical observations and satellite based in-situ and optical
observations for more than four decades. So far these observations have not been
able to provide information of the EA in a global perspective. The IMAGE/FUV
instrument provides instantaneous global images of OI 135.6 nm nightglow with two
minutes time resolution. Because the OI 135.6 nm emission from the nighttime
ionosphere is determined by the line-of-sight integrated plasma density, those
images are useful in investigating the nighttime low latitude ionosphere globally.
By using the IMAGE/FUV 135.6 nm observations from March to June, 2002, we have
examined the global characteristics of the EA, by constructing the constant local
time map (LT map), in which pixels with an assigned local time are extracted from
the IMAGE/FUV nightglow images obtained for three or more days, and put together
to compose a global distribution map of emission intensities. These LT maps show
that the development of the EA has significant longitudinal dependence, that is,
during this data period, the most enhanced EA development was observed at
longitudinal regions centered near 30%¾, 120%¾, 210%¾E, while the development of
EA was relatively suppressed at regions at 60%¾, and 150%¾ longitudes. Because the
IMAGE satellite located over the Northern hemisphere during the period, coverage
of the Southern hemisphere was limited. Due to large southward excursion of the
geomagnetic equator in the American sector, we do not have good deal of data in
this region in terms of geomagnetic latitude coverage. Thus, by ignoring this
region, the observed longitudinal variation of the EA may have the global wave
number of 4. The observed longitudinal dependence of the EA could not be
completely explained with the electric field model, the geomagnetic declination
angle, and the displacement of the geomagnetic equator from the geographic
equator. In order to explain the observed fine scale variation of the EA, it is
necessary to incorporate other effects, for example, forcing from the lower
thermosphere, such as tide.

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Presented at the 35th COSPAR Scientific Assembly, Paris, France, July 18-25, 2004.