Global view of the nighttime low-latitude ionosphere by the IMAGE/FUV 135.6 nm
observations

Eiichi Sagawa and Takashi Maruyama
Communications Research Laboratory,
Tokyo, Japan

Thomas J. Immel, Harald U. Frey, and Stephen B. Mende
Space Sciences Laboratory, University of California,
Berkeley, USA


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.

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Geophysical Research Letters, 30(10), 1534, p. 41-1, doi:10.1029/2003GL017140, 2003.