Global view of the nighttime low latitude ionosphere by the 135.6 nm
OI observation with IMAGE/FUV

E. Sagawa, T. Maruyama (CRL), T. Immel, H.U. Frey, and S.B. Mende
(SSL/UC Berkeley)

The aurora camera onboard the IMAGE satellite (FUV) has been taking
snapshots of earth's atmosphere.  The FUV/SI (Spectral Imager)
instrument takes images at two wavelengths (125.6 nm (SI12) and 135.6
nm (SI13)).  The SI13 measures 135.6 nm emissions, which is emitted
by atomic oxygen, with a narrow spectral resolution of 8.0 nm. 
During nighttime, the 135.6 nm emission comes from atomic oxygen in
excited state generated mainly from radiative recombination and
additionally from reactions involving negative oxygen ion (Dymond et.
al., 1997),

O+ + e- -> O*
O + e- -> O-  &  O+ + O- ->O* + O.

Therefore the intensity of the 135.6 nm emission is a good indicator
of ionospheric O+ ions integrated along the path.  Because the
latitude of IMAGEÕs apogee drifts toward lower latitudes, FUV is now
viewing the low latitude region of the earth. In particular, the SI13
image is clearly showing the nighttime "subtropical UV arcs"
corresponding to the ionospheric equatorial anomaly (EA).  The two
minutes cadence of the instrument provides a unique data set of the
low latitude ionosphere during nighttime, although the instrument
sensitivity is barely enough to observe the low latitude ionosphere
at limited area near the equator, and not enough to observe the
mid-latitude ionosphere.  In this report, we present IMAGE
observations of the low latitude ionosphere and compare it with the
model calculation of the 135.6 nm emission rate based on the SAMI-2
ionosphere model (Huba et. al., 2000). 

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Fall 2002 Meeting of the American Geophysical Union
San Francisco, CA, USA, 6-10 December 2002