Analysis of FUV Perigee Limb Data Š Dayglow and Moon

Gladstone, G. R., T. J. Immel, H. U. Frey, S. B. Mende

The IMAGE FUV instrument has made routine observations of EarthÕs limb
during perigee passes. The vertical resolution at the limb is about
8-12 km for WIC images and about 16-24 km for SI-12 and SI-13 images
obtained when IMAGE is at an altitude of 1000-2000 km. The total field
of view of each imager at the limb, for the same range of spacecraft
altitudes, is about 1000-1500 km on a side. Away from the auroral
regions, images of the limb may be used to study the vertical structure
of the far ultraviolet dayglow. Conversely, using sophisticated dayglow
models allows these data to be used in an independent test of the
standard instrument calibration, which is mainly based on International
Ultraviolet Explorer observations of several bright stars (of spectral
types B and A). The limb observations indicate the presence in the WIC
images of a substantial amount of non-dayglow emissions (i.e., Rayleigh
and/or particulate scattered light), probably resulting from a small
but finite throughput in the 200-220 nm wavelength range. While this
sensitivity to Rayleigh scattering will complicate dayglow studies with
WIC images, this is due to the very steep increase in the solar flux in
the FUV, and analyses of auroral emissions should not be affected.
Neither the SI-12 or SI-13 imager shows any sensitivity to Rayleigh
scattered light. We present here the first detailed simulations of the
vertical structure in IMAGE perigee observations of the far ultraviolet
dayglow. In several of the perigee limb-viewing images the Moon is
observed, and using reasonable estimates for the MoonÕs far ultraviolet
albedo and phase function, these data provide yet another independent
test of the instrument calibration.

_______________
To be presented at the Magnetospheric Imaging Workshop, 
Yosemite National Park, California, U.S.A., Feb. 5-8, 2002.