A Look at the Lyman-alpha Geocorona with the FUV/GEO Photometers on IMAGE

G. R. Gladstone
(SwRI, San Antonio, TX 78238-5166; randy@whistler.space.swri.edu)
S. B. Mende, H. U. Frey, S. P. Geller
(SSL, UC Berkeley, Berkeley, CA 94720)
H. Lauche 
(MPAe, D-37191 Katlenburg-Lindau, Germany)


Three Lyman-alpha-sensitive photometers (the FUV/GEO experiment) onboard IMAGE
are designed to obtain 360 degree swaths of the sky every 2 minutes, at 0 
degrees and 30 degrees from the spin/orbital plane of the spacecraft.  Each 
swath will have 1 degree resolution with an integration time of 0.3 s per 
measurement.  The sensitivity of the photometers is 0.4 counts/s/Rayleigh, 
providing excellent S/N ratios for geocoronal observations throughout the 1000 
km by 7 R_E altitude (and 13.5 hr period) IMAGE orbit (e.g., S/N ratios from
~10 to ~70 for zenith and bright limb viewing, respectively). The FUV/GEO 
measurements are intended to yield exospheric H density distributions to be 
used in the analysis of energetic neutral atom measurements also being made by 
experiments onboard IMAGE. Using the lyao_rt geocoronal radiative transfer 
model of Bishop (e.g., J. Quant. Spectrosc. Rad. Transf.}, 61, 473, 1999) and 
the interplanetary medium model of Pryor (e.g., J. Geophys. Res., 103, 26,813, 
1998), we have simulated the signal expected for the FUV/GEO experiment for a 
variety of conditions and look directions. These model results will be compared 
with initial data from the mission.

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Presented at the Spring American Geophysical Union Meeting, 
Washington, D.C., May 30-June 3, 2000