Satellite observations of upper atmospheric discharges with the ISUAL
instrument on the ROCSAT-2 satellite

S. B. Mende, Space Science Laboratory, University of California,
Berkeley, CA 94720.

L.-C. Lee, National Space Program Office, Hsin-chu City, Taiwan

J.-L. Chern, R.-R. Hsu, H.-T. Su, Department of Physics, National Cheng
Kung University, Tainan 70148, Taiwan

H. Fukunishi, Y. Takahashi, Tohoku University, Sendai, Japan

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment
on ROCSAT 2 satellite will make the first synoptic spacecraft based
measurements of lightning induced luminous phenomena.  Science
objectives of the ISUAL instrument are the determination of the spatial,
temporal and spectral properties of lightning induced upper atmospheric
optical flash transients (sprites, elves, blue jets etc.) and the global
(geographic/seasonal) distribution of upper atmospheric optical flash
transients. The ISUAL instruments consist of a limb viewing low light
level imager with a set of bore sighted photometers and a dual channel
high time resolution array photometer.  The ISUAL imager
spectrophotometer and array photometer will measure the spectral
content, the spatial and temporal intensity distribution of the optical
flashes uninhibited by the effects of lower atmospheric absorption. 
Intensity ratios of the various emissions will be used to determine the
energetics of the excitation processes.  The investigation will take
advantage of coordinated observations with ground based simultaneous
measurements of electric fields and VLF waves and optical imaging and
photometry.  The ISUAL payload flying at 891 km altitude in a 1000-2200
local time near sun-synchronous near polar orbit will be able to provide
the first unbiased survey of lightning induced optical flashes over the
entire globe in the midnight sector.  From such a survey we will be able
to determine whether the occurrence frequency of sprites depend on
factors other than the magnitude of the storms such as the prevailing
tropospheric or mesospheric conditions.