Study of Ionospheric Depletions Observed by the Far UV Imager GIMI

The Global Imaging Monitor of the Ionosphere (GIMI) experiment aboard the Advanced Research and Global Observation Satellite (ARGOS) includes a gimbaled far-ultraviolet camera capable of both astronomical and airglow observations. When trained on the atmosphere at a fixed look zenith angle, GIMI performs half-orbit imaging of the nightglow appropriate for tomographic retrieval. On the July 12, 1999 GIMI observed one, and then two horizontally separated, spatially compact regions of substantially reduced 1356 Å emission amid and otherwise unremarkable nightglow signature. Because nighttime 1356 Å radiation is produced by O⁺+e^- recombination, the observed columns of reduced emission correspond to regions of ionospheric depletion. Nearly simultaneous observations by another ARGOS instrument, the Low Resolution Airglow and Auroral Spectrograph (LORAAS), can provide independent vertical and in-track ionospheric density profiles. LORAAS observations have frequent (5.6° in-track) soundings and spectral information, complementing the more infrequent GIMI image sequences that provide vertical, in-track, and cross-track measurements integrated over its filter passband. The sequence of GIMI images will be analyzed in conjunction with LORAAS data produce a three-dimensional tomographic map of the extended depletion region. The ionosphere map will be compared to models of localized ionospheric depletions.