Electron densities determined by inversion of ultraviolet limb profiles


  • K. F. Dymond,

  • S. A. Budzien,

  • A. C. Nicholas,

  • S. E. Thonnard,

  • R. P. McCoy,

  • R. J. Thomas


We present electron density profiles derived by inversion of ultraviolet limb radiances observed on November 24, 1999, by the Low-Resolution Airglow and Aurora Spectrograph instrument on the Advanced Research and Global Observing Satellite. The solar 10.7-cm radio flux was 181 solar flux units, and the daily ap was 21, indicating moderate geomagnetic activity. The O+ density profile, which is approximately equal to the electron density profile in the F region ionosphere, was determined by inverting the limb radiance profile of O I 911-Å emission of atomic oxygen. The 911-Å emission is produced by radiative recombination of O+ ions and electrons. Sounding rocket and satellite measurements of the Earth's extreme ultraviolet dayglow indicated significant contamination in the 900- to 920-Å passband by emissions from atomic, molecular, and ionized nitrogen [Gentieu et al., 1979, 1981; Chakrabarti et al., 1983]. As a result of these observations, the radiative recombination emission was thought to be of little use for ionospheric sensing during the daytime. Feldman et al. [2001] have recently measured spectra in the 905- to 1184-Å passband and show that the contamination at F region altitudes is less than that present in the earlier observations [Gentieu et al., 1979, 1981; Chakrabarti et al., 1983]. We found the contamination of the O I 911-Å emission to be negligible at F region altitudes and have been able to use the 911-Å emission to accurately characterize the ionospheric state. We have compared the peak electron density and peak height determined by inversion of the 911-Å altitude profiles with nearly coincident ionosonde measurements, and we find the measurements from the two techniques to be in good agreement, demonstrating the accuracy of this technique for sensing the ionospheric state.