Correlations Between FORMOSAT-2/ISUAL 6300A Airglow and FORMOSAT-3/GOX Electron Density

We present the study of the ionospheric airglows in the 6300A wavelength observed by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) aboard the FORMOSAT-2 satellite during 02/2007 and 01/2008 and compare their monthly variation with the global three-dimensional electron density structure obtained by the GPS radio occultation experiment (GOX) of the FORMOSAT-3/COSMIC satellites. The 6300A emission in airglow comes mainly from the metastable O(1D) which results from the dissociative recombination of electrons and O2+ which comes from the reaction of O2 and O+. The ISUAL observations in low latitudes show the 6300A airglow enhancement with single and double peaks appeared mainly in equinox. The MSISE and IRI model calculations were performed to simulate the ISUAL airglow events with single peak enhancement reasonably well, but the model calculations mostly fail to reproduce double peak airglow events. Because the O+ density is about the same as the electron density in the F-region, we carried out the correlation of study between the ISUAL airglow observation and the concurrent electron density observation made by the GOX. Concurrent observations between ISUAL and GOX show the 6300A airglow enhancement occur at locations of stronger electron density in equinox. Monthly correlation between ISUAL 6300A and GOX electron density shows oscillation with period of about six months with correlation coefficient peaked in equinox at 0.8 in March, and lowest in solstice at 0.3 in June. The oscillation in correlation is mainly due to seasonal variations in the distribution of electron and O2 densities. In low latitude region, electron density is higher during equinox than during solstice, and O2 density distribution is rather uniform during equinox. But during solstice, O2 density is higher in the summer hemisphere due to more sunlight than in the winter hemisphere. Therefore, the correlation between airglow and electron density is high in equinox because O2 distribution in low latitude is uniform and can be considered as background and thus the correlation is directly due to electron density. In solstice the O2 distribution becomes non-uniform in low latitudes and thus the correlation between airglows and the electron density is low.