STUDY OF SOME IMPORTANT ATOMIC OXYGEN EMISSIONS IN AIRGLOW

Abstract

The solar EUV radiation has direct influence on thermospheric -the_ and mesospheric processes. Virtually a major part ofilsolar EUV radiation is absorbed in the thermosphere and mesosphere. This energy input is fundamental to the photochemistry and airglow emissions of this part of the atmosphere. The atoms and molecules in their neutral and ionized forms in the upper atmosphere are excited by various processes and emit characteristic radiation (radiation in the visible as well as the UV and infrared (IR) regions) that are signatures of important processes in the upper atmosphere. A quantitative understanding of airglow emissions and photochemistry is therefore essential to study the dynamics of mesosphere esosphere and thermosphere. A number of measurements (Shepherd et a1.,1993; Torr et al., 1993; Kita and Ogawa, 1992; Takahashi et al., 1990; McDade et al., 1986; Thomas and Young 1981; Hays et at.,1973). have been conducted using satellites and rockets to measure oxygen airglow emissions. The Wind Imaging Interferometer (WINDII) (Shepherd et al., 1993) which was launched on the Upper Atmosphere Research Satellite (UARS) on September 12,1991 has provided very useful data on airglow emissions, wind velocity and neutral temperature over the altitude range 80 to 300 Km. The airglow emissions which have been measured by WINDII are O(1S), 0(1D), 0+ (2P), OH and 02(A3E ). The O(1S), 0(1)) and 012P) emissions are important for the thermospheric studies while OH and 02(b1Eg+) which are confined to narrow layers in the altitude range of 85-97 Km are very useful for mesospheric studies. However, the lower peak of 0 (1S) emission near 100 Km may provide useful information about atomic oxygen density at the level of mesopause. The airglow emissions can be classified under three main categories depending on the time at which observations are made. Emissions that aredetectable from ground at night time the termed as nightglow and that observed during the day is termed as dayglow. When the sun is below the horizon at ground level, but is in view from an altitude of 50-150 Km the emission from the sunlit region of atmosphere, as seen by an observer at ground level is as termed as twilight glow. The twilight glow can be regarded beingjust the dayglow viewed from the earth's night side, clearly it will often be advantageous to observe dayglow features at twilight, when it is less difficult to discriminate against direct solar radia- tion. The measurement of twilight glow can provide useful in- formation about the thermosphere and electron density because most of the ions and emissions are generated above 150 Km. It is extremely difficult to measure twilight glow. Nevertheless, a few fl e. measurements have been reported in -1literature for atomic oxygen emissionat a wavelength48 c4" 446A in nightglow (Bahsoun-Hamade et al., 1989; 1994).