Molecular oxygen concentrations and absorption cross sections in the thermosphere derived from extreme ultraviolet occultation profiles

Abstract

A preliminary analysis of some of the Skylab extreme ultraviolet (EUV) occultation data is presented, Radiation at the wavelengths of three strong solar emission lines near 1335, 1216, and 1032 • is absorbed by O2 in the terrestrial atmosphere at altitudes between 90 and 160 km. It was found that the laboratory values for the absorption cross sections at these three wavelengths and the appropriate Jacchia model for the average atmospheric O2 densities needed only small adjustments to be consistent with the mean altitudes of unit optical depth determined from a number of the Skylab occultation profiles. H owevet, the altitudes of unit optical depth, h0, tended to increase with latitude (and magnetic activity) at 1355 and 1032/[, although only a small latitudinal effect was observed at 1216,4,. The values of h0 were also about 2 km lower at sunrise than at sunset at 1216/[, possibly a small diurnal effect remaining at 1335 and 1032 For future EUV occultation studies it is recommended that an instrument field of view of about I X I (arc min) 2 be employed both to provide good altitude resolution and to minimize spatially coherent intensity fluctuations from small features on the solar disk. al., 1974; Reeves et al., 1976, 1977] and will not be discussed in detail here. We will list, however, the features of the instrument which are fundamental to the analysis of the atmospheric occultation data. 1. The telescope mirror had a large collection area (338 cm •') and provided good signal statistics. 2. The detectors had a linear response to signal count rates in excess of 5 X 105 counts s-'. Typical dark count rates of the detectors were about 0.003 count s -1 during ground tests and about 0.1 count s -1 during orbital operation. The maximum dark count rate observed in orbit was about 2 counts s -1 during passage of the South Atlantic Anomaly and in-flight performance checks indicated no degradation of the detectors during the 8-month mission. Typical unattenuated count rates for the solar emission lines used in the occultation studies ranged from 11Y to 10 • counts s-1. 3. The spectrometer exit profile was trapezoidal with a wavelength band pass that varied between 1.6 and 8.3 ,• for the different detectors as shown in Table 1. The solar line width was small in comparison to the wavelength band pass for all exit slits in the primary polychromatic position and fell within the flat portion of the trapezoidal profile, except for the blue wing of the H Ly a line at 1216,4,, which fell on the slope of the trapezoid for detector 2.

Seasonal variations in molecular oxygen near 100 km

“…supporting no seasonal variation were taken at a lower level of solar activity near solar minimum [Oppenheimer et al, 1976;Kayser and Potter, 1976;Schaeffer and Noxon, 1975;Mayr et al, 1976;Weeks and Smith, 1968]. In addition, data supporting a sunrise-sunset variation in the molecular oxygen density were obtained near solar minimum [Weeks and Smith, 1968;Mayr et al, 1976;Garriott et al, 1977], whereas the Solrad 8 data showing little diurnal variation were obtained at higher levels of solar activity.…”

“…[1976] also assumed that their lack of observed seasonal variation extended to the 100-km region, and they suggested that the results of Robie and Norton [1972] could be explained as a result of a fortuitous sampling of the latitude variation. Preliminary results from a solar occultation experiment on the Skylab spacecraft during the period July-December 1973 [Garriott et al, 1977] show the molecular oxygen number density increasing with increasing latitude above 120 km, but the Lyman a solar occultation data show little latitudinal variation at 100 km. The Lyman a data, however, do show that the altitude of equal density values is 2 km lower at sunrise than at sunset, suggesting a diurnal variation with maximum values at sunset.…”