Empirical model of global thermospheric temperature and composition based on data from the Ogo 6 quadrupole mass spectrometer

Abstract

An empirical global model for magnetically quiet conditions has been derived from longitudinally averaged N2, O', and He densities by means of an expansion in spherical harmonics. The data were obtained by the Ogo 6 neutral mass spectrometer and cover the altitude remge 400-6(g) km for the period June 27, 1969, to May 13, 1971. The accuracy of the analytical description is of the order of the experimental error for ::He and O and about 3 times the experimental error for N2 and thus provides a reasonable overall representation of the satellite observations. Two model schemes are used' one representing densities extrapolated to 4•50 km and one representing densities extrapolated to 120 km with exospheric temperatures inferred from N2 densities. Features in the thermospheric structure brought out , by the model include a near 1000 hour local time maximum for He and a 1600 hour maximum for N2 at 450 km, a winter maximum in the O to N• ratio at 120 km,"and a 400øK summer to winter exospheric temperature difference with the diurnal temperature bulge located at high latitudes in summer. A global thermosphere model for quiet magnetic conditions has been generated by using in situ measurements of N•, O, and He obtained with the neutral particle quad-rupole mass spectrometer [Carignan and Pinkus, 1968] carried aboard the Ogo 6 satellite. The satellite was launched June 5, 1969, into an 82 ø inclination orbit with a 398-km perigee and 1100-km apogee and was operated until July 1971. The reduction of the raw data to ambient densities makes use of the usual thermal transp!ration equation in a moving coordinate system [Schultz et al., 1948; Horowitz and LaGow, 1957] and further includes corrections to the N• densities as a result of CO contributions to the mass 28 peak and corrections to atomic oxygen densities as a result of oxygen surface adsorption, recombination, and desorp-tion [Hedin et al., 1973a]. For each gas species a density value is determined every 9 s. The uncertainty in N• and O densities as a result of noise in thb data is always about 2-4%, but for He the uncertainty varies greatly (depending on the exact conditions) because of the generally low signal to noise ratio. Near perigee the uncertainty in density due to possible systematic errors in background subtraction and gas-surface interaction corrections is generally about 2% for N• and 6% for O (although it can be several times larger under very low density conditions). These systematic errors increase with altitude, particularly above 500 km. No densities are used for which the total uncertainty (random and systematic) is estimated to exceed 25% for N• and O and 50% for He. In addition, there is a laboratory calibration uncertainty of 10-15%. The model described here is a more recent version of a model presented by Hedin et al. [1973b] at COSPAR. The principal changes are inclusion of 8-hour local time com-Copyright¸1974Copyright¸Copyright¸1974 by the American Geophysical Union. ponents in the model formula, data from days with slightly higher magnetic activity, and data from the second year of satellite operation. The model is intended to provide in concise form as accurate a representation of the thousands of measured ambient densities as possible. At the same time the model reveals many features, both familiar and novel, in the global distribution of the various gases that are inherent in the data but would not otherwise be seen easily. DATA SELECTION AND COVERAGE To eliminate strong magnetic activity effects, data are selected from days that have a daily magnetic index A p of 7 or less and a 3-hour magnetic index ap of less than 12 for that day and 6 hours earlier. All the data obtained on a given day are grouped into 5 ø latitude bands (a distinction is also made between data taken on northbound or southbound passes, since these represent drastically different local times) and averaged over a 24-hour (universal time) period. All averages of N• and O densities used in the model are based upon at least 20 density points (10 points for He). The averaging procedure eliminates longitudinal-universal time effects [Hedin and Reber, 1972] in the data and reduces the number of data handled but does not eliminate local time effects., since the orbit plane changes quite slowly with respect to the sun (2ø/day). The latitude and local time coverage of the N• data can be seen !n Figure 1. The coverage for O is almost identical to that for N2, whereas that for He differs only in having more data in the winter hemisphere than in the summer hemisphere. In I year the latitude of perigee moved three times around the earth and through five diurnal cycles. Values of the mean (taken over three solar rotations) F•o.• flux vary from 108 to 168, the overall average being near 150. Departures of the daily F•o.•7 flux from the mean vary from-40 to 760. The overall average Ap value is 4. 215