Pergamou Adv. SpceRes. Vol. 17,No. Il.pp. (11)139~11)~42,19%

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TEMPERATURE DETERMINATION IN THE LOWER EQUATORIAL T~RMOSPHERE FROM OCCULTATION MEASUREMENTS, MADE ABOARD THE SAN MARCO 5 SATELLITE2

U. Zlender and M. Roemer

Institutflir Astrophysik und Extraterrestrische Forschung, Vniversitdt Bonn, Auf a’em Hiigel71,53121 Bonn, Germany

ABSTRACT

Preliminary results in temperature determination of the lower equatorial thermosphere are presented. Occultation data were obtained by two ASS1 spectrometers aboard of San Marco 5 in November, 1988. A new method in temperature determination from 0, absorption in the Schumann-Runge (SR) bands is described.

INTRODUCTION

Temperature is a fundamental parameter in all our attempts to understand atmospherical processes. A global database of temperatures and their changes over time, throughout all atmospheric layers is an important and desirable goal. The present situation is not by far satisfactory, especially for all atmospheric layers obove the regions which are reachable by balloons and below heights that are the domain of artificial satellites, the data are very sparse. Still too few rn~u~~nts have been made in equatorial regions by satellites, rockets or ground based equipment. To improve this situtation the San Marco 5 satellite was launched at Match 18,1988 from the coast of Kenya into a near-equatorial orbit (2.9” inclination, 262 km perigee and 619 km apogee at the begin of the mission). Aboard it had two Airglow-Solar Spectrometer Instruments (ASS1 A and B) which were operated until December 6, 1988. Among other scientific goals it was decided to obtain occultation data for temperature determinations. Measurements of such kind have been made by other groups in the past/l/, but predominant in the infrared nl.

INSTRUMENTATION

The instruments are described by Schmidtke /3/, only a brief summary can be given here:

Detectable wavelength ranged from 20 to 700 nm in 18 channels, partially overlapping for ~d~d~ce. A full spectrum was obtained by stepping the grating through all 112 scanpositions, which took about 20 minutes (measuring mode 3). Spectral resolution was 1 to 3 nm (wavelength dependent), spatial resolution 10.6”. The detectors were counting photons for periods of 250 msec, with estimated relative errors of less than 1%. The two instruments were mounted in opposite view to each other in the equatorial plane of the nearly spherical spacecraft. Since this was spinning at 6 r.p.m. (spin axis in parallel with the polar axis of the earth), the spectrometers sensed all directions from zenith to nadir every 10 seconds.

OCCULTATION DATA

If we compare vertical resolution in the atmospheric column at the point of minimal tangent ray height at the start and at the end of the ~ssion, it is obvious that better conditions are found at the end, when the orbit is

(I ltilto tJ. Zleader and M. Roamer

uealy circular as can be seen from Figure 1. If the spigot is for example located in a) at a height of 600 km the resolution in height H would be 22.4 km, while in case b) where height above the earth is 200 km, h would be 9.5 km For this reason most of the data were collected in November. Many of these data files are currently of reduced value, because the complex dyn~i~~ behavior of the spacecraft makes it difficult to interpret the measurements. In fact we only got two or three datapoints which are affected by attenuation during one sun rise or set.

Fig. 1. Geometric relations of the ASS1 occultation experiment

Angle of view 10.64 deg !’

~~l~tiou events were observed in mode 1, that means ~oun~ng photons at a fixed grating position, ~o~pond~~ to a preselected wavelength of e.g. 177 nm in channel I4 of ASS&B, for periods of 250 ms,

TEMPERATURE DETERMINATION AND DISCUSSION

At 177 nm molecular Oz is the main atmospheric absorber in the lower thermosphere” The photolysis in the spectral range of the Schumann-Runge bands (175 to 200 nm), is caused by predissociation of the bound B3 Z;, state via a number of unbound states, namely %I u , %I It , ‘II, and % u where each is in correlation with two ground state oxygen atoms. As can be seen from Figure 2, the absorption has a strong temperature dependence at 177 nm due to the underlying SR continuum, which is caused by thermal populatiou of exited rotational and vib~tion~ states, This fact is used to derive the absorbers ~rn~~~~ corresponding to the point of ~nirn~ tangent ray height, Figure 5. As Beers law holds for monocromatic radiation only, it can not be used for the broad spectral resolution of the instrument in the band system, seen in Figure 2. Calculations of the spectral features must be carried out on a much finer scale (0.5 cm-l) before averaging to the ASS1 resolution. f4,5/

Different atmospheric models like MSIS or CIRA72 can be used to give 0, densities at this point and along the line of sight, to compute atmospheric transmission when occultation occurs. Comparison between computed and observed transmission yields the temperature in an iterative procedure. The following points must be consider

Position of sun and SIC, limb crossing geometry, the angular diameter of the sun, because a certain fraction might be already shadowed or attenuated while others are not yet affected. The solar spectrum in the Inm measurement window, incident at the top of the atmosphere, solar and geomagnetic activity and other parameters which are effecting the column 02 density and the instrument sensitivity function (1 nm FWHM)_

Temperature Dete~ation in the Lower I3qoamial Thermmphere (1 I)141

Fig. 2. Variation of the computed absorption cross section of 0, in the 15-O band as the temperature is raised from 260 K to 560 K. For 560 K the con~bution of the SR continuum is obvious

Fig. 3. Derived tem~m~res. Shown is a net of parameter lines resulting from differences in compute countrates corresponding to fixed ternp~~~s (200 K-470 K) to the 350 K line in steps of 30 K. Also shown are 4 measured values, and the curve nz&ing from a computation with temperatures, which were taken from an atmospheric model. Data are for DOY 335, orbit number 3658 and 3668

6ooao I I I ,

* * I 104 110 120 130

Minimal tangent ray height (km) 140

(II)142 Cl. Zlender and M. Roemer

The main source of errors in the derived temperatures are positional uncertainties in the satellites orbit. With 50 km error in geographical length the minimum tangent ray height varies by 9 km, propagating the error through transmission and fmaiy to temperature determination.

CONCLUSIONS

Although the San Marco experiment provided an enormous amount of data, the ah-eady mentioned uncertainties do not yet allow to derive temperatures from the majority of the files. The few results shown in Figure 3 are in agreement with thermospheric model temperatures.

This work was supported by the DARA Grant 50 OT 9201~ZA.

REFERENCES

1. SK. Atreya, T.M. Donahue, W.E.Sharp, B. Wasser, J.F. Drake, G.R. Riegler, Ultraviolet stellar occultation measurement of the H, and 0, densities near 100 km in the earths atmosphere, Geophys. Res. Lat., 3,607-610, (1976).

2. J.C. Gille, J.M. Russel III: The limb infrared monitor of the stratosphere: Experiment description, performance and results, J. Geuphys. Rex, 89, D4,5125-5146, (1984).

3. G. Schmidtke, P. Seidl, C. Wita, Airglow-solar spectrometer instrument (20-700nm) aboard the SAN MARCO D/i satellite, Appiied Optics, 24,3206-3213, (1985).

4. D.P. Murtagh, The 0, Schumann-Runge system: new calculations of photodissocation cross-sections, Planet. Space Sci., 36, 819-828, (1988).

5. M. Nicolet, R. Kennes, Aeronomic problems of molecular oxygen photodissociation - VI. Photodissociation frequency and ~ansmit~ce in the spectral range of the Schum~n-Runge bands, Planet. Space Sci., 37,459- 491, (1989).