-~'uo S ' ,ae , : 9e-s. V o l . 5 . No . 3 , r i p . 1 9 5 - 1 9 9 , 1985 t ) 2 7 3 - ~ 1 7 - / 6 5 5tJ.O(~ + _50 P r i n t e d i n G r e a t B r i t a ' - n . A l l r i g i , t s r e s e r v e d , t ' , p , , r~ ' _ ,h t ' tOS_'kP,

SPACE TELESCOPE OBSERVATIONS OF THE EARTH'S UPPER ATMOSPHERE BY STELLAR OCCULTATIONS

John Caldxvell

Earth and Space Science Depr.tme,u Stale Um~el~rt~ ol Ne. York, Stou~ BIook, NY 11794. U S A

ABSTRACT

Stellar occultations provide a useful means of measurlng the trace gas composition of the Earth's mesosphere with a sensitivity of order one part per billion. The operational details will differ from those of other astronomical observations by ST, because of the difficulties In guiding near the Earth's limb. Two specific trace gases of interest to atmospheric stu- dies, C~ and CZO, are discussed in thls paper.

SUMMARY OF ~BBREVIATIONS

Angstrom unit, 10 -8 cm HRS AOV Stellar spectral type AO, luminosity Mg II

class V MgF 2 C; Chlorine NO CLO Chlorine ~noxide 02 CLONO 2 Chlorine nitrate 0 3 FGS Fine Guidance Sensor OAO3 FHST Fixed Head Star Tracker FOS Faint Ob3ect Spectrograph OAO-A2 ~O 3 Nitric acid OCS

ST

High Resolution Spectrograph Magnesium once Ionized ~agn=~ium Fluoride NIErlc oxide Molecular oxygen Ozone Orbiting Astronomlcal Observatory 3

((opernlcus) Orbiting Astronomical Observatory-A2 Carbonbl sulfide Space Telescope

INTRODUCTION

The Space Telescope (SY) has been designed to measure precisely the light from celestial objects. The scientific objectives described in this paper certainly involve the precise measurement of starlight, but in thls case, the physlca[ ~nt~Ev being studied is much closer =han typical ST targets - it is the mesospherlc region of the Earth's atmosphere.

As viewed from the ST, almost every star in the sk~ is ocuulted by the Earth's atmosphere during every spacecraft orbit.* In principle, thxs permit, a classical absorptlon experiment, where the star is the lisht source, the ST is the detector, and the mesosphere is the "sample" in which spectral features may be sought. The initlal complement of flve scientific Instru- ments includes two which could be used for this purpose the High Resolution Spectrograph (HRS) and the Faint Object Spectrograph (FOS).

The technique has been employed previously. Atreva /i/ h a ~ reviewed earlier work by two or- bltal predecessors of ST: the OAO-A2 and the Copernicus spacecraft. The former employed broadband photometers to measure mesospherle 03 and thermo~pherlc q2; the latter used a hlgh resolution spectrometer to observe or place upper limits on the following trace constituents of the mesosphere: H2, C[ and NO.

verx similar type of observation has been made by the NASA deep-space Voyager probes of the giant planets Jupiter and Saturn /eg. 2,3/. In these mlsston~, the ultraviolet spectro- meters detected various hydrocarbon molecules at much greater altitudes than was otherwise possible. The observations were made during the near-encounter phases of these mlsslons.,~*

~'A minor exception is the "continuous viewing zone", which ~s a small area of the sky near each orbital pole, and which moves at constant declination as the spacecraft orbit precesses. However, this exception is too small to influence the observaclons considered here.

~*Both Earth-orbital and deep space occultations are slgnlf~cantly different in practise than Earth-based observations of stellar occultatlons by other planets. In the former, the detec- tor is far from the occulting atmosphere, and dlffercntaal rcfractlon extlngulsI1es the star's llght. In this case, one measures the vertical varlatlo~l of the dlstaut atmosphere's

195

196 J. Caldwe ii

Another rela~ed category of e~:perlment ~s Earth-orbital Solar occultatzons (eg/4,5/).

ADVA~XTAGES OF EARTH-ORBITAL OBSERVATIONS OF STELLAR OCCL~LTATIO~]S

There are many reasons why this type of observation is attractive. It probes an altltude reglon that ~s both chemically ~mportant and otherwise difficult to observe. Because of the large number of suztable stellar sources~ multiple observations can be made. This permits the study of diurnal and secular effects in the Earth's mesosphere. It also permits opera- t~onal flex~b~l~ty and efficiency ~n scheduling, because the target star can be selected on the cr~terlon of proximity ~o other ST targets. Further, there are enough very bright ultra- v~olet stellar sources that data with an excellent slgnal-to-nolse ratio should be obtainable.

By beginning the observation sequence lust before the target star goes into occultation, it • s possible to obtain a contemporary calibration for every event. The quantity of interest is the ratio of the stellar spectrum in occultation to that before the occultation. Thls quantity ~s entirely ~ndependent of the detector's characteristics. Although the horizontal resolution zs llmlted, the vertzcal resolution can be extremely h~gh, depending on the cycle t~me cf the detector. Such is not the case for Solar oecultat~ons, where the f~nite angular s~ze of the Sun l~mits vertical resolution. Solar occultatlons also are restricted only to local t~mes of 6 h and 18 h, but no such restriction occurs for stellar occultat~ons.

Finally, the tangentlal Iimb-viewxng geometry provides a factor of ~ 50 galn over the vertical llne of szght column abundance. This significantly offsets the negative aspect of the low horizontal resolution of the technlque~ and should permit the detection of trace gases w~th a concentration of ~ 1 part per 109 , or less.

PRACTICAL ASPECTS OF OBSERVING OCCL-LTATIONS UITH ST

Observing occulta[~ons is not a primary goal of the ST program, and it wzll require some non- standard operational procedures. The most important questlon concerning the abil~ty of the ST to observe stellar occultations by the Earth is the spacecraft pointing system.

The ST guldance system normally operates by tracklng on gulde stars in two of the three Fine Guldance Sensors, which observe the periphery of the field of vlew of the five sclentlflc ins~ruments. The detectors ~n the FGSs are interferometers which are very sensitive to stray light, and which will not work near the limb of the Earth

Durlng normal operatlons, the FGSs w111 hold the spacecraft pointing steady to wlth~n ~0.01 arc sec. There Is a back-up gyroscoplc control system which can maintaln the polntlng durlng those intervals when the FGSs are not operating. The actual drift rate of the gxros w111 not be kno~m preclsely untll after launch, but it will probably be ~0.01 arc sec per second of tlme. Thls drlft rate would preclude keeplng the target star wlthln the smallest apertures of the sczentlflc instruments (0.i - 0.2 arc sec) for any signlficant tlme, and would also lead to loss of target wlthln several minutes for the larger spectrographic apertures (2 - 4 a r c s e c ) .

The next level of guldance is the Fixed Head Star Tracker (FHST) system, whlch does not use the prlmar2 optlcal system of the ST, but instead uses very small detectors whlch look in other dlrectlons. Indlvldual measurements with the FHST are nolsy, but the errors are rando~ not cumulatlve, and can be reduced by increasing the integration tlme for the slgnal from the FHST. Inltlal estimates are that an accuracy of ~ i arc sec can be achieved in i0 seconds of integratlon time. Thls would be adequate to keep the target star wlthln the larger apertures.

It would be deslrable t o use as small an aperture as possible, to reduce Raylelgh-scattered llght from the Earth's atmosphere along the line of sight. The optimum aperture slze for ob- servatlons w111 be determined in orbit, where real performance from the secondary guidance systems will be determined. It should be noted however that if a star ~,ere to drlft out of the aperturt durlng an oceultatlon, but were then recentered, the resulting record would include good data and gaps whlch would be clearly recognizable. The intervals of good data would not be eontamlnated by the bad ones.

A SA~,[PLE PROGRA~I OF OCCULTATION OBSERVATIONS

To be a worthwhile subject for ST, a candidate atmospheric gas should have the following characteristics: it should be sclentifically interesting; it should have narrow absorption bands, not a broad contlnuum, within the ST spectral range; and these bands should not be obscured by the continuum absorption of major species such as 02 and 03 .

refractlvlty, wlth very high spatial resolution. Comblning this wlth the conditlon of hydra static equillbrium, one can infer details of the thermal structure of the atmosphere, but no~ of ~ts trace constltuents /6/.

Observations of the Earth's Upper Atmosphere ]97

One topic whlch meets these crlteria is the chemistry of chlorine in the Earth's mesosphere. This has been reviewed recen[iv by Solomon et al. /7/, and only a brlef surmary of thelr dls-

cussion is repeated here.

Chlorine monoxide is believed to play an important role in modifying the ozone chemistry of the Earth's mesosphere by the following catalytic process:

03 + C£ ~ CZO + 02 (i)

C£O + 0 ~ C£ + 02 (2)

A diurnal variation is imposed on thls cycle by the following nightime, three-body reactlon"

C~O + NO 2 + M ~ C£OXO 2 + M (3)

React ion (3) is itself reversed in the daytime by photodissociation:

CZONO 2 + hu ~ Cf + NO 3 (4)

Of all the species in these four reactlons, Op is so dominant that its spectroscopic measure- ment is pointless; O 7 is better studied by dedicated satellites such as the Solar-Mesosphere Explorer; O and NO 2 features are overwhelmed by 02 and O 9 continua; NO 3 has apparently not been measured in the ultraviolet in the laboratory and CZONO 2 has only a broad continuum

absorption /8/,

However, the species CZ and C20, which are of major interest, are potentially observable by ST. Solomon et al. describe their ground-based microwave observations of C£O, including Its diurnal variations. Their results are in good agreement wlth theoretical calculations, and they cite supporting observatlonal evidence by others.

There is one discordant note in the literature on mesospheric C~O. ~lulmna et al. /9/ were unable to detect C£O using an infrared heterodyne technique, and placed an upper limit that was lower by a factor of 7 than other observations. The significance of their nondetection is greatly enhanced by thelr positive detection of HNO 3 and OCS. However, there are so many independent confirmations of C:O that it seems reasonable to assume, for the purpose of plan- ning observations, that the C:O is present. I~at the ST will do better than other experi- ments is to provide a homogeneous data set to investlgate secular, diurnal, and latitudinal effects.

288 4

S T RANGE

l 2852 2822 2 7 9 6

WAVELENGTH (nm)

Fig. i. The absorption spectrum of C£O, reproduced from Jourdain et al. /i0/. Also shown is the range that a single grating setting of the ST HRS covers at moderate resolution (3x 104). Both higher and lower spectral resolution is available.

19b J Caldwell

The A - X system of C~O, part of which is ~llustra~ed in Figure i, should be observable from ST. The data ~n Figure 1 are reproduced from Jourda~n et al. /i0/. The strongest subbands occur between 2700 and 2800 ~. Although some bands do occ_r above 3000 ~, they have not been seen from the ground because of their relative weakness and the r~ia[zvely short path length of zenith observations. The laboratory observations o~ Figure i have a spectral resolution comparable to the moderate resolution capabil~tv of the HRS. 3 x 104 . Also sho~n zn the f~gure is the wavelength range that can be covered by a s~ngle grating setting of that instru- ment. As may be seen, a time resolved series of spectra, ~itn the spectral range fixed as illustrated, could simultaneously record absorption by twc of ~he strong subbands

2 _ X2~3/2 ) 0 (~ -3/2 and one or two of the weaker subhands (~-- - X2-, ) The indlv~dual spectra would consmst of 500 independent diode measuremen~s,-each covering 0.09 ~, for a total range of 43 ~. This would provide a pos~tlve ident~Tzcat~or, if the gas ~s measurable.

Alternately, a h~gher resolution (~ 105 ) spectrum could b= obtained by the HRS covering only I0 ~, or approximately one strong subband. Since reliable ~neoretical models of the band exist, the higher resolution data could be ana]vaed, even ~elthout comparable laboratory spectra. The optimal choice of resolution w~l] be determined emp~r~callv by ~he achievable slgnal-to-noise ratio durlng real occ~itatlon events.

If lower resolution ~s desirable to ~mprove s~gnal to nols=, there is a mode w~th a resolu- tlon of ~ i000 in the FOS. It is clearly desirable to get as far away from the peak 03 absorption at 2550 ~ as possible, keeping In mlnd that th= strength of the subbands Is de- creasing rapidly toward longer wavelengths A reasonable f~rst strategy would be to observe the (i0, O) subband, at 2796 ~ at the highest HRS resolution Th~s is near the peak strength (see Table I of reference /i0/) and has the advar~age of being ~n the broad Solar Mg II absorp=ion feature. At th~s wavelength, therefore, Ra~'lelgh-scattered sunlight will be m~nimal.

Using the cross-sectlon reported in /10/corresponding to ~o~erate resolution in the HRS, with a vertical column ahundance of C70 of I 0 × i0 I~ molecules c~ -2 an~ a conservative factor of 50 enhancement w~th tangential llmb v~ewlng geomtrv, ~t ~s found that the optical depth of the absorption feature is ~ 0.03, which should be readil~ o~serva~le. The peak absorption would be greater at the highest ST spectral resolution.

The design goal for the HRS is to achieve a signal to no]== ratlo Jer diode o~ i0 in 200 seconds in the moderate resolution mod= for a 15 tb magnitu~e AOV s~ar This ~s equivalent to a signal to noise ratio ot i00 in 20 millls~conds for a zero magnitude AOV star, of which several exist in the sky The maximum temporal r~solut~o- ef the PRS is 50 mill~seconds. At this speed, the HRS could therefore eaqIl\ r~c~rd ~n a%~orptlon ~Jptlcal depth of 0.03 in a bright star as estimated above.

Slnce the vertical veloclt', of the tangential lln~ o~ slg~ to a se[tlng ~tar as seen fro- ST is 7 kan/sec, it is clear that sub-kilometer vertical soatzal r~solutlon should be posslble, if the positive claims of C70 detectlon are correct. One zs therefore optlmlstic that the ST will be successful in thi~ study

Rlegler et al. /ii/ used tnc Copernicus satelllte to plac~ an upper !imit on the abundance of C[ in the mesospherc. Ihe~ sought the ground stat~ tra~sltlon at 1188.775 ~, which occurs near a mlnlmum o[ O~ contlnuum oDaclt}. %l~hough i-is wav~length is near the short- ward end of effectlve ST ~pectral response, b~cause of Lh= "lgF 2 coating of the primary optics, both the HRS and FOS will b~ able to m~ke observations there The throughput efflclencv ~eill be determlned in orbit. The observed upper llmlt to the C mlxlng ratio by Riegler et al of 3 x 10 -9 ms a factor of "- 300 above the in sltu meaqur<-ent of %nderson et al. /12/. At this time, It is not possible to predict whether or not t~e shore ~avelength sensltlvlt\' of the ST wlll permit it to achleve a compensstlng l~provemw~l over Copernicus performance Therefore, the usefuln~ss of the ST for monltorlng th~ Earth's stratospheric C[ coneen- tratlon is uncertain

ACKNOWLEDC, E,XIEL I

The author appreciates very helpful dlscusslons ~Ith I. kos~luk, D. Demlng, R. de Zafra and C. Xurre. Thls work ]~ su;~ported by \~5A contract "~AS 8-32904.

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Observations of the Earth's Lpper Atmosphere 199

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