The natural background at shuttle altitudes

0273—1177/87 $0.00 + .50 Copyright © COSPAR

Adv. Space Res. Vol. 7, No. 5, pp. (5)141—(5)151, 1987 Printed in Great Britain. All rights reserved.

THE NATURAL BACKGROUND AT SHUTITLE ALTITUDES Marsha R. Torr,* J. K. Owens,* J. W. Eun,** D. G. Torr and P. G. Richards**

**

*Space Science Laboratory, NASA/Marshall Space Flight Center, Huntsville, AL 35812, U.S.A. **Department of Physics, University of Alabama at Huntsville, Huntsville, AL 35899, U.S.A.

ABSTRACT

Optical measurements made from the Space Shuttle include several sources of emission, each modified according to viewing configuration, Shuttle altitude, solar activity, local time, and latitude. These sources include the atmospheric emissions and emissions of non— terrestrial origin (such as stellar, interstellar, and interplanetary), together with any contamination emission induced by the Shuttle itself. In order to make astronomical observations from the Shuttle, the observer needs good information on the intensities and spectral characteristics of these various sources. In this paper we present a model spectrum for one of these components, the natural airglow background. The spectrum is modeled over a wavelength range extending from the extreme ultraviolet to the near infrared. This model is based on our present knowledge of the upper atmosphere. The effect of different viewing configurations is illustrated, together with day to night variations. The results synthesized here assume an ideal vehicle in the sense that no contaminant emissions are induced by the Shuttle and payload. These Spectra therefore represent a baseline which can be used to locate unanticipated or non—ambient features. INTRODUCTION Astrophysical observations made from spacecraft carry the considerable advantage of being above most of the absorbing and emitting atmosphere, thus permitting observations far into the ultraviolet and infrared, in addition to viewing to greater distances and of smaller objects. However, unless the observing platform is at very high altitudes, there will still be a natural atmospheric component which needs to be well identified. In this paper we present some representative model atmospheric spectra for a Shuttle altitude of 250 km. These model spectra cover the wavelength range from 400 to 13,000 A. Three viewing configurations have been selected for illustration. One of these has the field—of—view passing at a grazing altitude of 150 km from the surface of the Earth on the dayside. The second looks straight upward (radially away from the Earth) on the dayside, while the third is the most optimum for this altitude, namely straight upward at night (Figure 1). DETERMINATION OF THE AMBIENT EMISSION SPECTRUM Intensities The present understanding of the composition and processes occurring in the thermosphere (altitudes above —100 km) is good. We have developed a comprehensive model of the upper atmosphere which includes current knowledge of the major constituents, the VUV solar flux, the photoelectron processes, and transport of species /1,2/. This model is local time, latitude, solar flux, and magnetic index dependent. Thus, from this model we can compute the production and loss rates of most of the thermospheric emissions. Some examples are given in Figure 2 and are representative of summer, low—latitude conditions for large solar zenith angles (x 83°). These conditions are typical of segments of the Spacelab 1 orbit. Once the excitation rates are known, we can estimate the brightness of a particular emission along a given optical path as follows: 11 where

ii

A[X1

(1)

is the volume emission rate, A is the Einstein coefficient, and (Xl is the

concentration of the emitting specie. (5)141 JASS 7:5—3

(5)142

M. R. Torr et al.

250’Km150’Km~(

Fig. 1.

Illustration of the two viewing geometries for which model spectra are generated. 400 380

2

MODEL BRIGHT EMISSIONS

-

340

-

6300

-

5755 320 3®

5200

280-

1493

260

E

240 220200

-

180

-

180 140

1200 7320 -

6584

120i

02 ATM

N

2VI(

i

i

III

I

I

I

I

2

I

I

I

io

EMISSION RATES, CM3 SEC1 Fig. 2.

Excitation rates of several of the major thermospheric emissions.

The intensity, I, is given by I

=

filds

(2)

where the integral is along the line of sight. The intensity along a slant path through the atmosphere is amplified by the longer path length through the emitting layer. For atmospheric observations, a long path length through the atmosphere is an advantage. The amplification provided by a grazing incidence viewing geometry of 150 km over vertical viewing is substantial (3O) for an observing altitude from above the layer. However, for astronomical observations vertical viewing minimizes the natural atmospheric emission background.

Natural Background at Shuttle Altitudes Synthetic

(5)143

Spectrum

The emission intensities must be spectrally distributed across the various bands of a molecular system or between cotsponersts of atomic features. The synthetic spectra are computed using the line—by—line approach for diatomic molecules and atoms developed by Arnold et al. /3/ and modified for our purposes. Some detailed examples are given below in which we discuss the model spectrum starting with the near infrared wavelengths and progressing to shorter wavelengths. Each band system or atomic feature is synthesized separately with appropriate vibrational distributions and intensity scaling, and then a composite spectrum is generated for the wavelength range of interest. In order to generate spectra similar to those that would be measured by a representative spectrometer, the spectra, which contain many tens of thousands of lines, have been convolved with a slit function corresponding to each of the spectrometers of an instrument flown on Spacelab 1, the Imaging Spectrometric Observatory /4/. These slit widths are summarized in Table 1. In a separate publication /5/, we have made a detailed comparison of certain spectra measured with that instrument. TABLE 1

Summary of Slit Functions

FWHM (A)

Wavelength Range (A)

8,000—13,000 4,000—8,000 2,000—4,000 1,150—2,000 400—1,150

8.8 8.5 5.0 2.5 8.0

In the following paragraphs, we discuss the features that are included in the model. We first discuss the 150 km tangent ray height case and then comment on the other two cases (i.e., vertically upward from 250 km, day and night). This discussion has been kept very brief. For a detailed discussion of the terrestrial airglow and its sources, the reader is referred to the review by Torr and Torr /6/. 8000 to 13,000 A. The principal emission species in this wavelength and altitude region are from the molecular band systems of N 2(1P), N2+(M), 02 atmospheric and 02 infrared atmospheric, in addition to atomic lines of 0, N, and He. From the excitation rates computed by our code, the total slant path intensity for the entire N2(1P) system is approximately 2 kR for the solar zenith angle stodeled here. This total intensity must be distributed across the bands. To do this we have used the population distribution for the upper vibrational states computed by Cartwright B/7/, state and cascading the Cimpact state to the including direct from electron (Figure 3). The resulting spectrum for the N 2(1P) bands is shown in Figure 4. When looking upward from 250 km on the dayside, the total band intensity drops to approxistately 137 R, and at night there is no source of excitation from the N2(1P) system and so the intensities are zero.

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thermosphere both as are a result of direct The N2+ Meinel bands excited in the electron impact ionization—excitation from the ground state of N 2 and also resonance fluorescence of sunlight from the X state of fluorescence the N2+ molecule is the to larger the A state. of the Resonance two sources, and so knowing the g—factor for a particular band and the N allows us to estimate the 2+ brightness for a concentration particular hand, and the spectrum can be scaled to this band. We have used the population distributions for the A state /8/ and shown in Figure 5. The synthetic spectrum is computed (Figure 6) and along

E

a’

~

16 0

2

3n~ 6 c4

U

B3flq

8

10

12

14

16

VIBRATIONAL QUANTUM NUMBER Fig. 3. Population distributions of the N 2 B state (from /7/).

-

(5)144

M. R. Torr et a!.

with the other contributing species in this wavelength region is merged into the composite spectrum shown in Figure 7. In the case of the upward looking observations the intensity of the 2—0 band at 7854 A drops from approximately 113 R to approximately 45 R, and at night there is no source of N 2+(M). 80

~

z w I-

~

I

,

70

-

60

-

50

-

40

-

I

•

I I

30

A

~

5000

______

6000

7000

8000

9000

10000

11000

12000

13000

WAVELENGTH (ANGSTROMS) Fig. 4.

Synthetic spectrum for the N2 First Positive

~4/v..3

g

I

I

system.

I

285

-

245

-

205

-

165

-

I-. -I

~

1251h1 u 10

20

30

40

50

I 60

I 70

I 80

I 90

100

N~(A) % POPULATION Fig. 5.

Vibrational distributions for the N2~A state (from /8/).

At 150 km the principal source of excitation of the 02 atmospheric bands is energy exchange from 0(~D). The total band system will have a slant path intensity of approximately 13 kR which is distributed over the vibrational levels v’ = 0, 1, and 2 in the ratio of 1:0.8:0.2 /9/. For vertical viewing the total band intensity is 58 R, and at night the intensity drops by a factor of 100 to less than 1 R

Natural Background at Shuttle Altitudes

(5)145

130 120

-

110

-

100 90 ~ >.

I—

-

80 70

-

60

-

50

-

C’,

2 ~j

I-

~

hII~

~III

6000 6500 7000 7500 8000 8500 9000 9500 10000 10500 11000 11500 12000 WAVELENGTH (ANGSTROMS) Fig. 6.

Synthetic spectrum for the N

2+ Meinel system.

-

120

-

100

-

‘rr

l••••l••1•~•IIlIIIIIIIIIlI

IIIIIIUIIIIIIIIIIIIIIII

140

ii”rr

250 km VERTICAL DAYTIME

800 I—

60-

C,,

2 C,,

I

c~ 20

~ (Al

I

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I

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L

150km

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>.

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~~LAL~LLIII,.

8000

8500

~

9000

9500

10000 10500 11000 11500 12000 12500 13000

WAVELENGTH (ANGSTROMS) Fi~. 7. Composite synthetic spectrum for the 8000 to 13,000 A region including N2(1P), N2 (M), 02 atmospheric, 02 IR atmospheric, and atomic lines of 0, N, and He. While there is no informati?n in the 02 IA atmospheric bands in the thermosphere, collisional quenching of O( D) by 02 could produce intensities at 1.27 Mm similar to those estimated above for the 02 atmospheric system.

(5)146

M. R. Torr eta!.

The helium line at 1.08 Mm is excited by photoelectron impact and resonance fluorescence and has been measured at —4 kR vertical intensity in the twilight. We estimate the intensities here to be -10 kR along the 150 km TRH slant path, 2 kR for the vertical dayglow, and zero for the nightglow. For the forbidden NI line at 1.04 Mm, we estimate 10 kR in the slant path and 1 kR in the vertical dayglow. It is difficult to estimate intensities for all the DI, 011, MI, and Nil lines throughout the spectrum because so little information is available on the electron impact collision cross sections. In the range 8000 to 13,000 A, we have scaled the 0 and N features to a slant path brightness of 5 kR for the permitted lines at 7774 and 8446 A. The intensities drop to 200 A in the vertical dayglow and are not excited in the nightglow except for the tropical arcs where 0+ + a recombination is a nocturnal source of atomic oxygen emission. 4000 to 8000 A. The principal dayglow features at a tangent path altitude of 150 km in this part of the spectrum are the N 1S), O~(P), 2+(M), N2(1P), 02 atmospheric, and 0 and N emissions already N+( 5), and together N~(D). with the N2~(1N)and the metastable features, 0(10), 0( dis~ussed, For a solar zenith angle of 83°, the N 2F(1N) 0—0 band will be -700 R at 150 km TRH. We have scaled the bands relative to this intensity and have used the N2+ B state population distributions reported by Torr and Torr /8/. This system is illustrated in Figure 8. For upward viewing from an orbit altitude of 250 km, the 0—0 band intensity drops to 210 A and is zero at night. The intensity of the metastable atomic features is summarized in Figure 2, and the permitted lines are roughly scaled to the 7774 A feature as before. 900 800

700 600

V

500 >.

400

z 300 200 II

3200

I

3400

3600

Ii•

3800

I

4000

•~I

4200

I

4400

i~iI

4600

I

4800

5000

•

5200

5400

WAVELENGTH (ANGSTROMS) Fig. 8.

Synthetic spectrum for the N2~First Negative system.

A composite of all of these emissions is shown in FIgure

9.

spectral region is summarized in 2000 to 4000 A. In addition to the N2+ (iN), 2P). 0, N, This 0+, and N+ features, we now must add Figure along NOy, with the shorter N2(2P), 10 N2(V1C), Q(15), !4g+, wavelengths. o+i2~,,and N( In the case of N 2(2P) and N2(VK) we have computed a total brightness of 3 kR and 1.75 kR, respectively, and have used the vibrational distributions of the C and A states computed by Cartwright /7/. For vertical viewing, these intensities drop to 250 and 210 R, and there are no sources of either of these two emissions at night. 2D) intensity to be less than 10 R on the limb, the N(2P) to be 40 R, measurements We and have from computed the 0~( the Mg+ line is ‘.250 R. From the g.-factor and our calculation of the NO concentration at 150 km, we estimate the slant path brightness of the NOy(1—O) band to be 828 A. This system is the result of resonance fluorescence. This will decrease to ‘.1 R for upward viewing and there is no source at night. However, we have not been able to find any determinations of the vibrational distributions. An apparent vibrational temperature of 8000°K crudely approximates the relative intensities of the observed bands.

Natural Background at Shuttle Altitudes

30

-

20

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-

250 km VERTICAL AT NIGHT

V

V

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~ 0 ~100I-

(5)147

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150 km TANGENT RAY PATH DAYTIME

350

V

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z 2

200

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150

V

100

1. 4000

A . I

I

LIII

I

5500 6000 6500 7000 7500 8000 WAVELENGTH (ANGSTROMS) Fig. 9. Composite synthetic spectrum for the 4000 to 8000 A region including N 2(1P), N2+(M), 02 atmospheric, N2+(1N), and atomic lines of DI, DII, NI, and Nil. 200

-

180

-

160

-

4500

5000

250 km VERTICAL DAYTIME

140 120 100 ~

80

h ~

~20 ~

~

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150 km TANGENT RAY PATH

~200

~E0

_____________

2000

2200

2400

2600 2800 3000 3200 3400 WAVELENGTH (ANGSTROMS)

3600

3800

4000

Fig. ba. Composite synthetic spectrum for the 400 to 4000 A region including N2+(1N), N3(2P), N2(VK), N2(LBH), N2(BH), NOy, NO~, and atomic lines of 0, 0 , N, N+, H, He, and He+.

M. R. Torr et al.

(5)148

80 60

-

250km VERTICAL AT NIGHT

40 20

I

I

120 100

I

I

I

250 km VERTICAL DAYTIME

80 ~

60 40

-j

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a

—

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150 km TANGENT RAY HEIGHT

z 200 150

DAYTIME

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~:Ll~L~LL~A~ ~ V

100

V

1200

1300

1400

1500 1600 1700 1800 WAVELENGTH (ANGSTROMS) Fig. bOb.

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600 I

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1900

I

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2000

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1

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250 km VERTICAL DAYTIME

500 450 ~400-

:: >

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AIA~I

A.

150 km TANGENT RAY PATH DAYTIME

200 ~ 2

150

~100 IA 400

500

600

700

.

A

800

~

900

1000

1100

1200

WAVELENGTH (ANGSTROMS) Fig. lDc. 400 to 2000 A. In this wavelength region the bright features for the altitudes of interest are the NO6 bands, the N2 Birge-Hopfield and Lyman—Birge—Hopfield bands, and numerous lines of N, 0, H, and He and their ions. In the case of NO~as with the NOl’, as we do not have the vibrational distributions of the upper states, we have simulated the relative band intensities by means of an apparent vibrational temperature. For the N2 LBH system we have computed a slant path intensity of 4.2 kR and have used the vibrational distributions of Cartwright /7/. The total intensity drops to 250 A for the vertical dayglow above 250 km. 11T~transitions as weN2doBirge—Hopfield not have Franck—Condon for estimated the b’ E ~nd system do not the upper The systems arefactors not well only and include the know b level vibrational population distributions for either.

Natural Background at Shuttle Altitudes The atomic lines have been scaled where possible to measured features, in general are quite uncertain.

(5)149 but the intensities

Other Viewing Configurations Only three of many observing conditions possible on a single Shuttle mission have been illustrated above. Slant paths at night penetrating to tangent ray paths below a 250 km altitude will encounter progressively more features and increasing intensities. At 90 km very bright bands of the 02 atmospheric, infrared atmospheric, Chamberlain, and Herzberg systems will result from three—body recombination of atomic oxygen, and many of the atomic metastable lines are bright at night. At these altitudes a broad blue-green continuum due to NO 2 is seen and a weak NIR continuum may also exist. For tropical latitudes, even upward viewing at night will see the very bright atomic oxygen features resulting from 0+ recombination (Figure 11). For dayside observations for fields— of—view that penetrate below 80 km, the increasingly bright Rayleigh—scattered solar spectrum with its superimposed Fraunhofer and terrestrial absorption features rapidly dominates even the brightest emission features over the spectrum down to below 3000 A.

Fig. 11. Image of the atomic oxygen tropical arcs on the nightside taken from the lunar surface (from /10/). SOME COMPARISONS WITH SHUTTLE OBSERVATIONS Our assigned topic

for this paper was the natural airglow background at Shuttle altitudes. A comparison of measured spectra with models such as this must necessarily be detailed and will be dealt with elsewhere (for example /5/). However, a few comments will be made here on preliminary assessments made so far of the data and the model. A model such as has been presented here has proved to be very necessary to even begin to determine whether the measured spectra contain any major surprises, as much of the dayglow spectrum has been rather sparsely measured prior to the Spacelab 1 mission, and even that mission only allowed tantalizing samples to be obtained in many areas due to the rather poor (near—terminator) orbit that was eventually flown. For the most part, the basic features of our measured 150 km TAN dayglow spectrum agree with the predicted spectrum (see for example, Figure 12). The spectra do, on occasion, differ in interesting and perhaps somewhat exotic detail yet to be explained, and do contain some unidentified features. A pronounced difference between the spectral measurements from Spacelab 1 and expectations is in the N2+(1N) bands, which show quite unexplained vibrational distributions (Figure 13). At times the particulate environment surrounding the Shuttle is severe /11/ (Figure 14) and during water dumps takes on the appearance of a snow storm that lasts for periods of 30 to 60 minutes (Figure 15). These particulates on the dayside can superimpose a scattered sunlight signature on the data. A typical atmospheric spectrometer has a sensitivity threshold of 1 to 10 R. As a result underlying continua at the ~1 R/A level are not significant and so we can say little about the presence of induced weak continua. Tennyson et al. /12/ have reported that between

(5)150

M. R. Torr et al. 1200 to 3200 A, any such continuum must be

a few tenths of a Rayleigh.

At other

wavelengths, the question is still to be answered.

1200 1100

-

1000

-

900

>.

800

-

700

V

600

V

I•500~ 400

300200

-

100

-

I

4500

5000

f~ih1

~1AJk

~

5500

I

6000

6500

I

7000

I

7500

8000

WAVELENGTH (ANGSTROMS) Fig. 12. Visible spectrum observed from Spacelab 1 (solar zenith angle 92°, 6°S, 162°E) for a tangent ray height of 150 km.

600 ISO DATA

LAT = ~520

I

SZA=78°

500

400

300 —BATES’ MODEL

200

100

4100 4150 4200 4250 4300 WAVELENGTH (A) Fig. 13. Vibrational distributions for the N2+ B state determined from the Spacelab 1 mission vs. those predicted by theory.

Natural Background at Shuttle Altitudes

Fig. 14.

Fig. 15. /11/).

(5)151

Photograph of particulates in the Spacelab 1 environment (from /11/).

Photograph of ice particulates in a water dump during the Spacelab 1 mission (from

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

E. R. Young, D. G. Torr, P. G. Richards, and A. F. Nagy, Planet. Space Sci. 28, 881 (1980). P. G. Richards and 0. G. Torr, 3. Geophys. Rem. 90, 5261 (1985). 3. 0. Arnold, Whiting, E. E., and G. C. Lyle, 3. Quant. Spectrosc. Radiat. Transfer. 9, 775 (1969). M. R. Torr, R. W. Basedow, and D. G. Torr, Applied Optics 21, 4130 (1982). 3. K. Owens and M. R. Torr, The dayglow at 150 km tangent ray altitude from Spacelab 1, 3. Geophys. Res., submitted (1986). M. R. Torr and D. G. Torr, The terrestrial airglow, Rev. Geophys., submitted (1986). 0. C. Cartwright, 3. Geophys. Res. 83, 517 (1978). M. A. Torr and D. G. Torr, Rev. Geophys. Space Phys. 20, 91 (1982). N. R. Torr, B. Y. Welsh, and 0. G. Torr, 3. Geophys. Res. 91, 4561 (1986). G. A. Chrruthers and T. Page, Science 177,788 (1972). N. A. Torr, 7. K. Owens, and 0. G. Torr, The optical environment of the Spacelab 1 mission, 3. of Spacecraft and Rockets, submitted (1986). P. 0. Tennyson, P. D. Feldman, R. C. Henry, and 3. N. Murthy, EOS 67, 322 (1986).