Zodiacal Light Surface Brightness Measurements by Atmosphere Explorer-C

ICARUS 40, 49--59 (1979)

MARSHA R. TORR*, D. G. TORR*, AND R. STENCEL~ Space Physics Research Laboratory, University of Michigan, Ann Arbor, Michigan 48109 Received October 13, 1977; revised February 13, 1979

Using the visible airglow photometer on the Atmosphere Explorer-C satellite, we have mapped the zodiacal light surface brightness at the wavelengths monitored by the instrument: 3371, 4278, 5200, 5577, 6300, and 7319 .~. The study constitutes a survey over this wavelength range, covering most of the celestial sphere, from altitudes above the atmospheric emissions, and free from atmospheric scattering and attenuation. The intensity variations reveal enhancements near elongations of 130°, and possibly near 60°, at all wavelengths. The intensity of the zodiacal light near the ecliptic pole is found to be -30 S,0. The color ratio with respect to the Sun is found to be redder than the Sun (0.7) at all elongations.

1. INTRODUCTION as a function of ecliptic latitude (/3) and Much of our knowledge of dust and par- heliocentric ecliptic longitude (~) for 3371, ticulate matter in the solar system comes 4278, 5200, 5577, 6300, and 7319 ~. The from observations of sunlight scattered off results are compared with a Mie-type scat- these particles. An addition to the data base tering model and with previous measure- of surface brightness measurements that ments. We determine color ratios, relative have been accumulated over the years is the to the Sun, for several pairs of wavelengths. large volume of measurements, made at 2. MEASUREMENTS high altitude and at several wavelengths, by The surface-brightness measurements the Visible Airglow Experiment (VAE) on used in this paper were made by the visible the Atmosphere Explorer satellites (Hays et airglow photometer on the AE-C satellite. al., 1973). This instrument has been described by Satellite measurements of the zodiacal Hays et al. (1973). The photometer contains light have an advantage over ground-based six interference filters, with bandwidths of measurements because of freedom from -20 A, mounted in a filter wheel so that any contamination by atmospheric emissions one of these can be moved into the optical and the problems of scattering and extinc- path. In addition, background and calibration in the lower atmosphere. tion positions are available. The photometer The AE satellites have two modes of op- has two optical channels, oriented at right eration: despun, and spinning about an axis angles to the spin axis. The measurements normal to the orbital plane. This motion, discussed here were made with the wider of together with the precession of the orbital these channels, which had a half-angle field plane, allows almost complete coverage in of view of 3 ° . ecliptic latitude and longitude. In this paper The data base analyzed in what follows we present maps of the surface brightness was accumulated over a 10-month period between January and October 1974, during * Also affiliated with the National Institute for Tele- which the AE-C was in an elliptic orbit, and communications Research of the C.S.I.R., Johannes- burg, South Africa. measurements were made from altitudes t Present address: Goddard Space Flight Center, above the atmospheric emissions. In a Greenbelt, Md. 20771. separate paper (Torr et aL, 1977) we have 49 0019-1035/79/100049-11$02.00/0 Copyright © by Academic Press, Inc. All rights of reproduction in any form reserved. ~A Z [-

Z < m 0

0 N ©

~Z ,.-]

> f~

,.-]Z

~; : ~ :i ~ i ~/i/~ii!!~ i ¸I

FIG. 1. Surface brightness maps on a polar coordinate system, as a function of ecliptic latitude and heliocentric ecliptic longitude. The units are R/,~. (a) 3371 /~; (b) 4278 ,~; (c) 5200 ,~; (d) 5577/~; (e) 6300 A; (f) 7319 A. 52 TORR, TORR, AND STENCEL

addressed the question of stability of the galactic component has also been mapped sensitivity calibration of the instrument and will be reported separately.) over this time period. By monitoring an in- Thus sorted, the data have been averaged flight calibration source (as a check on the into 5 ° x 5 ° bins. The results are shown in stability of the photometer gain), and by Fig. 1 (a-f). We have assumed symmetry performing some coordinated measure- about the Sun in the ecliptic plane, and ments with ground-based stations, we are have averaged northern and southern eclip- confident that the sensitivity did not change tic latitudes, and east and west heliocentric significantly. longitudes. This is duscussed further below. Considerable care has been taken to Some gaps exist in the coverage. As a avoid measurements made under any condi- routine operational precaution, the photom- tions that might introduce spurious data. eter was not turned on when the orbital Some of these selection constraints have plane was within ~ 12° of the Sun. A result been discussed in a separate paper (Torr et of this was that no data were obtained al., 1977). We have not used measurements within 12° of the antisolar direction, pre- made below 500 km in altitude on the day cluding mapping of the gegenschein. The side, and below 450 km on the nightside. In orbital inclination of the AE-C satellite was addition, we have used only data measured 68.4 ° . This precluded observations being when the photometer was looking into the taken in the vicinity of the ecliptic pole. upward hemisphere, so as to avoid any Other unshaded areas of the maps were left slant observing paths downward through unfilled either because the sample was the atmosphere. The AE photometers have small (< 10) or the standard deviation was a two-stage baffle system, designed to at- large (>40%). Each 5 ° bin is therefore an tenuate scattered light from outside the field average of anywhere between 10 and sev- of view by 10 tz (Hays et al., 1963; Torr et eral hundred individual measurements. We al., 1977). This system makes it possible to hope to complete the maps at a later date by measure the zodiacal light at a surface combining information from the AE-D and brightness of-1 R/~ in the presence of AE-E satellites. sunlight (--5 × l0 H R//~ if viewed di- As the data have been carefully screened rectly). We have not used data taken closer to avoid atmospheric emissions and radia- than 35 ° to the Sun and 20 ° to the Moon. tion belt effects, the residual major non- We have avoided measurements made in zodiacal components are due to individual the vicinity of the South Atlantic Radiation stars and diffuse starlight. We consider the Anomaly by rejecting any data between effects of each of these in turn. For an in- geographic longitudes 100 ° W to 50°E, and strument operated from a platform that var- between geographic latitudes 15°N to 55°S. ies slowly with time, such as a ground- Outside this region, we have not used data based instrument, a first-magnitude star in a at altitudes above 1000 km, where fast 3 ° half-cone angle field of view would con- charged particles in the radiation belts tribute -134 SI0(V). A contribution of might affect the instrument (Torr et al., this magnitude would be comparable with 1977). the zodiacal component at large elongations The attitude of the satellite is known and thus would require removal for studies nominally to _+ l °. From this information we of these regions. However, the effect is have calculated the viewing direction of the much less severe for a spinning satellite- photometer in ecliptic coordinates and in borne instrument that progressively sweeps galactic coordinates. For the purposes of the celestial sphere. First, the spinning mo- the zodiacal maps, we have not used data tion of the vehicle effectively smears the within 30 ° of the galactic equator. (The field of view over the integration period, so ZODIACAL LIGHT MEASUREMENTS BY AE-C 53

that it is --54 square degrees, rather than 28 3.1. Variation in the Ecliptic Plane (for a 3 ° half-cone angle). The contribution The surface brightnesses shown in Fig. 1 of a first-magnitude star is thus ~<70 S10(V) are in units of rayleighs per angstrom, R//~ rather than 134 S10(V). A more significant (a unit commonly used in aeronomy, where point, however, is the fact that the star is 1 R = 106/4,n- photons cm-'-' sec-' sr-'). The only in the field of view for ~<0.13 sec, the most commonly used unit in zodiacal light duration of the integration period. The next work is $10, the number of tenth-magnitude time (days, weeks, or months later)that the stars per square degree. In Table I we show same 5 ° × 5 ° portion of the zodiacal light is a conversion between these two units at the scanned by the instrument, the stellar back- wavelengths considered in this paper. ground will be different. On the average Much of the earlier work has been re- 100 integration periods (accumulated over stricted to the plane of the ecliptic. Leinert the l0 months of measurement) comprise an (1975) has shown a comparison of intensity average surface brightness shown in a given measurements made by 16 workers since 5 ° x 5 ° bin at a particular wavelength. Thus 1964, as a function of elongation, ~' (where the contribution of the first-magnitude star ~' is equivalent to heliocentric longitude, E, is reduced to -0.7 S10(V) which corre- in the ecliptic plane; cos ~' = cos ~ cos/3). sponds to 3 x 10 -a R//~ at 5577 A and is not In Fig. 2 we show the variation of the AE significant. Stars of smaller magnitude will data for the six wavelengths analyzed here, not be detectable. Only large magnitude and we have superimposed on this the en- stars that were encountered on successive velope of the spread of the earlier data. spins would degrade the data, and the num- The AE data are in general agreement ber of such stars and the nature of the ob- with the earlier observations. However, in servations makes this source of error small Fig. 2 the AE observations show an en- for the purposes of this study. hancement near e - 130 °. This can also be We consider next the contribution from seen in Fig. 1, and extends to higher diffuse starlight. According to Shectman latitudes. A comparable feature was indi- (1974) and Roach and Megill (1961), the cated in the data of Smith, et al. (1975) and contribution from numerous faint-field stars also in the data presented by Chiplonkar at high galactic altitudes is -28 S~0(V). Dif- and Tillu (1967), but is generally absent fuse galactic light above b = 30 ° amounts from the results shown by other workers. to -20 S~0(V) (Roach and Smith, 1964). We discuss this further in Section 3.3. Thus the total is -48 S10(V) which is less As we mentioned earlier the data maps than 0.2 R/,h, at 5577/~. This contribution shown in Fig. l assume symmetry about the is 25% of the lowest surface brightness level Sun in the ecliptic plane. Several analyses shown in the zodiacal maps at elongations. have concluded that the axis of symmetry is At smaller elongations, the contribution is not significant. TABLE I 3. COMPARISON OF RESULTS WITH EARLIER WORK h No. of $10 (,~) per I R//~ The most recent review on the zodiacal light is that by Leinert (1975), and we refer 3371 772 readers to this work for a bibliography of 4278 334 papers in the field. In this section we shall 5200 242 compare the data shown in Fig. 1 with re- 5577 234 6300 229 sults previously published, and address 7319 244 some of the main issues. 54 TORR, TORR, AND STENCEL

I • 33 j'?SA I I - the pole. In the case of 6300 A, we estimate ° 42?8 60 $10 uncorrected for star background. • 5200 * 5577 From the tables by Roach (1960) and Roach • 6500 • 7319 and Megill (1961), the integrated starlight in \\ _--_-_ ENVELOPE OF OTHER OBS.- this region should be -30 S~0. We therefore .\. : .\ conclude from the AE data that the zodiacal ,o~ ~\ \~ component in the vicinity of the ecliptic \* \ ~; .x.,~ pole is -30 S~0. The data are sparse at high latitudes. However, no significant north- south asymmetry is apparent at 20-30 ° from , x * *** a I J the poles.

I I I I ,1 I % 5o* -to* ~* ,,o* n3o* ,5o* ,to* 3.3. Intensity Variations out of the Ecliptic c In Section 3.1 we compared the AE sur- FIG. 2. Variation of zodiacal light surface brightness face brightness measurements in the eclip- vs heliocentric ecliptic longitude for the six wavelengths analyzed. Also shown (dotted lines) is the en- tic plane with earlier measurements. We velope of a compilation of measurements made since pointed out an enhancement near ~ - 130 ° 1964. For the sources of these see Leinert (1975)• and mentioned that this feature (present at all the wavelengths studied) extended to the invariable plane rather than the ecliptic higher ecliptic latitudes. The feature ap- plane. The inclination, however, is smaller pears to be related to elongation. A second, than the bin size selected for averaging the although more subtle, enhancement can be data shown here. An inspection of the data seen in Fig. 1, at higher latitudes near from the northern and southern hemi- - 60 °. This appears to be the feature re- spheres does not reveal any hemispherical ported by Smithet al., (1965), but which has asymmetry detectable within the statistical been largely ignored because it was not evi- variation of the data. This is in agreement dent in most other observations. In Fig. 3 we with measurements by Sparrow and Ney compare our results at 6300 /~ with the (1972) and Leinert et al. (1974). An east- isophotes shown by Frey et al. (1974). The west symmetry is generally assumed to feature near ~' - 60 ° would not have easily exist, corresponding to the rotational sym- emerged from the Frey et al. results be- metry of the cloud. In this respect, these cause of the low ~' cutoff in their data. data are in agreement with findings by Frey However, the enhancement near ~' - 130 ° et al. (1974), Dumont (1965), and Leinert et in the AE data, corresponds to a very defi- al. (1974) which confirm this symmetry• We nite minimum in the Frey et al. data. (It therefore do not believe that the averaging should be noted for the purposes of this has introduced errors of significant mag- comparison, that the AE data are not cor- nitude for the purpose of this study. rected for diffuse starlight.) Also, the enhancement seen in the AE data near 3.2. Brightness Near the Ecliptic Pole ~'-60 °, only emerges at higher ecliptic Earlier measurements of the surface latitudes and at longitudes for which Frey et brightness of the zodiacal light near the al. do not show data. ecliptic pole have ranged from 50 to 123 $10. Clearly, if real, these features contain As was mentioned earlier, the AE-C very significant information about the na- photometer was not able to make measure- ture of the interplanetary dust. We have ments right at the ecliptic pole. However, therefore devoted considerable effort in at- we did obtain data -- 10° away, and the scale tempting to ascertain whether the en- of the variations is such that we can obtain hancements are real or artifacts of the data. from this an estimate of the brightness at As was mentioned earlier, we have not in- ZODIACAL LIGHT MEASUREMENTS BY AE-C 55

HELIOCENTRIC ECLIPTIC LONGITUDE .•0 90 I00 I10 ._ ..At,/ ,,'/.~ ~X~l ! .~ //, ,o/11,,'If,fr~ll YAi "1 I .#&' 1 / y sO rio

o"1 I I r /I,," f 18o 0" 20 40 60 80 90 70 50 50 I0° ECLIPTIC LATITUDE FIG. 3. Isophotes (in SIo) of the AE measurements at 6300 ,~ superimposed on the measurements made by Frey et al., 0974). eluded in the surface brightness data shown m ~ 2.5 log F. here, any averages for which the standard Stars fainter than V = 1.0 do not appear to deviation was ~>40% or the number of sam- dominate the extended surface brightness in ples < 10. From an examination of the data the field of view. We therefore conclude in the enhancement regions, it does not ap- that the enhancements are due to real fea- pear that these brighter regions are due to tures of the zodiacal light, but we require undersampling. further measurements in order to confirm A possible source of these brighter re- this. gions might be stars in the field of view of We shall comment further on these re- the instrument. We have discussed this gions in the following section. question earlier and generally bright stars would not degrade the observations unless 3.4. Comparison with Theory present in the field of view for a significant number of the samples comprising the Models of the zodiacal light surface average brightness in any given bin. The possibility of this occurring is small, but in case this might contribute to the enhance- '. y]' io2 ments in question, we have analyzed all the data falling in the region of the enhance- / ,, / ments on an orbit-by-orbit basis. We have / mapped the viewing direction in right as- 0 cension and declination. In each case where O0 a star could possibly have been in the field of view, we have determined the difference between the surface brightness in the enhancement region and that of the surround- i0 } i I I I i 5 4 3 2 I 0 ing regions. We have plotted this difference my vs the apparent magnitude of the star (Fig. FIG. 4. The points show the surface-brightness en- 4). The results indicate that only the very hancements at times when stars were in the field of brightest stars (V < 1.0 m) obey the usual view. The dashed line shows the response anticipated relation between flux and apparent from the m a 2.5 log F relationship, if the stellar con- magnitude tribution dominated the signal. 56 TORR, TORR, AND STENCEL

brightness are generally based on Mie scattering theory (scattering by spherical particles). Such models are clearly approxima- tions, in that the particles are not perfect spheres. However, they do allow useful first-order modeling of the large-scale variations of the dust cloud. Because so many models have appeared (and are appearing) in the literature based on this approach, in this section we compare the AE data with sf Mie theory. Such models assume a power Re, "~ law for the dependence of particle number Fxo. 5. From Frey et al. (1967). Relation between density, N, on solar distance, r, Sun-particle distance, p, particle-Earth distance A, and scattering angle O. S represents Sun, E, Earth, dN ~- r-~o~-kdot, (1) and P, scattering particle. where v is the exponent of the radial density distribution, ot is the size parameter Giese (1972) has published the results of models for a range of sizes with a = 2rra/X, (2) ~mln -~ ~ ~ 120 (Otmin = 1,2,4,10,60), for and a is the radius of the particles. flat (k = 2.5) and steep (k = 4) size spectra, Using the notation of Aller et al. (1967), and complex refractive indices m = illustrated in Fig. 5, the surface brightness rnl- m2i with rnl = 1.33, 1.5, 1.7 and of the zodiacal light at a particular wave- m2 = 0, 0.01, 0.05, and 0.1. A scan of his length is given by numerous plots of the calculated surface brightness vs e shows that he is able to re- lxidto = Ex(~k2//8,ff 2 (Ro2/p 2) produce the enhancement at E --- 130 ° using m = 1.33, t~ = 60-120, for k = 2.5 or 4.0. N(r,z)tri(O,k) dA dos. (3) We have similarly modeled the zodiacal Ix t is the monochromatic solar flux at the surface brightness as a function of e' and/3, Earth. Introducing an average density distri- and in Fig. 6 we show illustrative results in bution given by the same polar coordinate system as we have shown the data. In these calculations N(r,z) = No(Ro/r) ~ exp (-Kz/Ro), (4) we have used a radial density distribution in where No is the density of scatterers at R0, accordance with the Pioneer measurements and performing several angular transforma- of Weinberg (1976), namely, r -~. The scale tions, Aller et al. (1967) show that height factor is rather less well determined. Aller et al. (1967) use K = 5, while Dumont Ix'(e,/3) = ExNoRo(h2 /8zr 2) (1975) derives an eUipsoidal cloud model of axial ratio 7:1. Our calculations indicate that K between 5 and 6 adequately matches f~ [(sin O)p/e'(sin e') p + (cos/)~] the Ix(e,O)/e slope for 35 ° < e < 90 °. We • ~ri(O,m,k) × exp [-K{[sin (O have used ~r(O) presented by Geise (1972) for size distribution defined by dN ~ ct -k dot

- e')]/sin O} sin/3] dO, (5) for 60 <- t~ <- 120/z, k = 4.0, for Mie theory where K is the scale height factor (in AU) in calculations with index of refraction the density distribution above the ecliptic m = 1.33 and m = 1.33 - 0.01i. plane, cr(O,m,k) is the scattering cross sec- The enhancement near E' - 130 ° can be tion, and re(k) is the complex index of re- seen in these results, and if real indicates fraction of the material. larger particles with m-~ 1.33. The AE ZODIACAL LIGHT MEASUREMENTS BY AE-C 57

E 9O

O 180

FIG. 6. Isophotes of surface brightness calculated from Mie-type scattering theory. The upper half is for m = 1.33 and the lower half for m = 1.33 - 0.01i (relative units).

data, however, also indicate a second bright and using this, we can determine the color feature near •'-60 ° and this does not ratio from the data contained in Fig. 1 for emerge from the theoretical calculations. various elongations. In Fig. 7 we show the These features would correspond to scatter- results for eight wavelength pairs. As can ing angles of 150-160 ° . We may therefore be be seen from the figure, the color ratio with seeing a halo effect associated with the respect to the Sun is found to be 0.7 peaks in the scattering cross section near (___20%), i.e., redder than the Sun, for eight these angles. color pairs, and for all elongations. Leinert et al. (1974) found a reddening of the 3.5. Color of Zodiacal Light zodiacal light, smaller than that shown The spectral distribution of the zodiacal here, for e < 40 °. Also, indications of a light is typically compared with that of the reddening of the zodiacal light have been Sun. This is because departures from the reported in the corona [see, for example, solar color would directly reflect the size Gillett et al. (1964)]. However, Frey et al., distributions of the particulate matter. Pre- (1974) found no deviation from solar color vious reports have ranged from redder than for • > 45 °. As has been discussed earlier, the Sun to bluer than the Sun, and some the AE results have a residual component have found the color ratio to vary with due to starlight amounting to -30% for elongation (Leinert et al., 1974). We can • ~> 90%. Thus at larger elongations, the define the color ratio relative to the Sun by reddening found in Fig. 7 may be enhanced by the stellar component. However, for C(XI,X2) = smaller elongations, the stellar component [lzL(hl)/lo(hO]/[IzL(h2)/Io(hz) ] (6) should not be significant. 58 TORR, TORR, AND STENCEL

1.2 I i I I I m = 1.33, 60-----a<- 120, and k=2.5 or I.I I c(~.7319) 5 c(5-.¢77.6300) 2 C(4278,6300) 6 C(6300,7319) 4.0, but other explanations may exist. 1,0 3 C(5200~6300) 7 C(5200.5577) 4 C{5577,7319) 8 C(5200,7319) The intensity near the ecliptic pole agrees ,9 with the lower range of values reported by N -__ j2%~----~i other workers, i.e., ~30 $10. The color ratio "< .71 ,.< • with respect to the Sun is found to be -0.7, t.) i.e., redder than the Sun, for eight color .5 pairs and for all elongations. .4 .3 ACKNOWLEDGMENTS .2 I I I I I I 50 70 90 I10 1'30 150 170 We are grateful to Dr. P. B. Hays for the Atmo- £' sphere Explorer-C photometer data, from which this study was made. This research was supported at the FIG. 7. Average color ratio with respect to the Sun University of Michigan by Grant NAS5-24331. for eight wavelength pairs, as a function of elongation. REFERENCES ALLER, L. H., DUFFNER, G., DWORETSKY, M., GUDEHUS, D., KILSTON, S., LECKRONE, D., 4. CONCLUSIONS MONTGOMERY, J., OLIVER, J., AND ZIMMERMAN, E. We have presented here maps of the (1967). In Zodiacal Light and Interplanetary Me- zodiacal light surface brightness as a func- dium (J. L. Weinberg, Ed.), p. 243. NASA SP-150. tion of ecliptic latitude and heliocentric CHIPLONKAR, M., AND TILLU, A. (1967). Photometric evaluation of the zodiacal light and gegenschein. ecliptic longitude, at six wavelengths ranging Ann. Geophys. 23, 17. from the near ultraviolet to the near in- DUMONT, R. (1975). Ground-based observations of the frared. The measurements are made from zodiacal light. In Interplanetary Dust and Zodiacal above the atmosphere from the Atmo- Light (H. Elsasser and H. Fechtig, Eds.), p. 85. IAU sphere Explorer-C satellite. This study con- Colloquium 3 I, Springer-Verlag, Berlin. FREY, A., HOFMANN, W., LEMKE, D., AND THUM, C. stitutes the first phase of the reduction of (1974). Photometry of the zodiacal light with these data and some initial findings are pre- balloon-borne telescope THISBE. Astron. As- sented. A second phase will attempt to trophys. 36, 447. achieve a better separation of the zodiacal GIESE, R. (1972). Single component zodiacal light models. Dudley Obs. Report No. 7. and stellar components. However, for the GILLETT, F. C., STEIN, W. A., AND NEY, E. P. (1964). purposes of this study, the stellar contribu- Observations of the solar corona from the limb of the tion is less than 30% of the intensities sun to the zodiacal light, July 20, 1963. Astrophys. J. shown in the maps. 140, 292. The AE data have produced intensity vari- HAYS, P. B., CARIGNAN, G. R., KENNEDY, B. C., SHEPHERD, G. G., AND WALKER, J. C. G. (1973). ations that are basically in accord with ear- The visible-airglow experiment on atmosphere lier measurements in the plane of the eclip- explorer. Radio Sci. 8, 369-378. tic. However, fairly subtle enhancements LEINERT, C. (1975). Zodiacal light. Space Sci. Rev. 18, are found in two regions of the celestial 281-339. sphere at all the wavelengths analyzed. At- LEINERT, C., LINK, H., AND PITZ, E. (1974). The thermal emission of the dust corona during the tempts to explain these enhancements in eclipse of June 30, 1973. Astron. Astrophys. 37, 81. terms of artifacts in the data or contamina- ROACH, F. E. (1960). Diffuse galactic light. In Modern tion by other sources have not been suc- Astrophysics. Gauthier-Villars, Paris. cessful. We are therefore left with the con- ROACH, F. E., AND MEGILL, L. R. (1961). Integrated clusion that the enhancements may be real starlight over the sky. Astrophys. J. 133, 228. features of the zodiacal light. The en- SHECTMAN, S. A. (1974). The small scale anisotropy of the cosmic light. Astrophys. J. 188, 233. hancement near ~ -~ 130° is the more dis- SMITH, L. L., ROACH, F. E., AND OWEN R. W. (1965). tinctive of the two. This can be reproduced Absolute photometry of the zodiacal light. Planet. in a Mie-type scattering model by using Space Sci. 13, 207-217. ZODIACAL LIGHT MEASUREMENTS BY AE-C 59

SPARROW, J. G., AND NEY, E. P. (1972). Observations observations with the Atmosphere Explorer satel- of the zodiacal light from the ecliptic to the poles. lite. Planet. Space Sci. 25, 173-184. Astrophys. J. 174, 705. WEINBERG, J. L. (1976). Light of the night sky. InlAU TORR, M. R., HAYS, P. B., KENNEDY, B. C., AND Reports of Astronomy, (G. Contopoulos, Ed.), Vol. WALKER, J. C. G. (1977). Intercalibration of airglow 1, p. 135. Reidel, Dordrecht.