A listing and discussion are given of balloon, rocket, satellite, and space probe observations of the zodiacal light. The paucity of space observations in several critical areas (e.g., in the ultraviolet and near the sun) is noted and suggestions are made for experiments to meet these needs.

The Z.L. experiment consists of 3 photometers which are mounted rigidly into the s/c with orientations of about 15°, 30° and 90° south of the s/c – XY-plane, which coincides in orbit with the ecliptic plane (see Fig.1). Helios is spinning uith 1 Hz, and the integration time of the experiment is 513 revolutions. The 90° – photometer always looks to the south ecliptic pole and one revolution of the s/c is divided into 8 sectors to get information on the polarization of Z.L. The polarization is measured by a fixed polaroid foil within the photometer which is rotated by the s/c. In the other two photometers one revolution is split into 32 sectors with different angular resolution. Near the antisun where the gradient in Z.L. intensity is small, the sector length is 4 times the length near the sun (see Fig.2). In these 2 photometers the polarization is obtained by 3 differently oriented polarization foils moved by stepping motors. Intensity and polarization of Z.L. is measured in 3 different colors, which are near the international UBV – system (Ažusienis and Straižs 1969), the effective wavelength shifted by about 100 Å to the blue end.

Helios A was launched on December 10, 1974 into a highly elliptical orbit with a perihelion of 0.31 A.U., which was reached on March 15, 1975. The zodiacal light experiment on Helios, described in the preceding paper, worked flawlessly and provided the first observations of the zodiacal light from inside the Earth’ orbit. A typical example from the raw data of the 15° – photometer is shown in Fig.1. There is a strong intensity increase towards the sun and a remarkably flat intensity distribution at large elongations. The Milky way is superimposed on the zodiacal light at longitudes 135° to 180° and 315° to 360°. Due to the orbital motion of Helios the star background is being shifted with respect to the zodiacal light, which will facilitate its separation from the total observed intensity. The peak at the right side of Fig.1 is due to the star α CMi, which we intend to use for the calibration of the instrument in addition to the ground calibrations. A preliminary evaluation showed less than 20% difference between the two calibrations. This is typical for other stars and for the other photometers, too, and gives a safe upper limit for the accuracy of the absolute calibration. Temperature effects are comparatively small and have not been corrected so far.

Sky maps made by the Pioneer 10 Imaging Photopolarimeter (IPP) at sun-spacecraft distances from 1 to 3 AU have been analyzed to derive the brightness of the zodiacal light near the ecliptic at elongations greater than 90 degrees. The change in zodiacal light brightness with heliocentric distance is compared with models of the spatial distribution of the dust. Use of background starlight brightnesses derived from IPP measurements beyond the asteroid belt, where the zodiacal light is not detected, and, especially, use of a corrected calibration lead to considerably lower values for zodiacal light than those reported by us previously.

Star counts in the sky region observed by Pioneer 10 at 4.64AU from the sun, where the contribution of zodiacal light is negligible, were made using the Palomar Sky Survey Atlas. Brightness of the integrated starlight derived from our star counts agrees, in general, with the Pioneer 10’s observation.

A source for some of the discrepancies among reported zodiacal light measurements is suggested, and a plea is made for more uniformity in the choice of units used to report zodiacal light results.

A brief description is given of the Skylab ten color photoelectric photometer and the programs of measurements made during Skylab missions SL-2 and SL-3. Results obtained on the polarized brightness of zodiacal light at five points on the antisolar hemisphere are discussed and compared with other published data for the north celestial pole, south ecliptic pole, at elongation 90 degrees on the ecliptic, and at two places near the north galactic pole.

We report on new measurements extending the spectral range of our earlier photometry (Frey et al. 1974) to the near ultraviolet. The residual extinction caused by atmospheric ozone was found to be 0.m25 ± 0.m13 (2950 Å) and 0m36 ± 0m13 (2150 Å) at 41.5 km float altitude. Within the errors of 10-30% arising from calibration and the reduction procedure our measurements at 5000, 3450, and 2950 Å are compatible to a colour of the zodiacal light not different from that of the sun. Our result obtained at 2150 Å is an upper limit, since no reduction of airglow and integrated starlight has been done yet at that wavelength. This upper limit is 30% above a solar-like spectrum. This result is not in contradiction to the 0A0-2 measurements (Lillie 1972). The strong intensity increase he found occurs at wavelengths below 2150 Å.

The Minnesota zodiacal light experiment on board OSO-5 was reactivated in July of 1974, after having been off for 1-1/2 years and operated successfully through August 1975 at which time the satellite was again turned off. This gives another yearte worth of data to compare with that taken by the same experiment from January 1969 to January 19731,2,3. This paper reports on the seven year comparison.

From observations made with a photometer placed on board D2A-Tournesol from April 1971 to June 1973 at ε = ± 90° for all ecliptic latitudes, we had deduced that the intensity of the zodiacal light is not constant, but shows fluctuations, some of which we correlated with cometary debris. The measurements were carried out by scanning the celestial sphere in a plane orthogonal to the Sun-Earth direction with a one minute period (LEVASSEUR et BLAMONT, 1973). BURNETT, SPARROW and NEY (1974) have pointed out that our data could have been contaminated by stray moonlight; we have therefore refined our data analysis and conclude that indeed fluctuations of the intensity of the zodiacal light do exist, even when the Moon is much below the horizon.

Surface brightness photometry of the night sky with experiments aboard several rockets and spacecraft indicates a significant enhancement of the brightness of the zodiacal light relative to the sun in the 1700 to 3000 A spectral region. This enhancement is most likely due to Mie scattering by non-absorbing (dielectric) particles with a mean radius of ~0.05µ and a real index of refraction which increases rapidly from ~1.4 to ~2.0 at ~2000 A, where most optical materials have an absorption edge (and exhibit a similar phenomenon). Assuming the visible zodiacal light is produced by 10 to 30µ particles, the number density of 0.5µ particles must be ~3 × 105 times greater, in good agreement with size distributions from crater counts and space probe particle detectors. Ultraviolet observations of comets indicate the enhancement is not due to a bulk property of unmodified cometary dust grains. The most probable source of submicron particles is the breakup of large agglomerates of small (~1000 A) spherical particles like those found by particle collection experiments.

The orbital track of the Apollo Command Service Module (CSM) around the moon during the flights of Apollo 15, 16, and 17 provided unique opportunities to observe the solar corona and zodiacal light. Photographs of the lunar sunrise and sunset were made with a 70 mm electric Hasselblad camera, 80 mm lens at f/2.8, using Eastman Kodak 2485 High Speed Recording film.

During the Skylab mission – May 1973 through February 1974 – the High Altitude Observatory’s white light coronagraph observed the sum of the F-corona, electron scattered K-corona, and instrumental stray light between 0.4 and 1.6 degrees from the sun. In searching for temporal variations in the F-corona, measurements were confined to the solar polar regions to minimize the effects of the K-coronal component. Changes in instrumental stray light were eliminated by restricting measurements to a single region within the instruments’ field of view. The largest source of error is the photometric calibration of the individual rolls of film. Frames were specifically selected to encompass periods of time ranging from a few days to eight months. Generally no variation in the total radiance greater than three percent was detected for intervals on the order of a few weeks. This level of stability holds for most of the eight-month period, excepting a few instances when deviations of up to eight percent were observed where the calibration is most uncertain. A preliminary study of the asymmetry in the F-corona close to the sun and the possible effect of solar eruptions (e.g., flares and prominences) upon the F-corona will be discussed.

The white light corona (K + F) between 3 and 10 Rs was observed from October, 1971 through June, 1974 by the NRL coronagraph orbiting on 0S0-7. Daily images from October, 1971 through December, 1972 have been calibrated to yield the daily variation of the coronal brightness as measured through a segmented polarizing plate. This plate had segments of tangential and radial polarization which enabled the separation of the observed brightness into its polarized and unpolarized components at about 5 and 7 Rs. Each image was scanned at 35 heights above the solar limb between 3 and 10 Rs and at every 1° in heliographic position angle. The brightness of selected points in the corona was plotted to obtain the time history of the variations. Curvilinear regression of the radial brightness distributions for each month shows fluctuations in the gradient, magnitude and standard deviation from regression associated with solar activity. Taking means over the entire set of observations yields curves representing the average, the minimum and the maximum coronal brightnesses. The minimum curve derived for each position angle is interpreted as an upper limit of the F coronal brightness. The equatorial minimum curve agrees extremely well with the F model of Saito (1970) in both shape and absolute value after a corrector for stray light has been subtracted. However the minimum curve for the polar regions lies approximately 3 standard deviations (30%) below that for the equator. This implies that the F-corona is not independent of azimuth or alternatively, the minimum equatorial curve may contain a contribution from the K-corona.

Observations of the F-corona have been made in the 10 µ region of the infrared during the eclipse of June 30, 1973. Use of the supersonic aircraft “Concorde 001” permitted 74 min of observation during totality and greatly reduced problems due to sky noise. The plane of the ecliptic was scanned over heliocentric distances of from 3 to 19 solar radii off the east limb. Bright features previously observed at shorter wavelengths, notably emission at 4 solar radii are evident on the 10 µ scans, strongly indicating that the radiation is due to thermal emission by dust. The specific intensity in the 4 R⊙ feature is 5 µ W cm−2 sterad−1 µ−1; higher than the intensity 22 arcmin above the ecliptic. Spectra were taken at one region in the ecliptic and tentatively attributed to silicate-type material. Complete details may be found in Astron. & Astrophys. 37, 75-79 and 81-86, 1974.

The 0S0-6 Zodiacal Light experiment has provided evidence for the backscatter of light from the Earth-Moon libration regions at 5000 A. Additional measured data at 4000 A and 6100 A show similar characteristics. The combined three-color data lead to libration region color indices slightly bluer than the average surrounding sky background. The differential b-v for the L4 region is approximately −0.05.

Observations to determine the radiance of forward scattered sunlight from particles in lunar libration regions have been attempted with the white light coronagraph on Skylab. The libration regions could not be distinguished against the solar K + F coronal background; the upper limit to the libration cloud radiance is determined to be 2.5 × 10−11 Bo, where Bo is the radiance of the mean solar disk. Employing models of the particle type and size distribution in the libration clouds, density enhancements have been calculated on the basis of the upper limit of the forward scattered radiance presented herein, and on the basis of earlier observations of the libration region backscattered radiance. The cases where the power law particle size distribution exponent K and complex index of refraction m are 2.5, 1.33-0.051 and 2.5, 1.50-0.051, respectively, are inconsistent with the forward and backscatter observations. Finally, the brightness contrast of remaining possible models of the libration clouds with respect to the K- and F-coronal background is calculated, and is shown to be a maximum in the vicinity of elongation angle ~30°.

The french astronomical satellite D2B is to be launched on September 1975. During the dial of the orbit, one axis of the satellite points to the Sun, with an accuracy of ± 30 arcmin; during the night of the orbit, the satellite keeps this stabilization by gyroscopic effect. The perpendicular axes -one of them is the optical axis of the Z.L. (Zodiacal Light) instrument- roll around the Earth-Sun direction, once every 4 minutes. Because of the Earth’s motion around the Sun, the celestrial sphere will be scanned along ecliptic meridians, every orbits; the complete exploration of the sky will be achieved after six months. The Z.L. instrument is a photometer analyzing galactic and zodiacal fluxes, in the range λλ 3100-750, in five spectral intervals 200-500 wide, with a field of 2.8°□. For a single scan, the limit of detection by the instrument will be the 6th visual magnitude for early-type stars (0-B).

The French astronomical satellite D2B is to be launched on September 1975. The spin axis is constantly oriented towards the sun with an accuracy of ± 30 arc min. The ERC experiment (field recognition) wii provide imager in three spectral intervals of a region centered at the anti-sun point and having a diameter of 10°. The scanning of this region is realized by a combination of the spin motion of the satellite and internal electronic scanning. This experiment includes a catadioptric objective [optimized Kern telescope) with a field of 3° × 6° and an image dissector which analyses the image.

At the present time, when spaoe experiments bring us more and more information of increasing quality, it might appear questionable whether ground-based zodiacal light observations are still of interest. Nevertheless, any detailed examination of the available data (see, for example, the review paper of Leinert 1975) shows that a considerable part of our present optical knowledge of interplanetary dust has been contributed by ground-based programmes. Moreover, new available parameters arising from spaoe data, mainly in the field of heliocentric dependence of brightness, open new important abilities for a better interpretation of ground-based observations.