We discuss some aspects of the study of the Zodiacal cloud based on brightness observations. The discussion of optical properties as well as the spatial distribution of the dust cloud show that the description of the dust cloud as a homogeneous cloud is reasonable for the regions near the Earth orbit, but fails in the description of the dust in the inner solar system. The reasons for this are that different components of the dust cloud may have different types of orbital evolution depending on the parameters of their initial orbits. Also the collisional evolution of dust in the inner solar system may have some influence. As far as perspectives for future observations are concerned, the study of Doppler shifts of the Fraunhofer lines in the Zodiacal light will provide further knowledge about the orbital distribution of dust particles, as well as advanced infrared observations will help towards a better understanding of the outer solar system dust cloud beyond the asteroid belt.

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Photometric surveys of the Zodiacal Light (ZL) already allowed to retrieve features of interplanetary dust space distribution and optical behaviour. Of the brightness “gathering” function dZ = Ð(α)/m along each line of sight (LOS), (α being the phase angle, m the subsolar distance of the LOS, Ð the local scattering coefficient), two approximate values could be derived, based on the constraints provided by the two observed values of its integral Z, when the LOS is in-ecliptic. This “nodes of lesser uncertainty” method (Dumont, Levasseur-Regourd, Renard, 1985 to 1996), however, lowered but did not rule out question marks upon the phase function. To improve this inversion, additional constraints can be found in ZL surveys from deep space probes. We show that both the Pioneer 10 (Toller and Weinberg, 1985) and (despite their lack of in-ecliptic scans) the Helios (Leinert et al., 1982) data imply the phase function to weakly depart from isotropy, at least in the 30°-150°range. The latitudinal dependence f(β, r = cst) of the space density (less well known than the heliocentric, Ð(r, β_??_ = 0)) can be tracked through the brightness ratio, at the same elongation ε, aiming in the helioecliptic meridian, against in the ecliptic. At ε = 90°, this ratio 0.3 would lead-in the improper assumption of a single, homogeneous cloud-to fit the latitudinal density drop by a cos¹² β_??_ function. The resulting brightness ratio at ε < 90°, which should be equal to ∫LOS cos¹² β(α) Ð(α)·d α/∫LOS Ð(α)dαLOS LOS turns out to be much lower than the ratio observed in the 60° > ε > 15° range (again ≈ 0.3). This contradiction is solved with a steeper exponent (20-22?) for cos β_??_, and by assuming the flattened cloud to coexist with another one, spherically symmetrical, which contributes 15-25 S10 at ε = 90°, 50-80 S10 at ε = 60°, 100-160 S10 at ε = 45° and 250-450 S10 at ε = 30°.

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We summarize the results of our Gegenschein observations at Norikura (altitude 2876 m, Japan) in Sept. 18, 1996, and at Kiso (altitude 1130 m, Japan) in Feb. 9-14, and March 4 and 5, 1997. The instrument consisting of fish-eye lens or the wide angle lens attached to the cooled CCD camera was used with green filter. It is found that the position of the maximum brightness near the antisolar point (the Gegenschein) is slightly deviated approximately 0.°4 from the antisolar point to the south in September, whereas nearly 0.°7 to the north in February/March. Furthermore, a gradient of the relative intensity along a line perpendicular to the ecliptic is remarkably different with changing seasons, i.e. the brightness decreases faster towards the north in September, in contrast with its quick decrease towards the south in February/March. Our observed evidence suggests that these variations of the peak position and the brightness gradient of the Gegenschein are caused by the annual motion of the Earth with respect to the plane of the zodiacal cloud, and does not conflict with the predictions deduced from the cloud model having its symmetric plane beyond the Earth's orbit coinciding with the invariable plane of the solar system.

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The history of zodiacal light spectrometry follows faithfully the history of optical detection technology. Each new design effort has resulted in the harnessing of new and more efficient detectors to the problem, and with each new development the quality of the spectra obtained has improved. This article traces the development of the technique from its beginnings in the 1950's to the spectrographs of the present day.

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We discuss our present knowledge about the brightness of the solar F-corona in the wavelength range from the visible to the middle infrared. From the general trend of the observational data, the F-corona is regarded as the continuous extension of the zodiacal light at smaller elongation of the line of sight. A contribution of thermal emission from dust is indicated by the increasing F-coronal brightness in comparison to the solar spectrum towards longer wavelength. As compared with the F-coronal brightness, the polarization and color in the visible regime are not well determined due to the high sensitivity of these quantities to the observational accuracy. Aside from observational problems, our present interpretation of the F-coronal brightness is also limited due to ambiguities in the inversion of the line of sight integral. Nevertheless, the measurements and model calculations of the brightness can be used to deduce some physical properties of dust grains. We show that the hump of the near-infrared brightness at 4 solar radii, which was sometimes observed in the corona, is related rather to the physical properties of dust grains along the line of sight than to the existence of a dust ring as previously discussed. We also show that the appearance or disappearance of the near-infrared peak in the coronal brightness cannot be described in any periodic cycle for each wavelength range.

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We have calculated the brightness of zodiacal emission by using the three dimensional optical models of zodiacal cloud. By comparing the calculated brightness distribution with the IRAS observations, we found that the cosine model is the best out of the three for describing the helioecliptic latitude dependence of dust distribution. We also found best parameters for the heliocentric variations of the dust temperature and volumetric absorption cross-section. Inclination and ascending node of the symmetry plane were deduced from annual variations of i) the peak offset latitude and ii) the pole brightness difference. Longitudes of the ascending node derived from i) and ii) are shown to be significantly different from each other.

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We present the mid-infrared spectrum (3-12 μm) of the zodiacal emission obtained by the Infrared Telescope in Space (IRTS), the first Japanese cryogenically cooled orbital infrared telescope. The Near-Infrared Spectrometer (NIRS) on board IRTS provided the spectrum of 3-4 μm, while that of 4.5-11.7 μm has been observed by the Mid-Infrared Spectrometer (MIRS). In this paper we present the data reduction and results of the observations by MIRS. Spectra of the background emission at high galactic latitudes (|b| > 30°) have been extracted from the MIRS observations by excluding point sources. The observed sky brightness has a clear dependence on the ecliptic latitude, indicating that the zodiacal emission dominates in the mid-infrared sky brightness. On the other hand, the spectral shape does not show any appreciable dependence on the ecliptic latitude for β = 0°-75°. The spectrum combining the NIRS and MIRS observations can be fitted by a grey body radiation at 250 K, but excess emission is seen in the 3-6 μm range. Alternatively, the spectrum of the zodiacal emission can be reproduced fairly well by a grey body at 280 K with an excess around 10 μm. In this case the excess may be attributed to a silicate emission band. Other than these excesses, no spectral features above the 10% level are seen in the MIRS spectrum.

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Retrieval algorithms for scattering particles are shown based on photopolarimeric measurements of sky light over the ocean and multiple scattering simulations of the polarization field. Polarized components of the atmospheric constituents have been measured by a photopolarimeter (named PSR1000) with spectral bands set up to correspond to the ADEOS/POLDER. The POLDER is the first sensor on board the satellite to be designed to observe polarization. It is shown that heterogeneous grains are better than homogeneous models to explain polarimetric properties of atmospheric aerosols, and a Maxwell-Garnett mixing rule for small water-soluble (WS) inclusions in an oceanic (OC) matrix is available to interpret the polarization measurements of atmospheric aerosols over the Seto Inland Sea. We also found during our observations that the value of refractive index of the aerosol, i.e., its chemical composition, varies with time and place rather than particle size.

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On the basis of numerical approaches for the collisional and orbital evolution of dust particles, the number density distribution of interplanetary dust with the mass range of m≥10^-12g is investigated. The slope of the mass distribution of dust particles strongly depends on the radial dependence of dust production by their parent bodies, and the collisional interaction between particles. Specifically, the m^-7/3 dependence of the number density distribution at 1 AU for m≥10^-6g can be explained through the balance between the collisional loss of particles and the dust supply, whereas the m^-4/3 dependence for 10^-12g ≤m ≤10^-6g particles is derived from simple Poynting-Robertson orbital decay. A possible model of the dust populations of asteroidal, cometary, and Edgeworth-Kuiper belt origin that is consistent with the observed dust flux at a solar distance of 1 AU is presented.

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We calculate the temperature of dust grains produced in Edgeworth-Kuiper Belt (EKB) based on the grain model for water-ice and silicate mixtures. The dust grains with radii ranging from 0.1 μm to 1 mm have low temperatures of about 20 K to 50 K in EKB, depending on their size, solar distance, and a volume mixing ratio of silicate to water-ice. We also estimate the thermal radiation from dust cloud in EKB. The result of thermal emission shows the spectral feature of water-ice at the wavelength of about 60 μm. Although it is difficult to estimate the possibility to detect the thermal emission spectrum of EKB dust cloud, due to large uncertainties in its spatial density, we found that the thermal emission of dust cloud in EKB lies below the IRAS data of foreground zodiacal emission. The maximum value of the thermal emission derived from the acceptable dust cloud model in EKB, however, becomes to be comparable to that of foreground zodiacal emission in far-infrared and submillimeter wavelength domains. Since the EKB dust cloud seems to concentrate near the ecliptic plane, a scanning of infrared observation along a line perpendicular to the ecliptic plane may reveal the presence of such dust cloud in the future.

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Recent results of analytical and numerical modelling of the interplanetary dust (IPD) distribution are described. They have been obtained with a new techniques employing the continuity equation written in the space of orbital coordinates. A3-D structure and the corresponding 2-D slices for the IPD cloud governed by the Poynting-Robertson drag are computed in the framework of our ‘reference model’, accounting for almost all of the known major sources of dust (5000 asteroids and 217 comets). We discuss also the origin and structure of the ‘dust bands’ and the resonant ring near Earth.

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Like the solar photons the solar wind particles induce a drag force onto the zodiacal dust grains in the heliosphere. For the distant solar wind with high Mach numbers the drag coefficient is a constant, but close to the Sun, where Mach numbers become small, the drag coefficient is a complicated function of the ion sound speed, density and temperature. We discuss the dynamics of dust particles due to this drag force and compare it with that in the distant solar wind. Especially in the near solar wind the eccentricity varies in a complicated way with the inclination of the orbits, also the semimajor axis decreases faster closer to the Sun. These variations are quite different in the distant solar wind.
In addition, we apply an analogous mathematical formalism to the dust dynamics in the outer region of the heliosphere (>20 AU) where the neutral gas density becomes comparable or larger than that of the solar wind plasma. Here the neutral hydrogen gas induces a drag force onto the dust particles similar to the plasma Poynting-Robertson effect. But different to the radial solar wind, the velocity of the interstellar gas is mono-directional, and hence with respect to the inflowdirection of the interstellar material this introduces an axial-symmetric force onto the dust particles. This force acts asymmetric in the orbit, and causes the eccentricity to increase fairly fast. The lifetime for dust grains in the Edgeworth-Kuiper Belt is no longer determined by the electromagnetic Poynting-Robertson lifetime, but by that of the neutral gas and is in the order of half a million years for a 10 μm sized particle.

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New model calculations for the dynamical evolution of dust particles at several solar radii around the Sun are presented. We choose a fractal aggregate consisting of either silicate or carbon as a representative of dielectric and absorbing fluffy particles. We take into account a large array of forces and effects acting on the dust particles-solar gravity, direct solar radiation pressure, Poynting-Robertson effect, sublimation, and the Lorentz force, with a special emphasis given to the latter. The Lorentz force was computed on the base of modeled grain's charges and a model of the actual solar magnetic field from 1976 to 1996. We have investigated the dynamics of individual grains, obtained radial and vertical density profiles of different-sized particles, and used the computed dust density distributions to calculate the expected F-corona brightness during the periods of weak and strong magnetic field. We have found that the solar magnetic field and its variations do not affect the dynamics and spatial distribution of carbon aggregates, which are thought to produce the peak features of the near-infrared F-corona brightness that were sometimes observed. On the other hand, the variations of the solar magnetic field may alter the latitudinal distribution of silicate aggregates. However, the effect is not strong enough to account for the observed temporal variations in the brightness. Thus we can rule out the correlation between the appearance or disappearance of a peak feature and the solar activity cycle.

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Zodiacal dust evolves from cometary debris through a stage called a meteoroid stream. Meteoroid streams produce meteor showers if a node of the stream is near 1 AU. On occasion, Earth encounters a stream of meteoroids that has not dispersed wide enough to be detected annually. A rare and often short lived enhancement of rates is observed during which the meteors typically have smaller radiant dispersion and sometimes anomalous fragmentation properties and end heights. Here, we summarize recent observations of these meteor outbursts and discuss how the results constrain our knowledge of the early stages of meteoroid stream formation. These stages tie meteoroid streams to cometary dust trails and are an important step in the dynamical evolution from cometary to zodiacal dust.

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The orbital element distributions of meteoroids detected during the Harvard Radio Meteor Project, 1968-69 Synoptic Year Program, have been reanalysed to remove selection effects associated with the radar observations. Corrections are made for the observing schedule, antenna beam patterns, the radio diffusion ceiling, speed dependence of ionization production, the flux enhancement due to the Earth's gravity and the probability of encounter with the Earth. These render the eccentricity, aphelion distance, and inclination distributions for meteoroids larger than 10^-4g (radius -200 μm), with orbits that cross the ecliptic near 1 AU.

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We review recent progresses in light scattering for non-spherical particles. Special attention is paid to cluster of spheres in order to improve our understanding of interplanetary dust particles. For scattering by non-spherical particles the discrete-dipole approximation (DDA) has widely been used in many scientific fields. However mainly due to the requirements of large computing memory and long computing time, the applicability of this theory is practically limited for rather small particle compared with wavelength. In order to overcome this practical problem, i.e., the large particle can not be calculated by the DDA, we recently developed the a1-term method, which is a modification version of the DDA where the dipole polarizability is determined by the first term of scattering coefficient in Mie theory. Accuracy of this method is tested by comparing the solutions by the a₁-term method with those by modal analysis, which gives the analytical solutions for cluster of spherical monomers. According to the error analysis mentioned above, the applicabilities of the a₁-term method are established as follows. The maximum size parameter of the monomer in the cluster is 1 and the total size parameter of the cluster can exceed X - 100 when the N - 10⁶ dipoles are used. We show the extinction efficiencies and asymmetry factors for cluster of spheres whose size parameter is larger than the wavelength, e.g., the volume equivalent size parameter X is larger than 30. Finally we summarize the applicabilities of DDA, T-Matrix, modal analysis and the a₁-term method. The a₁-term method can partly fulfill a gap where both DDA and the ray tracing technique based on geometrical optics can not be applied when the target is cluster. However for the target which has edges remains to be problematic. This would be the topic which should be focused on future research.

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Dust particles in the disk of Beta Pictoris (β Pic) is modeled by fluffy aggregates consisting of slightly modified interstellar grains. We calculate the optical properties by Discrete Dipole Approximation with a1-term method. It is found that the characteristic size of an aggregate is defined by the radius of volume-equivalent sphere. The wavelength dependence of the scattering efficiency in visible becomes flat when the volume-equivalent sphere exceeds 1 μm irrespective of the monomer size, while the infrared silicate feature is present even for aggregates larger than 10 μm. Therefore, our model can account for the coexistence of the neutral scattering and silicate feature in the β Pic disk without detailed tuning of the dust size distribution. Due to the enhancement of geometrical cross section, aggregates generally show higher scattering and absorption efficiencies in visible compared with the volume-equivalent sphere. In contrast, the absorption efficiency for an aggregate is comparable to that for the volume-equivalent sphere when the size is smaller than wavelength. These properties are also consistent with the observed superheat and high albedo of the dust particles in the β Pic disk.

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Atmospheric correction algorithm, which means a procedure to remove scattered light in an atmosphere from the spaced-based data, are shown for ocean color data given by the satellite ADEOS. In order to achieve better atmospheric correction, this paper proposes two subjects; one is howto determine aerosol characteristics by referring to both of radiance and polarization, and the other is introduction of atmospheric correction coefficients.
At first it is shown that a heterogeneous grain model according to Maxwell-Garnett mixing rule as small watersoluble (WS) inclusions in an oceanic (OC) matrix is available to interpret ADEOS/OCTS and POLDER data observed over the Arabian Sea. Our algorithm is based on an idea that aerosol characteristics can be estimated in terms of scattering behavior in the polarization field. Then atmospheric correction, which is based on radiative transfer process in an atmosphere-ocean model involving the retrieved aerosol model, is applied to ocean color data given by ADEOS/OCTS. Finally our atmospheric correction provides an expected chlorophyll map near the sea surface.
It is of interest to mention that retrieval of atmospheric aerosols is improved by combination use of radiance and polarization, moreover atmospheric correction process is progressed by using the correction coefficients.

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Herbig Ae/Be (HAeBe) stars, young stars surrounded by dust shells, are believed to be precursors of β Pic-like stars, and the dust around them is thought to be a possible source material for the formation of planets. A group of the HAeBe stars (UX Ori, WW Vul, etc.) shows large brightness variations. The dust in the vicinity of these stars is responsible not only for their excess emission in the infrared, anomalous extinction in the ultraviolet and visual, and specific spatial distributions of the intensity and polarization, but also for the “blueing” effect in the colour-magnitude diagrams and the intrinsic polarization increase observed in deep minima.
In contrast to the previous studies, we take advantage of a simultaneous modelling of all the observational data mentioned. Monte-Carlo simulations of polarized radiation transfer in the shells with a spheroidal density distribution have been performed for different dust grain models. The results are compared with observations of a typical HAeBe star WW Vul. Some effects related to a possible porosity of the circumstellar grains are considered.

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Progress achieved in zodiacal light studies not only allows the estimation of the brightness and polarization to be expected in a given direction for an observer in the ecliptic plane; it also provides information about the radial dependence of some local optical properties, and about their phase curves. It is nevertheless necessary to stress the high level of the uncertainties, with, as an example, a value of (0.15 ± 0.08) derived from two approaches for the local geometric albedo at the Earth. Besides, various open questions remain, possibly because of the lack of space observations since the early eighties. This paper would like to warn the reader against crude extrapolations of the available results; it also emphasizes the need for further observations and new computational tools, and presents new laboratory measurements, such as the microgravity series of experiments now under development.

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