Introduction

For optical astronomers 'zodiacal light' is a dim glow visible with the naked eye after sunset or before sunrise. This conspicuous phenomenon, first studied by Cassini in 1683, is caused by the scattering of sunlight by dust particles orbiting in the interplanetary space. For infrared astronomers, however, zodiacal light is the thermal re-radiation of sunlight absorbed by the same particles. The cloud formed by these particles, called the Interplanetary Dust Cloud (IDC), occupies the inner solar system extending out to at least the asteroidal belt. It contains 1016 - 1017 kg of dust, equivalent to the mass of a large comet. In spite of the low density of the IDC (« 10~19 kg m~3), in most directions the zodiacal light dominates the brightness of the infrared sky in the 3-70 /¿m wavelength range. The absorption and scattering of solar radiation decreases the orbital velocities of the particles due to the Poynting-Robertson effect, limiting the lifetime of individual grains to 104 — 105 years. The most likely source to replenish the evaporated dust is the destruction of comets and asteroids. The basic observational facts on the zodiacal light and interplanetary dust are reviewed in [1,2].

Since the Earth is orbiting within the IDC, there are direct ways to study individual dust particles, such as optical and radio observations of meteors, collection of particles in the upper atmosphere or in the Antarctic, in-situ capture of dust by space probes,

"Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the P/I countries France, Germany, the Netherlands and the United Kingdom) with participation of ISAS and NASA.

and the study of lunar micro-craters [1], To determine the large scale structure of the IDC, however, the analysis of the global brightness distribution of the zodiacal light is the only available tool. Although the observed zodiacal light is an integral along the line-of-sight, via repeated mapping of the zodiacal light at different orbital positions of the Earth, i.e. at different dates during the year, the 3-dimensional structure of the IDC can be determined. The brightness distribution of the zodiacal light contains also substructures (asteroidal bands, cometary trails, rings of resonantly trapped dust) which carry important information on the origin of the interplanetary dust particles. Finally, the shape of the spectral energy distribution of the zodiacal light gives information on the properties of the dust particles averaged over the whole IDC. In this respect the mid-infrared spectrum has a special importance because in the 5-15 fim range the spectral shape is very sensitive to dust properties, and in case individual spectral features were identified even the dust composition can be determined.

The zodiacal light, however, is only one component of the infrared sky brightness, and it has to be separated from the galactic cirrus emission and from the extragalactic background light (not to mention the possible instrumental straylight). The all-sky multifilter photometric surveys performed by the IRAS satellite and by the DIRBE instrument on-board the COBE satellite have produced extensive databases of the brightness distribution of the infrared sky [3,4], Based on their data, both the IRAS and the COBE/DIRBE teams developed methods to extract the zodiacal light from the total sky brightness [3-5]. The DIRBE data have special importance due to their high quality and due to the dedicated observing strategy of repeated mapping of a significant fraction of the sky every week during the 9 months cooled operational phase of the instrument. To extract the zodiacal light the DIRBE method used the fact, that the zodiacal light is the only component which exhibits annual variation due to the Earth's orbital motion. The results from the two satellites have changed the traditional view of a smooth and symmetric zodiacal light distribution to the picture of a structured cloud containing substructures and asymmetries (e.g [6]).

ESA's Infrared Space Observatory (ISO) [7], operated between Nov. 1995 and April 1998, was not designed for all-sky surveys. It was an observatory performing pointed observations of mainly point sources. However, the cold focal plane allowed to perform absolute sky brightness measurements, and the two and half year mission resulted in a large set of individual observations of the extended sky brightness at mid- and far-infrared wavelengths, in both photometric and spectrophotometric modes. The beam of < 3' was considerably smaller than the beam of DIRBE (42'), and could efficiently avoid point sources and cirrus structure. ISO's good filter coverage in the 3-200 fim range as well as the possibility of performing mid-infrared spectrophotometry provide better spectral coverage than the previous satellites had done (recently the IRTS satellite could also observe the near- and mid-infrared low-resolution spectrum of the infrared zodiacal light [8]). The rejection of straylight from the Sun, Earth, and Moon has been proved to be excellent [9], Via its absolute photometric flux calibration, ISO can give an independent determination of the absolute brightness level of the zodiacal light (between the IRAS and DIRBE absolute calibrations in the far-infrared there is an unresolved zero point difference [3]).

ISO had two focal plane instruments sensitive enough to measure the extended emission of the sky: the spectro-photopolarimeter ISOPHOT in the 2.5-240/mi wavelength range [10], and the mid-infrared camera ISOCAM at 2.5-18/mi [11], The sky background turned out to be too faint for the on-board spectrometers SWS and LWS. The highest quality data were obtained by dedicated measurements of selected dark sky regions, but also off-source observations in the vicinity of point sources, the serendipitious survey of ISOPHOT [12], and the parallel mode observations of ISOCAM [13] provided valuable data. Most ISOCAM measurements of compact sources could also be used by taking pixels off the source. So far mainly the dedicated observations have been analysed, but we will set up a database of all suitable ISOPHOT off-source measurements. This database will also be useful for studies of the galactic cirrus and the extragalactic background light. Data from the ISOPHOT Serendipity Survey at 170/im and the ISOCAM parallel mode in the mid-infrared are reduced, but have not yet been used for zodiacal light studies.

In this contribution we discuss the ISO results at the present status of calibration as well as the expected future contribution from the new data. Since the authors are related to the ISOPHOT team, the main emphasis will be on the ISOPHOT results.

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