One of the main characteristics of the zodiacal light is the large-scale smoothness of its brightness distribution over the sky. Little is known of the inhomogeneities in the zodiacal light brightness at small spatial scales. Structure must exist at some level because the zodiacal light is produced by particles which are replenished from localised sources, comets and asteroids.
We mapped a few fields of w0.5°x0.5° at 25 ¡¿m with ISOPHOT in order to look for fluctuations in the zodiacal light at low, intermediate and high ecliptic latitudes. These fields were selected for low cirrus emission, and bright infrared point sources were avoided. Figure 6 shows one of the maps with pixel size of 3'. The image smoothness is quantified in the central panel of Fig. 6 which shows the histogram of the map. The width of the almost perfectly Gaussian distribution - after removing the instrumental noise contribution - is about 0.2% of the total brightness level. The right panel of Fig. 6 displays the structure function <(Ii - Ij)2> calculated from the measured single-pixel brightnesses in the map. The flat distribution with no dependence on the spatial scale indicates the complete lack of large scale coherent structures in the brightness distribution. The very low fluctuation and the lack of correlated structures support the concept of a generally smooth zodiacal light distribution. More details of this study can be found in .
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