One of the main characteristics of the zodiacal light is the large-scale smoothness of its brightness distribution over the sky. Variability due to Thompson scattering of solar radiation at interplanetary plasma clouds is usually a small effect, related to high solar activity, even at visual wavelengths (Richter et al. 1982). The high incidence of localised brightness structures, observed by Levasseur and Blamont (1973) from the satellite D2A at 653 nm and attributed by them to meteor streams, is not typical for the zodiacal light in general. However, two types of elongated structures detected by the infrared satellite IRAS are now considered as typical phenomena: the asteroidal bands and the cometary trails. The asteroidal bands are located at low ecliptic latitudes, , have a brightness enhancement of several percent of the zodiacal light and a width of two to four degrees (see, e.g. Reach 1992). They are thought to result from debris created in collisions between members of asteroid families (e.g. Sykes 1990). The cometary dust trails are fainter and narrower, having a peak brightness of about 1% of the zodiacal light and a width of one or a few arcminutes (Sykes and Walker 1992). They are due mainly to mm-sized dust particles released from the comet at lower velocities than the particles constituting the tail and which therefore concentrate in the comet orbital plane.
Little is known on the graininess of zodiacal light brightness at small spatial scales. It has to exist at some level since the zodiacal light is produced from particles which are replenished from localised sources, comets and asteroids. Depending on amplitude and scale of such fluctuations, the sensitivity of deep source counts and the usefulness of fluctuation analysis to recover the flux distribution of still fainter sources may be reduced. This may adversely affect some of the extragalactic studies to be performed with infrared satellites.
Therefore, as part of the ISOPHOT guaranteed time programme on infrared diffuse sky brightness, we mapped a few fields of at 25 m 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 avoided any brighter infrared point sources. Since in the wavelength range of 7 m to 60 m the zodiacal light is at its maximum, any detected fluctuation probably would be due to the zodiacal light.
© European Southern Observatory (ESO) 1997
Online publication: March 26, 1998