IRAS MEASUREMENTS OF DIFFUSE SOLAR-SYSTEM RADIATION - ANNUAL SKY BRIGHTNESS VARIATION AND GEOMETRY OF THE INTERPLANETARY DUST CLOUD

We present an overview based on IRAS data of the properties of infrare d emission from the large-scale component of the interplanetary dust c loud. To assist in separating the smooth component of the interplaneta ry emission from Galactic emission, discrete sources, and the zodiacal emission bands, and to characterize the properties of the interplanet ary emission in compact form, we introduce an empirical function whose adjustable parameters have simple geometrical interpretations. This f unction is fitted in a lower envelope sense to IRAS scan data; the fun ction represents the data well, with rms residuals at 25 mu m of only similar to 0.3 MJy sr(-1) less than 1/2% of the peak emission. We use the parameters thus obtained at 12, 25, and 60 mu m to produce a simpl e analytical prescription for the interplanetary dust emission near a solar elongation angle of 90 degrees at any time of year. The nearly 1 yr duration of the IRAS observations permits the study of time variat ion of the infrared sky brightness due to the Earth's motion, and of t he geometry of the interplanetary dust cloud. We employ our representa tion of the interplanetary dust emission to examine by several methods the location of a possible surface of symmetry of the interplanetary dust cloud. Discrepancies between these methods, generally consistent with results from other analysis approaches and sometimes other collec tions of data, indicate that such a surface must have substantial devi ations from a plane.