Phase function and polarization curve of interplanetary scatterers from zodiacal light photopolarimetry

Abstract

Wide-angle ecliptic measurements of zodiacal light brightness ( Z) and polarization ( P) lead to fundamental results about optical properties of interplanetary scatterers, under a few reasonable assumptions (that they depend upon heliocentric distance by a r − n law, and suffer no significant distortion of their scattering indicatrix between 0.5 and 2 a.u.): 1. The phase function σ(θ) is expressed (Equation 6) as a function of n and of ( Z) data. 2. At the elongation ϵ = 90°, the derivative dZ dϵ yields an absolute determination of the intensity T scattered at right angles from the Sun by a single unit-volume of interplanetary medium (Equation 7). 3. The polarization degree P (θ) of the sunlight scattered by a single volume is derived (Equation 12) from n and from ( Z + P) data. For two special values of the scattering angle θ, n vanishes in Equation (12), so that a fair knowledge of the polarization curve (Fig. 2) is reached prior to any assumption, or any forthcoming Jupiter-probe measure, about the value of n. Should n be provided by the Pioneers, then a thorough treatment of the whole problem of phase function and polarization curve can be performed by means of Equations (6) and (12) supplied with available zodiacal light photopolarimetric observations.