Zodiacal light gathered along the line of sight: Retrieval of the local scattering coefficient from photometric surveys of the ecliptic plane

Abstract

A clue towards a retrieval of the zodiacal brightness gathering along a line of sight in the ecliptic plane consists in introducing the other intersection of that line with the terrestrial orbit (Fig. 1). The distribution of the elemental contribution to the brightness, or of the local quantity D [directional scattering coefficient, i.e. cross-section of the unit-volume, which gives very simple expressions (1), (2) for the brightness integral] can then be approached with reduced uncertainty. The assumptions-steady state of the zodiacal cloud; smooth distribution of D —are strongly suggested by the observations, and are much less controversial than the classical assumption of uniform composition and size everywhere. The scattering coefficient may vary along the line of sight as seen in Fig. 3 : an uncertainty bar highly dependent of the abscissa, and considerably reduced in the vicinity of two “nodes”. Both in abscissae and in ordinates, these nodes are conspicuously insensitive to the arbitrary choice of a mathematical model (Table 1). The node exterior to the Earth's orbit (“martian node”) remains at r ≅ 1.5 a.u. from the Sun (Fig. 4). It gives access to a range of the scattering phase function near Mars' orbit, deconvolved from any radial dependence of that function (Fig. 5). The backscattering effect obtained is a new confirmation of the non-terrestrial origin of the gegenschein. The node interior to the Earth's orbit remains located not far from the middle of each chord (“quasi-radial node”. Fig. 4). It allows to retrieve the radial dependence of D , partly deconvolved from its angular dependence, between 0.5 and 1 a.u. (Fig. 6 and Table 4). The uncertainty bars on D at the two observing locations yield two uncertainty bars of the phase function σ( θ) at 1 a.u. (Fig. 7). At θ = 30°, the forward scattering efficiency (normalized to θ = 90°) cannot exceed 6 and more likely 4. This disagrees with higher values obtained assuming spherical particles, and even obtained in part of the more realistic studies (assuming irregularly shaped particles, or mainly observational) reviewed in Table 5. All of these results are derived, with fair agreement, from three independent observational sources.