This short communication is part of a thesis at the University of Heidelberg, FRG.

It deals with some dynamical aspects of small interplanetary particles, which are affected by non-gravitational forces like the Poynting - Robertson Effect.

The equations of motion of test particles are numerically integrated. The forces acting on these particles include the gravitational attraction of the Sun and the Planets as well as forces due to incident solar radiation.

The ephemerides of the Planets in the heliocentric rectangular 3-dimensional coordinate system used here are taken from the Development Ephemeris DE-102 (Newhall et al. 1983) . This restricts the time span of the integration to about 4412 years.

Measurements of longitudinal variations in the brightness and in the latitude of the solar system dust bands recently discovered by IRAS will determine the orbital elements of the particles involved and may discriminate between cometary and asteroidal models of the origin of these bands. The expected variations for bands of dust particles with common orbital elements and small eccentricity and inclination are calculated as functions of semimajor axis.

Direct observations have established the size distribution of interplanetary meteoroids at 1 AU as well as the dependence of the spatial density with respect to the distance from the sun. After evaluating the consequences of mutual collisions and the effect of radiation pressure the following conclusions can be drawn: 1. Catastrophic collisions dominate the lifetimes of meteoroids with masses m ≳ 10−5g. About 10 tons per second are lost within 1 AU (mostly in form of 10−4g to 10−1g particles). Under steady state conditions these meteor sized particles have to be replenished by other sources, e.g. comets. 2. After being crushed by collisions 70 to 85% of this mass will be in form of particles with masses 10−10g ≲ m ≲ 10−5g. Part of these “zodiacal light” particles (about 0.3 tons per second) are transported by the Poynting Robertson effect towards the sun where they will evaporate. However, since the collisional production of these intermediate sized particles exceeds their losses this population is presently not in equilibrium. 3. 15 to 30% of the collisional fragments have masses m ≲ 10−10 g. Most of these small particles will be injected into hyperbolic orbits by radiation pressure (β-meteoroids).

The scattering of dust grains orbits due to recurrent sectors of the interplanetary magnetic field is reinvestigated with a better formalism. Our method reveals the resonant character of the diffusion and is well suited for the problem. The spreads in the orbital parameters are found less important than believed untill now and to vary rapidly with eccentricity and semi-major axis. Only the small dielectric grains with size less than 0.5 μm may be scattered by the Lorentz force fluctuations; the main diffusion occurs in inclination and near the sun (20-60 R⊙).

Distribution of eccentricities, perihelion distances of interstellar particles and their concentrations in the Solar system have been computed in the assumption that their velocities are similar to ones of the nearby stars of late spectral classes. More than 75% of orbits have eccentricities not exceeding the value 1.1 and concentration of particles at the Earth’s orbit 3.5 times greater than in the interstellar space.

There is now a satisfactory convergence of the observational results for the absolute intensity of the zodiacal light for elongations larger than 20° to 30°. They are still discrepancies affecting the data for the F-corona but after carefully scrutinizing available data, Koutchmy and Lamy could obtain an axy-asymmetric non-spherical model for the equatorial and polar regions which satisfactorily bridges to the zodiacal light. However, a closer examination reveals several points of concern:

1. i) the gradient of the brightness which is of utmost importance for retrieving the phase function of interplanetary dust: the “Hawaï” data (Misconi and Weinberg), the PCN data (Nikolsky et al.) and the classical data of Dumont and Sanchez are in disagreement;

2. ii) a lack of data for the inner zodiacal light;

3. iii) insufficient out-of-ecliptic data; the on-going analysis of the “Hawaï” data (which benefits from an improved reduction method) and of the “South Africa” data will help partly remedy this situation. A consensus appears to be reached on the near solar color of the zodiacal light or, to be more precise, a slight reddening down to 2000 Å. The inner zodiacal light and the F-corona are clearly redder than the Sun. Some structure near 2800 Å reported by Feldman et al. is certainly worth of further investigation. Ultraviolet observations of Comet Halley may well provide adequate opportunities. The situation of the polarization of the zodiacal light is less satisfactory as large discrepancies persist. Of major importance for the properties of the dust grains are the behaviours at small and large scattering angles - in particular, the shape of the negative branch - as well as the angle of maximum polarization and its magnitude. The polarization is certainly color dependent but the present data are insufficient for a clear-cut conclusion.