Dust belt of the Earth

Abstract

The dynamical and light-scattering properties of small dust grains that orbit the earth are investigated. It is shown that the radiation pressure will cause a diffuse concentration of dust particles near the ecliptic plane. This symmetry with respect to the ecliptic plane is almost undisturbed by the gravitational perturbations beyond a geocentric distance of 7.7 earth radii. Inside 7.7 earth radii, the oblateness of the earth destroys the ecliptic symmetry of orbiting dust grains for particles larger than an approximately determined critical size. The critical size decreases with geocentric distance such that within 1 to 2 earth radii from the earth's surface essentially all ecliptic symmetry is destroyed. The radiation pressure also causes large-amplitude oscillations in the eccentricity of a dust particle orbit, which makes this perturbation the dominant factor in determining the lifetimes of the particles in orbit. When the area to mass ratio exceeds about 1500 cm2/g, the lifetime of an orbiting particle is less than 1 year. The secular Coulomb and Poynting-Robertson drags are negligible by comparison. The ecliptic symmetry of the orbiting dust implies that all the zodiacal light might be due to dust on such geocentric orbits rather than on heliocentric ones as is commonly believed. Observational evidence currently available does not distinguish between these possibilities. These observations include the dependence of brightness and polarization of the zodiacal light on elongation from the sun, lifetimes of Van Allen electrons and protons, attempted Doppler shift measurements of the Fraunhofer lines, and estimated infall rates of mass to the earth. Several experiments are discussed that may lead to a determination of the relative contributions from the two types of orbit. In an appendix a new method of estimating the photoelectron contribution to the electrostatic potential of a dust grain is used in determining the charged-particle drag as a function of plasma temperature and density. The sense of this Coulomb drag is reversed in the outer magnetosphere for forward-orbiting particles because of the corotation of the magnetosphere with the earth. It is shown that the energy addition to the particle orbit by this forward drag exceeds that dissipated by the Poynting-Robertson drag if the magnetospheric plasma temperature does not exceed a few thousand degrees.