The Interaction of Solid Particles with the Interplanetary Medium

Abstract

Craters on the lunar surface proved that a continuous spectrum of interplanetary particulates exists from km-sized boulders (small asteroids and comets) to submicron sized dust grains. The dominant force for all particles, except for the smallest dust grains is the solar gravitational attraction

$${\rm F}_{\rm grav}({\rm r})=\gamma{{\rm M}_{\odot}{\rm m}\over{\rm r}^{2}}=2.5\ {\rm s}^{3}\rho\left({{\rm r}\over{\rm r}_{\rm o}}\right)^{-2}$$

((1))

where γ = 6.67x10^-8 g^-1 cm^-3 s^-2 is the gravitational constant and M ⊙ = 1.99x10³³ g the solar mass. With particle radius s in (cm), its density p in (g cm^-3) and r_o = 1 AU the gravitational force is given in dynes. Scattering and absorption of solar radiation by an interplanetary particle leads to a radiation pressure force F_rad directed almost radially outward (cf. Burns et al., 1979). The ratio of radiation pressure to gravitational attraction (both forces have the same dependence with solar distance r) has for spherical particles the value (Dohnanyi, 1978)

$$\beta={{\rm F}_{\rm rad}\over{\rm F}_{\rm grav}}=5.7\times 10^{-5}{{\rm Q}_{\rm pr}\over\rho_{\rm s}}$$

((2))

With Q_pr being an effiency factor for the momentum transfer. Q_pr = 1 for a perfectly absorbing sphere, for real particles, however, Q_pr decreases for decreasing s below 10^-5 to 10^-4 cm (i.e order of the effective wavelength of the solar light.) Therefore β has its maximum value for absorbing particles (like carbon or magnetite) at β = 2 to 5 and for dielectric particles (like silicates) at β = 0.5 to 1. Radiation pressure may dominate in the size regime from 10^-5 to 10^-4 cm, below that it becomes less important again.