III.A.1 Processes Affecting Grains

The direct radiation "overpressure" factor b = F_rad /F_grav is a function of grain size, density, and albedo (Burns et al. 1979). Values of b greater than 0.5 result in expulsion from the solar system of a grain released from a parent body in a circular solar orbit. Such values obtain for silicate grains smaller than roughly 1 µm radius. The time scale for expulsion is only of order the orbit period, i.e. essentially instantaneous compared with other time scales. Thus, this process effectively produces a lower limit to the grain size commonly found in solar orbit.

Grains large enough not to be expelled spiral toward the Sun under the influence of PR drag. The PR time to de-orbit a black 30 µm-radius (typical) grain with density 3 g cm^-3 starting from 1 AU is about 10⁵ yr (Burns et al. 1979). This time scale is proportional to grain size for a given grain composition and proportional to the square of the initial orbit radius. The drift speed increases as r^-1. In a "wedge" geometry, equilibrium between a constant dust creation rate and destruction via PR drag would produce a surface density proportional to r⁰, approximately as observed in Earth's vicinity.

The dust mutual collision time scale is of order the orbit period divided by the cloud optical depth perpendicular to the symmetry plane (e.g. Backman and Paresce 1993). This is about 10⁷ - 10⁸ yr for the local zodiacal cloud, much longer than the PR drag time scale. This further indicates that local evolution is controlled mainly by PR drag. Those particles which do collide can produce fragments smaller than the radiation pressure "blowout" size and that material then exits the system rapidly.