EVOLUTION OF DUST PARTICLE ORBITS UNDER THE INFLUENCE OF SOLAR-WIND OUTFLOW ASYMMETRIES AND THE FORMATION OF THE ZODIACAL DUST CLOUD

The secular orbital evolution of zodiacal dust particles is mainly inf luenced by the electromagnetic and the plasma Poynting-Robertson effec ts. Whereas the first effect is radially symmetric, the second one, ca used by the dynamical friction of the dust particles in the ambient so lar wind flow, depends on heliographic latitude. This is because the s olar wind appears to have pronounced mass and momentum flow variations with respect to heliographic latitudes, with mass flows decreasing by about 50% from the ecliptic toward the poles. We take into account re lated asymmetries in the plasma Poynting-Robertson effect and study or bital evolution of dust particles orbiting with different inclinations . We derive equations describing the secular changes of the orbital el ements under these conditions. We show that the radial migration rates are greater by about 5 to 10% for particles close to the ecliptic as compared to those at higher inclinations. This leads to typical change s in the distribution of inclinations with decreasing semimajor axes o f the particles and also shows that the dust distribution function can not be factored into parts solely dependent on radial distance or incl ination alone. Furthermore, the kinetic equation for the distribution function of dust particles is solved numerically, yielding the distrib ution in orbital element space. The spatial density of the particles c an then be found as an integral over this distribution function after application of a Jacobian transformation of element space differential s into configuration space differentials and can be compared with earl ier results. Some observational consequences are drawn from this compa rison and are discussed in some more detail. It is also pointed out th at the plasma Poynting-Robertson effect in a two-temperature solar win d produces normal components of perturbation forces and thus tends to change the inclination of the dust particle orbits. This is of special importance in the solar wind regime inside the critical point where l ow-Mach-number flows prevail. (C) 1994 Academic Press, Inc.