DYNAMICS OF DUST NEAR THE SUN

In an effort to shed some light on the main features of the innermost part of the zodiacal cloud, the solar F-corona region, for which both observational and theoretical studies still give controversial results , we model the dynamics and physical evolution of dust grains at sever al solar radii (Ro) from the Sun. We take into account solar gravity, direct solar radiation pressure, Poynting-Robertson force, sublimation , and the Lorentz force. The latter is computed on the base of (i) the grain surface potentials derived from elaborate model calculations an d shown to vary from +3 to +12 V; (ii) a multipole radial model of the actual solar magnetic field for the period 1976-1996. The dust partic les are assumed to be porous and compact spherical grains, made of two types of material: dielectric (silicate) grains and absorbing (carbon ) ones. Our main results can be summarized as follows. The decrease of grains' sizes and the dynamics of particles in the orbital plane are well described by taking into account solar gravity and radiative forc es together with the sublimation process, being relatively insensitive to the electromagnetic force. The silicate grains typically move inwa rd in near-circular spirals until intensive sublimation starts and the y disappear at heliocentric distances from 2 to 3 R.. The carbon grain s intensively sublimate near 4R.. After several radial oscillations, t hey are eventually ejected out as beta-meteoroids, when they approach a critical radius of approximate to 2.4 mu m (for porous grains) or ap proximate to 0.5 mu m (for solid spheres), which corresponds to the ra diation pressure to solar gravity ratio beta equal to unity. The orien tation of the orbital planes of the particles is dictated by the Loren tz force. Both porous and compact carbon grains possess high beta rati os and must be larger than respectively 2.4 and 0.5 mu m to reach the near-solar region. For these sizes, the Lorentz force is relatively we ak, comes basically from the dipole zonal component of the field, and leads to low-amplitude oscillations of orbital inclinations and a prec ession of the lines of nodes. The same behavior is predicted for silic ate porous (compact) grains larger than 2 mu m (1 mu m) and 1 mu m (0. 5 mu m) for the periods of quiet and active Sun, respectively. From th ese sizes to smaller ones, the Lorentz force effectively broadens the initial distribution of inclinations of silicate grains. Submicrometer -sized particles easily get in polar and retrograde orbits well before the evaporation. On the whole, we find that the dynamics of near-sola r grains depend radically on their sizes, chemical composition, and st ructure and, in cases of relatively small dielectric grains, may be se verely correlated to the solar activity cycle. (C) 1998 Academic Press .