## 4. ConclusionsThis paper deals with Saturn's E ring dust grain dynamics. First, it extends the analytical theory of the circumplanetary dust dynamics by Hamilton & Krivov (1996). In order to account for the grain's charge variability, a reasonable approximation of the potential depending on the distance from the planet is suggested and its effects are considered when combined with the solar radiation pressure, planetary oblateness and the Lorentz force due to dipole magnetic field. It is found that the distance-dependent charge does not alter the eccentricity but causes precession or regression of the orbit with the rate depending on the charging model. The variable charge does not extend the set of the phase portrait topologies described by Hamilton & Krivov (1996). Like the constant charge, the variable one allows orbits to attain large eccentricities, with the charging model by Jurac et al. (1995) being a reasonable alternative to that of Horányi et al. (1992). Second, the action of the plasma drag is investigated analytically
in the approximation of small eccentricities and zero inclinations. On
the base of the plasma model for Saturn (Richardson (1995) it is shown
that direct drag due to the heavy ion fraction (usually referred to as
the oxygen ions) is the only drag force component which is necessary
to account for. Grain's interactions with other fractions and the
Coulomb drag due to the heavy ions can also be ignored in the plasma
drag model. The primary effect of the plasma drag on a grain's orbit
is the gradual growth of the semimajor axis. This in turn leads to the
long-term growth of the eccentricity when the drag is introduced
simultaneously with the radiation pressure. The rate of the semimajor
axis growth is rapid enough to let the submicron grains launched from
Enceladus on the circular orbit leave the sweeping zone and thus
escape from recollision with the parent satellite. © European Southern Observatory (ESO) 1999 Online publication: June 17, 1999 |