The Fate if Circumplanetary Dust Grains

Details The Fate if Circumplanetary Dust Grains The Fate if Circumplanetary Dust Grains

Oct 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.2209j&link_type=abstract

American Astronomical Society, DPS meeting #42, #22.09; Bulletin of the American Astronomical Society, Vol. 42, p.990

Astronomy and Astrophysics

Astronomy

Scientific paper

Charged dust grains in planetary ring systems experience both gravitational and electromagnetic forces. Large grains orbit the planet along Kepler ellipses, perturbed slightly by non-gravitational forces, while the smallest grains, controlled by Lorentz forces, spiral around planetary magnetic field lines. We explore the stability of dust grain orbits between these regimes, covering a wide range of charge-to-mass ratios and launch distances; first in an aligned dipole field and then in more complicated magnetic field geometries.
With an aligned dipole, some negatively charged grains are unstable to vertical perturbations via an instability first identified by Northrop and Hill (1982). The Northrop and Hill model ignores the effect of gyromotion which acts to partially stabilize the orbits of Kepler launched grains via the magnetic mirroring process. We include this effect and derive new conditions for instability. In addition, positively charged grains for which gravity and Lorentz forces are comparable have orbits that are radially unstable to escape at high speed (if launched outside synchronous orbit), or to crash into the planet (if launched within synchronous orbit). We map the stability regions of the giant planets as a function of grain size and distance, and analytically derive many of the boundaries between stable and unstable trajectories.
We also map the stability of grains orbiting around the four giant planets, with the planetary magnetic field accurately represented out to octupole components. In the non-axisymmetric fields of Jupiter, Uranus and Neptune, we find that negative grains outside synchronous orbit can still escape, but the process is much slower than for positive grains. We identify the cause of these escaping grains as destabilizing resonances driven by individual magnetic field components. Similar resonances affect both positively and negatively charged grains inside synchronous orbit. Our theories are in good agreement with our numerical simulations.

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