The Fate of Sub-micron Circumplanetary Dust Grains II: Multipolar Fields

TL;DR

This study investigates the stability of sub-micron dust grains around planets with complex magnetic fields, revealing how non-axisymmetric fields and variable grain charge influence dust escape and stability.

Contribution

It extends previous models by including non-axisymmetric magnetic field components and analyzing their effects on dust grain stability and escape mechanisms.

Findings

Non-axisymmetric magnetic fields cause significant dust instability.

Variable grain charge greatly affects micron-sized dust stability.

Uranus and Neptune's magnetic fields lead to higher dust instability.

Abstract

We study the radial and vertical stability of dust grains launched with all charge-to-mass ratios at arbitrary distances from rotating planets with complex magnetic fields. We show that the aligned dipole magnetic field model analyzed by Jontof-Hutter and Hamilton (2012) is an excellent approximation in most cases, but that fundamentally new physics arises with the inclusion of non-axisymmetric magnetic field terms. In particular, large numbers of distant negatively-charged dust grains, stable in a magnetic dipole, can be driven to escape by a more complex field. We trace the origin of the instability to overlapping Lorentz resonances which are extremely powerful when the gravitational and electromagnetic forces on a dust grain are comparable. These resonances enable a dust grain to tap the spin energy of the planet to power its escape. We also explore the relatively minor influence of different launch speeds and the far more important effects of variable grain charge. Only the latter are capable of significantly affecting the micron-sized grains that dominate visible and infrared images of faint dust rings. Finally, we present full stability maps for Earth, Jupiter, Saturn, Uranus, and Neptune with magnetic fields modeled out to octupole order. Not surprisingly, dust in the tortured magnetic fields of Uranus and Neptune show the greatest instability.