Dust-driven Dynamos in Accretion Disks

Abstract

Magnetically driven astrophysical jets are related to accretion and involve toroidal magnetic field pressure inflating poloidal magnetic field flux surfaces. Examination of particle motion in combined gravitational and magnetic fields shows that these astrophysical jet toroidal and poloidal magnetic fields can be powered by the gravitational energy liberated by accreting dust grains that have become positively charged by emitting photo-electrons. Because a dust grain experiences magnetic forces after becoming charged, but not before, charging can cause irreversible trapping of the grain, so dust accretion is a consequence of charging. Furthermore, charging causes canonical angular momentum to replace mechanical angular momentum as the relevant constant of the motion. The resulting effective potential has three distinct classes of accreting particles distinguished by canonical angular momentum, namely (1) "cyclotron-orbit," (2) "Speiser-orbit," and (3) "zero canonical angular momentum" particles. Electrons and ions are of class 1, but depending on mass and initial orbit inclination, dust grains can be of any class. Light-weight dust grains develop class 1 orbits such that the grains are confined to nested poloidal flux surfaces, whereas grains with a critical weight such that they experience comparable gravitational and magnetic forces can develop class 2 or class 3 orbits, respectively producing poloidal and toroidal field dynamos.