Pitch angle diffusion at Jupiter's moon Ganymede

Abstract

The magnetic field of Jupiter's moon Ganymede, provides a unique, large B value mirror point near the equator of Jovian field lines that intersect the moon. Ganymede creates a large loss cone in the distributions of all charged particles populating those field lines, emptying them of particles that mirror at radial distances < ∼ 8 RJ. Such a loss cone is seen when the Galileo energetic particles detector (EPD) views along the field line toward Ganymede. When the EPD views in the antimoon direction, the loss cone is nearly filled in for low energy electrons and remains empty at high energies. Because these electrons travel between their Ganymede mirror points and their near‐Jupiter mirror points much faster than they drift across the Ganymede magnetosphere, they are effectively trapped between the moon and Jupiter for several bounce periods. Thus the amount of filling observed in the antimoon viewing loss cone gives a direct measure of the amount of pitch angle scattering occurring during the electrons half‐bounce period. Using general wave‐particle scattering theory, we have fit the pitch angle distributions within the antimoon viewing loss cone and have extracted values of the diffusion coefficient and its variation with energy. We find weak diffusion to dominate, yielding lifetimes of ∼103 s at low energies to ∼104 s at high energies. The diffusion coefficient, assumed independent of pitch angle, varies as ∼E−1 or equivalently ∼γ−3.5 (E is the electron kinetic energy and γ is the relativistic correction factor) at energies above ∼50 keV. Finally, we note that the existence of loss cones in the antimoon direction is evidence that Jovian field lines intersecting Ganymede effectively move with the moon.