On Trapped Particle Dynamics in Rotating Frames

Abstract

The rapid rotation of Jupiter and Saturn, combined with internal source of plasma provided by their moons Io and Enceladus respectively, creates a magnetodisk structure of the planetary magnetic field. The magnetodisk looks like a stretched dipole magnetic field in the equatorial region, where centrifugal force is largest. The centrifugal force, originating in the rotating frame, is known to have large contribution to the magnetodisk structure in the Jovian and Kronian magnetospheres. In order to investigate deviations in the dynamics of charged particles trapped in a magnetodisk compared to a pure dipole magnetic field, this thesis studies how centrifugal force influences a trapped particle's bounce motion as described by the so-called guiding centre approximation.

Here a model characterising a trapped particle's bounce period in a rotating frame of reference is presented. It is evident that conservation of energy and conservation of first adiabatic invariant put constraints on the particle motion along the field line. The beta parameter is a boundary condition to the model that determines the rate of change between kinetic and potential energy along the field line, and describes to which degree the system is affected by rotation. The bounce period is larger than in a non-rotating frame when inverse parallel velocity component increases faster than mirror point latitude decreases, and shorter for the opposite case. How these components change in relation to each other varies as a function of beta. Small values of beta results in longer bounce periods for particles with small equatorial pitch angles, and shorter bounce periods for particles with large equatorial pitch angles. An effect of rotation when beta increases is that also particles with small pitch angles are confined towards equator, bouncing with shorter bounce periods compared to a non-rotating frame. The beta parameter, describing the ratio of centrifugal potential energy to kinetic energy at equator, is thus a prerequisite for the particle dynamics along the field line in a rotating frame.