The Effects of Large Amplitude Field Line Resonance Structure and Azimuthal Localization on Drift Resonant Electron Dynamics

We examine the effects of: i) strongly peaked radial structure, and ii) azimuthal localization of ULF wave field line resonances (FLRs, with specified frequency ω in the Pc5 range) on drift-resonant particle dynamics. Starting with an analytic MHD model in a dipole field to describe continuously driven/damped ULF wave modes, we use the bounce-averaged formalism of Northrop [1963] to obtain equations of motion for charged particles in the wave frame, and analytic solutions are found for the case of temporally and azimuthally constant ULF wave amplitude profiles (i.e. a single ω and azimuthal mode number, m). We demonstrate that, for sufficiently peaked FLR radial profiles, multiple drift resonances appear that are associated with the FLR peak. These are in addition to the well-known zeroth order drift resonance location, occurring when the unperturbed azimuthal drift speed dφ/dt satisfies the resonance condition (mdφ/dt - ω = 0). An example is shown in the attached figure , which shows drift-resonance islands for various electron magnetic moments associated with an FLR at L = 5.5 (with ω/2π=4.8mHz, m = 30). It is well known that internally and externally driven Pc5 ULF waves are very often localized in MLT. The effect of the azimuthal localization of wave fields on the particle motion is considered by introducing a discrete spectrum of azimuthal modes. Using a test particle model, we predict transient signatures of the altered drift-resonances in residual particle flux resulting from azimuthally localized FLR structures that should be observable in ultra-high-resolution MagEIS particle flux data of the Van Allen Probes mission. As shown previously (Degeling et al., Planet. Space Sci., [2007]), these azimuthal modes lead to stochastic particle dynamics and diffusive radial transport on long timescales, when the "2/3 rule" resonance overlap criterion is satisfied (Lichtenberg and Lieberman [1991], Chapter 5). Here we also examine how the additional resonance islands associated with the FLR alter the stochasticity threshold, the extent of the region of particle phase space available to stochastic dynamics, and the development diffusive radial transport.