Ion radial diffusion in an electrostatic impulse model for stormtime ring current formation

Guiding‐center simulations of stormtime transport of ring‐current and radiation‐belt ions having first adiabatic invariants μ ≳ 15 MeV/G (E ≳ 165 keV at L ∼ 3) are surprisingly well described (typically within a factor of ≲ 4) by the quasilinear theory of radial diffusion. This holds even for the case of an individual model storm characterized by substorm‐associated impulses in the convection electric field, provided that the actual spectrum of the electric field is incorporated in the quasilinear theory. Correction of the quasilinear diffusion coefficient for drift‐resonance broadening (so as to define ) reduced the typical discrepancy with the diffusion coefficients deduced from guiding‐center simulations of representative‐particle trajectories to a factor ∼3. The typical discrepancy was reduced to a factor ∼ 1.4 by averaging , , and over an ensemble of model storms characterized by different (but statistically equivalent) sets of substorm‐onset times.