Figure 4: Comparison of Fokker–Planck simulation results with observations.

(a,b) Observed evolution of the relativistic electron fluxes as a function of pitch angle at the same location (shown inside panels) during the simulation. (c,d) Starting with an initial condition representative of relativistic electrons (a), we show the relativistic (2–3.6 MeV) electron fluxes due to acceleration by chorus and MS waves from a numerical solution to the two-dimensional Fokker–Planck diffusion equation. Acceleration is most pronounced within the medium pitch angle region 30°–60° or 120°–150° produced by both chorus and MS waves, and noticeable around the high pitch angles induced by chorus waves. The combined acceleration by chorus and MS waves leads to the butterfly distribution both in the magnitude and the time scale (d) comparable to the observation (b). The current simulation provides a further support for wave-driven butterfly distribution of relativistic electrons during this storm.