  Effects of increasing whistler amplitude and propagation angle are studied
through a variational test particle simulation and calculations of the
resonance width. While high amplitude and oblique whistlers in typical 1 AU
solar wind parameters are capable of forming an isotropic population without
any additional processes, anomalous interactions with quasi-parallel whistlers
may be essential to the process of halo formation near the Sun. High amplitude
and quasi-parallel whistlers can scatter strahl electrons to low velocities
(less than the wave phase velocity) to form a halo population, as long as their
amplitude is sufficiently high. We also present in detail a careful treatment
of the sensitivity to initial conditions based on calculations of the phase
space volume, which is necessary for numerical calculations of highly
stochastic motion due to resonant interactions with large amplitude waves. Our
method ensures that the volume-preserving characteristic of the Boris algorithm
is consistently applied for simulations of both stochastic and non-stochastic
particle motion.
