  Recent study has demonstrated that electron cyclotron harmonic (ECH) waves
can be excited by a low energy electron beam. Such waves propagate at
moderately oblique wave normal angles (~70). The potential effects of
beam-driven ECH waves on electron dynamics in Earth's plasma sheet is not
known. Using two-dimensional Darwin particle-in-cell simulations with initial
electron distributions that represent typical plasma conditions in the plasma
sheet, we explore the excitation and saturation of such beam-driven ECH waves.
Both ECH and electron acoustic waves are excited in the simulation and
propagate at oblique wave normal angles. Compared with the electron acoustic
waves, ECH waves grow much faster and have more intense saturation amplitudes.
Cold, stationary electrons are first accelerated by ECH waves through cyclotron
resonance and then accelerated in the parallel direction by both the ECH and
electron acoustic waves through Landau resonance. Beam electrons, on the other
hand, are decelerated in the parallel direction and scattered to larger pitch
angles. The relaxation of the electron beam and the continuous heating of the
cold electrons contribute to ECH wave saturation and suppress the excitation of
electron acoustic waves. When the ratio of plasma to electron cyclotron
frequency wpe/wce increases, the ECH wave amplitude increases while the
electron acoustic wave amplitude decreases. Our work reveals the importance of
ECH and electron acoustic waves in reshaping sub-thermal electron distributions
and improves our understanding on the potential effects of wave-particle
interactions in trapping ionospheric electron outflows and forming anisotropic
(field-aligned) electron distributions in the plasma sheet.
