We investigate electrostatic waves in a magnetopause reconnection
diffusion region event. In the electron diffusion region on the
magnetospheric side, an oblique electrostatic wave is observed. The
local distribution exhibits fast non-gyrotropic electron beams with
drifts comparable to the electron thermal speed, but the wave has a much
lower phase speed. Response of ions and possible cold electrons may
contribute to wave excitation. Near the current sheet mid-plane,
parallel electron beam-mode waves are modulated by whistler waves. In
the separatrix region, parallel waves associated with field-aligned
electron beams and perpendicular electron cyclotron waves with loss cone
distributions exhibit modulation frequencies in the lower-hybrid wave
frequency range. We infer that lower-hybrid waves scatter electrons to
produce beams and alter loss cones to modulate electrostatic waves. The
results advance our understanding about the regimes and mechanisms of
electrostatic waves in reconnection and their coupling with
lower-frequency waves.