Observations of large amplitude parallel electric field wave packets at the plasma sheet boundary

We report the first observation of large amplitude electric fields parallel to the earth's magnetic field at radial distances of ∼6 R_E on magnetic field lines linking the plasma sheet boundary to the auroral zone. The parallel fields, with magnitudes up to 40 mV/m, occur in three different electrostatic structures: unipolar wave packets with a net potential drop, bipolar wave packets, and solitary waves. Examples from one boundary crossing are presented which show that the wave packets have durations of <1s, are traveling at velocities of ≤100 k/s, and are consistent with ion acoustic waves. The waves can contribute to the acceleration of auroral electrons and modify the pitch angle distributions. The wave packets occur in a region of large-scale field-aligned currents and are often associated with perpendicular waves near the lower hybrid frequency with strongly modulated amplitudes. The velocities determined for the solitary waves are faster than those of the wave packets.