OBSERVATIONS OF LARGE-AMPLITUDE PARALLEL ELECTRIC-FIELD WAVE-PACKETS AT THE PLASMA SHEET BOUNDARY

We report the first observation of large amplitude electric fields: pa rallel to the earth's magnetic field at radial distances of similar to 6 R-E on magnetic field lines linking the plasma sheet boundary to th e auroral zone. The parallel fields, with magnitudes up to 40 mV/m, oc cur 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 les s than or equal to 100 k/s, and are consistent with ion acoustic waves . The waves can contribute to the acceleration of auroral electrons an d modify the pitch angle distributions. The wave packets occur in a re gion of large-scale field-aligned currents and are often associated wi th perpendicular waves near the lower hybrid frequency with strongly m odulated amplitudes. The velocities determined for the solitary waves are faster than those of the wave packets.