Spiky electric fields in the magnetotail

Large-amplitude spiky electric fields which have been proposed to provide e nergization and heating of particles have been observed in many regions of the magnetosphere, including the bow shock, auroral zone, and plasma sheet boundary. We extend previous statistical studies of such fields in the magn etotail (which were limited to less than or similar to 22 R-E) to cover the region explored by the Geotail satellite. Results are presented concerning the occurrence frequency, location, and association with density gradients of spiky electric fields in the magnetotail at radial distances of similar to 10-200 R-E. Spiky fields do not occur beyond similar to 100 R-E, and th ey predominantly occur at or near the plasma sheet boundary (as indicated b y changes in the spacecraft potential) and are always associated With waves near the lower hybrid frequency. Spiky fields occur preferentially during periods of higher magnetic activity (Kp > 3). Statistical analysis of the d irection of the largest field amplitudes provides information on the polari zation of the waves and the spikes and therefore provides constraints on th e origin of the fields. Waves which accompany the spikes are primarily pola rized perpendicular to the magnetic field and are consistent with lower hyb rid waves, while the spikes are polarized obliquely to the magnetic field. The results are described and compared with various possible generation mec hanisms, including lower hybrid wave collapse, a leading candidate for expl aining these spikes.