Most of the BEN wave forms observed by GEOTAIL in the Plasma Sheet Boundary Layer (PSBL) appear as a series of isolated spiky pulses which are termed ''Electrostatic Solitary Waves (ESW).'' Comparison between the BEN observations and plasma measurements shows that the uppermost frequency of the ESW is closely related to the temperature of the flowing ions in the PSBI. We also find that the observed spike width of the ESW and the inter-pulse-time-span change very rapidly on time scales ranging from a few milliseconds to a few hundreds of milliseconds, suggesting that the speed of the ESW potential changes very rapidly. ¿ American Geophysical Union 1994