WELL-RESOLVED OBSERVATIONS BY ISEE 2 OF ION DISPERSION IN THE MAGNETOSPHERIC CUSP

During a prolonged period of southward interplanetary magnetic field on October 30, 1978, the ISEE 2 spacecraft observed an unusually prolonged and distinct region of ion energy-time dispersion in the dayside, dawn magnetosphere. Observed plasma features included the (1) presence of magnetosheath electrons and initial absence of magnetosheath ions and (2) subsequent arrival and energy-time dispersion of magnetosheath ions as the spacecraft transited outbound from 6.03 to 7.66 Earth radii. We use ISEE 2 ion measurements to illustrate these dispersive time-of-flight effects and to support our interpretation of a location in the North magnetospheric cusp. Ion energy dispersion, well documented at lower altitudes but generally only observable for a few minutes due to rapid cusp crossings by the spacecraft, in this case persists for approximately 35 min and includes well-resolved sequences of magnetosheath and magnetospheric ion distributions. We focus primarily on observations and time-of-flight analysis of ions from the magnetosheath, which are observed simultaneously entering and exiting the cusp at different energies. We discuss the observational support and shortcomings of various ion injection locations and durations and conclude that quasi-steady merging at the dayside magnetopause, with ion dispersion caused by spacecraft motion away from the last closed field line, is the most likely scenario.