Correlated Electric Field and Low-Energy Electron Measurements in the Low-Altitude Polar Cusp.

Correlated electric field and low-energy electron measurements are presented for two passes of Hawkeye 1 through the south polar cusp at 2000 km altitude during local morning. In one case the electric field reversal coincides with the boundary of detectable 5.2-keV electron intensities and the equatorward boundary of the cusp. In the other case the electric field reversal and the 5.2-keV electron trapping boundary coincide but the equatorward edge of the cusp as determined from the presence of 180-eV electron intensities is 5 deg invariant latitude equatorward of the eletric field reversal. We conclude that in the second case electron intensities associated with the polar cusp populate closed dayside field lines and hence the corresponding equatorward edge of these electron intensities is not always an indicator of the boundary between closed dayside field lines and polar cap field lines. Author