Near‐conjugate observations of polar cusp electron precipitation using DE 1 and DE 2

Abstract

Near‐conjugate plasma measurements from the DE 1 high‐altitude plasma instrument (HAPI) and the DE 2 low‐altitude plasma instrument (LAPI) instruments in the dayside polar cusp are examined to determine quantitatively the energization of precipitating cusp electrons. The plasma moments, including number flux, energy flux, number density, and average energy, are computed from the plasma measurements with energies between 15 and 13,000 eV. At the DE 1 altitudes (near 20,000 km) the typical polar cusp has a downward density around 1 cm−3, a peak value of the downward number flux around 109 cm−2 s−1, a peak value of the downward energy flux around 1 erg/cm² s, and an average energy around 150 eV. At the DE 2 altitudes (near 1000 km) the downward energy flux is 1–15 times that measured by DE 1. Using the DE 1 plasma data, the downward energy flux expected at the DE 2 altitudes is modeled according to the convergence of the dipole magnetic field lines and compared with the DE 2 measurements. Contributions to the total downward energy flux by secondary electrons and turbulent processes are neglected in the modeling. For two cusp passes, the DE 1 HAPI plasma data produce energy fluxes larger than the LAPI measurements after mapping to the DE 2 altitudes without downward electron acceleration. A downward acceleration is found to be needed to explain a more than 10 times gain of energy flux for the other four conjugate passes examined. Assuming adiabatic acceleration of electrons by an electrostatic field, the upper limit of the cusp potential drop is about 400 V. This estimate is consistent with the cusp potential drop inferred separately from the characteristics of distribution functions with small pitch angles. These results suggest that significant plasma acceleration sometimes occurs along field lines between the two DE satellites.