DE 1 observations of type 1 counterstreaming electrons and field‐aligned currents

In this paper we report that the Dynamics Explorer 1 satellite frequently detected type 1 counterstreaming electrons in the region of field‐aligned currents at high altitudes in the nighttime auroral zone. Type 1 counterstreaming electrons are generally detected at energies below a few hundred electron volts in association with energetic (> keV) precipitating electron fluxes. The plasma and magnetic field measurements from DE 1 are used to investigate the relative contributions of energetic precipitating electron fluxes and counterstreaming electrons to the field‐aligned current density. The current density is computed by integrating the two‐dimensional electron distribution functions in two energy ranges: 18–235 eV and 18–10,000 eV. In the case of downward current, low‐energy (18 < E < 235 eV) electrons are found to carry more than 40% of the total plasma current density integrated above 18 eV, while higher‐energy electrons carry the rest. In the case of upward currents, less than 50% of the total plasma current density integrated above 18 eV is found to be contributed by low‐energy (18 < E < 235 eV) electrons. In general, the observed pitch angle distributions show that fluxes of counterstreaming electrons are enhanced at both small (<40°) and large (>140°) pitch angles. The energy spectra of counterstreaming electrons in the field‐aligned directions are frequently found to be characterized by Maxwellian functions and occasionally by spectral peaks. It is thus concluded that counterstreaming electrons contribute to field‐aligned current density mostly because of imbalances in the flux enhancement in the upward and downward directions. The Scandinavian Twin Auroral Radar Experiment radar simultaneous observations indicated that the field‐aligned current observed by DE 1 was closed through Pedersen currents in the ionosphere.