Upward electron beams measured by DE‐1: A primary source of dayside region‐1 birkeland currents

Measurements made by the High Altitude Plasma Instrument on DE‐1 have shown that intense upward electron beams with energies from ≈20 eV to ≈200 eV are a common feature of the region just equatorward of the morning‐side polar cusp. Computations of the currents carried by these beams and by the precipitating cusp electrons show excellent agreement with the simultaneous DE‐1 magnetometer measurements for both upward and downward Birkeland currents. The data indicate that cold ionospheric electrons, which carry the downward region‐1 Birkeland currents on the morning side, are accelerated upward by potential drops of a few tens of eV at altitudes of several thousand kilometers. This acceleration process allows spacecraft above those altitudes to measure routinely the charge carriers of both downward and upward current systems.