UPFLOWING IONOSPHERIC IONS IN THE AURORAL REGION

Observations of upflowing ionospheric ions are obtained nearly simultaneously by DE 1 and DE 2 over the nightside auroral regions. At low altitudes, the mean value of the net upward ion number flux is of the order of 10(9) CM-2 s-1. The ionosphere is predominantly O+, and the flux of ions with energy greater than 5 eV is a very small fraction (less than 1%) of the total ion flux. At high altitudes, the upflowing ions are accelerated by a parallel electric field and heated (with characteristic energies of hundreds of electron volts). Comparing upflowing fluxes at high and low altitudes yields an estimated height of the bottom of the auroral acceleration region of 1400-1700 km for the region of peak potential drop. This low-altitude acceleration could either be from a parallel electric field or from perpendicular acceleration. The fluxes at the edges of the arc are mostly H+ thus implying a higher-altitude base of the acceleration region at the edges where the potential drop is lower.