We report observations of molecular upflowing ions in the high-altitude auroral ionosphere by the EXOS D (Akebono) suprathermal mass spectrometer (SMS). At Akebono altitude, the occurrence of molecular upflowing ions was rare and was often confined in latitudinal extent, in both the dayside and the nightside. In the dayside, molecular upflowing ions were observed up to about 85¿ &Lgr;, inside and up to ~10¿ poleward of the cleft. In the nightside, they were generally located at lower invariant latitudes (~60¿--75¿ &Lgr;) and were observed both inside and equatorward of regions of energetic (1--10 keV) electron precipitation. The observed molecular ions appeared as a minor component of the upflowing ionospheric ion population, with a flux typically less than 5% but at times as high as 15% of the total. They were composed of comparable fluxes of NO+ and N2+ ions; at times, smaller fluxes of O2+ were also observed. The observed NO+/N2+ ratio ranged from 0.3 to 1.3; the observed O2+/N2+ ratio was 0.1 or less. They were invariably accompanied by enhanced N+ ion flux, the N+/O+ ratio being 0.5--1.0. They were more intense (flux ~105 cm-2 s-1), had higher density (~0.1--0.2 cm-3) and lower energy (~5 to ~20 eV/q), and were more anisotropic and peaked near the upward field line direction, in the dayside than in the nightside.

Data from about 440 Akebono passes in the 7-month period from November 1989 to May 1990 were surveyed for their occurrence. Molecular ions were found in 14 passes, i.e., 3% of the passes. In most but not all cases, the Kp index was 4 or higher at the time of their observation and in the preceding several hours, suggesting that they typically occur in periods of sustained auroral activity. These observations are compared with previous low-altitude observations and discussed in terms of the transit and recombination times of the observed molecular ions, the orders-of-magnitude enhancement in neutral molecular densities and the corresponding N+, N2+ and NO+ ion production above the F region in periods of prolonged auroral activity, and the moderate energization of the ions immediately after their production. ¿ American Geophysical Union 1993