The background electron temperature and density at altitudes between 1000 and 8000 km at invariant latitudes >60¿ have been determined from swept Langmuir probe measurements from the S3-3 satellite. These plasma parameters are determined by fitting the measured probe current-voltage relation to the expected theoretical response. Statistically acceptable fits are found for ~20% of all measurements and do not include measurements within the auroral density cavity. The results indicate that the density varies as an inverse power law with increasing altitude which has a typical value of 10 cm-3 at 8000 km in altitude. The electron temperature shows a slight increase with altitude but is <5 eV for almost all measurements. These results suggest that the background plasma outside of auroral density cavities on high-latitude field lines below 8000 km is dominated by cold plasma of ionospheric origin which is at least an order of magnitude more dense than hotter magnetospheric components. ¿ 1998 American Geophysical Union