Ion compositions in the auroral ionosphere as observed by EISCAT

The method to extract information of the ion composition directly from incoherently scattered spectra measured in the ionospheric transition region between atomic and molecular dominated ions as described previously by Lathuillere et al. has been employed on measurements performed by EISCAT in the auroral zone. It is found that the hourly mean daily variation of the height (Z40) where the atomic O+ ion contributes 40% to the total ion density varies by the solar zenith angle in a similar manner as found by Oliver for the transition height Z50 where O+ contributes by half the ion density. The Z40 parameter is found to decrease on the average by 20 km from summer days to winter days also in qualitative agreement with the works by Oliver and Kelly and Wickwar. These results indicate that on a hourly mean basis the bottom side F-region auroral ionosphere is in photo-chemical equilibrium. Incidences are, however, found where auroral disturbances introduce larger variations in Z40 within an hour or so than are else observed between seasons, indicating that severe disturbances occur in the ion composition at F-region heights during such events. It is evident for several of these outstanding features that the ionosphere deviates from the photochemical equilibrium model. These discrepancies are discussed with reference to variations in other ionospheric parameters measured such as electric fields, particle precipitation and temperature. A new result concerns the effect of particle precipitation which can act either to increase the percentage of atomic ions, depending on the altitude of penetration