Ion composition in the auroral lower F-region inferred from residuals of ion temperature profiles observed with EISCAT Original Research Article

Ion composition in the auroral lower F-region has been obtained by correcting the EISCAT composition model with the ratio between the measured and modeled ion temperatures. The model profile of ion temperature was estimated by fitting a tan−1 function to the EISCAT measurements under the assumption that the temperature is almost free from any additional heating. In order to warrant this assumption, the data periods during which the effect of frictional heating was negligibly small have been selected using the criterion that the F-region d.c. electric field remained below 5 mV/m in magnitude. The result reveals that the transition altitude between molecular and oxygen ions has a clear diurnal variation of 18 ∼ 26 km with a minimum altitude of about 190 km around 13 LT in the summer and spring seasons. The diurnal variation of the transition width is clearly anticorrelated with the transition altitude in summer.