OBSERVATIONS OF THE STRUCTURE OF THE THERMOSPHERIC RELATIVE COMPOSITION N(H) N(O) WITH THE EISCAT-VHF RADAR/

Previous papers have shown, using EISCAT-VHF and -UHF measurements, th at the atomic hydrogen concentration given by semi-experimental neutra l atmospheric models, such as CIRA-86, is underestimated by a factor o f approximately five, and sometimes much larger. This factor has been derived at 600 km. We show in this paper that the factor varies with a ltitude, mainly because of the high-latitude transport effect acting o n the profile structure of the atomic hydrogen concentration. Our resu lts show that the model underestimates the atomic hydrogen concentrati on at lower altitudes, while at higher altitudes it overestimates the concentration. The transition altitude, above which transport effects start to become important, is calculated and its tentative dependence on magnetic activity is discussed.