Relative ion composition variation over the dip equator- a comparison of measurements with IRI

Abstract

Four ion composition measurements carried out over Thumba, an equatorial station during 1978-1982 in the early morning hours (χ =70° to 90°) are compared with the appropriate model (IRI, 1981). The model values are shown to underestimate consistently the ratio [N0⁺]/[O₂⁺] at least by a factor of 1.6. However, they agree with the measured values over mid-latitudes fairly well. The discrepancy over the equatorial latitudes is explained on the basis of ion dynamics. During the night as the background electron density is decreased the life-time of the dominant molecular ion NO⁺ becomes enhanced proportionately. As the usual drift direction during night-time is downwards, NO⁺ ions are brought down from above due to the Pedersen currents prevalent as a part of the electrojet current system and thus enhance its concentration down below. This feature is suggested to be the reason for the measured [NO⁺]/[O₂⁺] being consistently larger than the IRI predicted values. This should then be incorporated in the formulation of IRI in order to represent equatorial latitudes better. The method of estimating the densities of minor constituents like NO under the assumption of simple photochemical equilibrium (without transport effects) is also not valid for equatorial latitudes.