Annales Geophysicae, 17, 782-793, 1999.
We study the negative ion chemistry in the mesosphere from a number of EISCAT observations during high energy ion-precipitation (PCA) events around sunset and sunrise. In these conditions we are able to look in some detail at the relative importance of various photodetachment and photodissociation processes controlling the concentration of negative ions.
The data analysed are from several UHF GEN-11 determinations of the ion-plasma ACF together with the pseudo zero-lag estimate of the 'raw' electron density, at heights between 55 km and 85 km, less than 1 km resolution. The power profiles from the UHF are combined with the 55-ion Sodankylä model to obtain consistent estimates of the electron density, the negative ion concentrations, and the average ion mass with height. The neutral concentrations and the ion temperature are given by the MSIS90 model. These parameters are then used to compare the calculated widths of the ion line with the GEN-11 determinations. The ion-line spectrum gives information on the effects of negative ions below 75 km where they are dominant; the spectral width is almost a direct measure of the relative abundance of negative ions.