Negative ions in the auroral mesosphere during a PCA event around sunset

This is a study of the negative ion chemistry in the mesosphere above Troms phi using a number of EISCAT observations of high energy proton precipitat ion events during the last solar maximum, and in particular around sunset o n 23 October, 1989. In these conditions it is possible to look at the relat ive importance of the various photodetachment and photodissociation process es controlling the concentration of negative ions. The data analysed are fr om several UHF GEN11 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 kin, at less than I km resolution. The power profiles from the UHF are combined with the 55-ion Sodankyla model to obtain consistent esti mates of the electron density, the negative ion concentrations, and the ave rage ion mass with height. The neutral concentrations and ion temperature a re given by the MISIS90 model. These parameters are then used to compare th e calculated widths of the ion-line with the GENII determinations. The ion- line spectrum gives information on the effects of negative ions below 70 km where they are dominant; the spectral width is almost a direct measure of the relative abundance of negative ions.