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This is a study of the negative ion chemistry\udin the mesosphere above Tromsø using a number of EISCAT observations of high\udenergy proton precipitation events during the last solar maximum, and in\udparticular around sunset on 23 October, 1989. In these conditions it is possible\udto look at the relative importance of the various photodetachment and\udphotodissociation processes controlling the concentration of negative ions. The\uddata analysed are from several UHF GEN11 determinations of the ion-plasma ACF\udtogether with the pseudo zero-lag estimate of the `raw' electron density, at\udheights between 55 km and 85 km, at less than 1 km resolution. The power\udprofiles from the UHF are combined with the 55-ion Sodankylä model to obtain\udconsistent estimates of the electron density, the negative ion concentrations,\udand the average ion mass with height. The neutral concentrations and ion\udtemperature are given by the MSIS90 model. These parameters are then used to\udcompare the calculated widths of the ion-line with the GEN11 determinations. The\udion-line spectrum gives information on the effects of negative ions below 70 km\udwhere they are dominant; the spectral width is almost a direct measure of the\udrelative abundance of negative ions.  <b>Key words.</b> Ionosphere (auroral ionosphere; ion\udchemistry and composition; particle precipitation)

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