Negative ion composition in the auroral mesosphere during a PCA event around sunset

Carlos F del Pozo1, Esa Turunen2, and Thomas Ulich2

1University of Wales Aberystwyth, Wales, UK
2Geophysical Observatory, Sodankylä, Finland

We study the negative ion chemistry in the mesosphere above Tromso using a number of EISCAT observations of high energy ion-precipitation (PCA) events during the last solar maximum, and in particular around sunset on October 23 1989. In these conditions we are able to look in some detail at the relative importance of the 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, at less than 1 km resolution. The power profiles from the UHF are combined with the 55-ion Sodankyla model to obtain consistent estimates of the electron density, the negative ion concentrations, and the average ion mass with height. The neutral concentrations and ion temperature are given by the MSIS90 model. These parameters are then used to compare the calculated widths of the ion-line with the GEN11 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.