An artificial neural network (ANN) is applied to retrieve electron density at satellite height from cosmic radio noise measurements made near the plasma cutoff frequency in the topside ionosphere. Observations were made using the topside sounder on board the Ionosphere Sounding Satellite launched in 1978. The automatic gain control voltage of the sounder receiver supplied information on the spectrum of cosmic radio noise intensity. Local electron densities, which are the target of the ANN, were scaled from the plasma resonance spikes and the echo traces in the ionogram for several passes. Each pass contained about 100 frames of ionograms; it is not a very easy task to scale the local plasma parameters by an eyeball judgment for all passes. With the aid of a neural network application, we determined the in situ electron density for about 150,000 ionograms. The results were applied to global ionospheric mapping.