Perturbations in EISCAT electron densities visualised by normalisation

The European incoherent scatter radar installation EISCAT near Tromsø, Norway, yields electron densities from the D- to the F-region. Variations of the electron densities can be predicted by season, solar activity, and solar zenith angle; at auroral latitudes one needs additional parameters to quantify the common state of a disturbed ionosphere such as riometer absorption, a magnetic index and geomagnetic time. A neural network-based model using the above parameters as inputs to predict the ionosphere up to 150 km was recently developed. The ratio between actual measurement and the corresponding value from a perfect model should ideally be unity. Deviations from it can be due to inadequacies of the empirical electron density model, or simply reflect the fact that any model can only represent average conditions. Measurements taken from intervals of good EISCAT coverage of the lower ionosphere were normalised by their corresponding model values. Residual variations in the electron densities thus obtained emphasise features which may be due to a variety of conceivable processes.