Derivation and test of ionospheric activity indices from real-time ionosonde observations in the European region

New ionospheric activity indices are derived from automatically scaled online data from several European ionosonde stations. These indices are used to distinguish between normal ionospheric conditions expected from prevailing solar activity and ionospheric disturbances caused by specific solar and atmospheric events (flares, coronal mass ejections, atmospheric waves, etc.). The most reliable indices are derived from the maximum electron density of the ionospheric F2-layer expressed by the maximum critical frequency foF2. Similar indices derived from ionospheric M(3000)F2 values show a markedly lower variability indicating that the changes of the altitude of the F2-layer maximum are proportionally smaller than those estimated from the maximum electron density in the F2-layer. By using the ionospheric activity indices for several stations the ionospheric disturbance level over a substantial part of Europe (34 degrees N-60 degrees N; 5 degrees W-40 degrees E) can now be displayed online. (C) 2006 Elsevier Ltd. All rights reserved.