The interpretation of topside sounder ionograms

Most of the phenomena observed on Alouette I and Alouette II ionograms are identified and interpreted. Simple as well as complex ionograms are treated, but emphasis is placed on ionograms exhibiting very low electron density, spread-F and field-aligned propagation. It is shown that many of the ionograms normally considered too complex for analysis can yield useful geophysical parameters using the methods described in the paper. Also discussed are signals observed on Alouette II ionograms at frequencies well below foF2, due to the propagation of earth-based transmissions via either the whistler or the Z mode.