The concept of indices for long-term ionospheric predictions relies on the assumption that the important characteristics of the ionosphere, such as the critical frequencies of the various layers and the MLTF factor M (3000) F2, depend in a systematic way on certain measurable quantities concerned with solar radiation. It should however be noted that the correlation between these indices and the actual ionospheric characteristics does not necessarily imply a causal relationship, but rather an indication of associated phenomena. Changes of solar activity in general contain the components.