The differences in the relationship between ionospheric critical frequencies and sunspot number for different sunspot cycles

Analysis of data for Washington and Watheroo indicates differences in the relationship between ƒ°F2 and sunspot number for the current and preceding sunspot cycles. The sunspot number is therefore not entirely satisfactory as an index for ionospheric variations. Consequently, ionospheric data for the current cycle only should be used in preparing ionospheric radio propagation predictions whenever possible.