Comparative correlations of ƒ°F2 with “Ionospheric sunspot‐number” and ordinary sunspot‐number

Comparison was made between the correlation of monthly‐average ƒ° F 2 at several ionospheric stations with monthly‐average Wolf relative sunspot‐number as compiled at Zurich, and with a new index of solar activity which has been provisionally called the “ionospheric sunspot‐number.” This quantity is derived from the ƒ° F 2 observed at Washington, D.C.; Huancayo (Peru) and Watheroo (Western Australia)—stations having the longest time series of such data—for the hours 10 through 14 local time, using the well‐established trends of ƒ° F 2 with sunspot‐number for these times and places. The average of correlation‐coefficients between smoothed annual sunspot‐number and monthly average of ƒ° F 2 at noon for these stations for June, September, and December is 0.94. The stations for which comparative correlations were made were not those whose data were used in deriving this so‐called “ionospheric sunspot‐number”, but others, having a series of ƒ° F 2‐data extending over nearly as long a period of time. The correlation‐coefficients for these are given in Table 1.