Scatter plots of daily SOHO/SEM 26-34 nm EUV flux versus (a) the F_10.7 index and versus (b) the S index during the solar cycle 23; the parameter r is the correlation coefficient between the EUV flux and the indices. (c) Variations of mean midday f_oF₂ at Kokubunji; the red line for observed values, the blue line for F_10.7 fitted values, and the grey line for the S index fitted values with 1–day lag of f_oF₂ to S being included; r is the correlation coefficient between observed and fitted f_oF₂. (d) Difference between observed and F_10.7 fitted f_oF₂; the parameter STD is the standard deviation of the difference. (e) Same as Figure 2d but for the S index fitted f_oF₂.