The correlation coefficients of the linear regression of six solar indices versus 10.7 cm radio flux 𝐹_10.7 were analysed in solar cycles 21, 22 and 23. We also analysed the interconnection between these indices and 𝐹_10.7 with help of approximation by polynomials of second order. The indices we have studied in this paper are: the relative sunspot numbers – SSN, 530.3 nm coronal line flux – 𝐹₅₃₀ , the total solar irradiance – TSI, Mg II 280 nm core-to-wing ratio UV-index, the Flare Index – FI and the counts of flares. In most cases the regressions of these solar indices vs. 𝐹_10.7 are close to the linear regression except the moments of time near the minimums and maximums of the 11-year activity. For the linear regressions, we found that correlation coefficients 𝐾_corr(𝑡) for the solar indices vs. 𝐹_10.7 and SSN dropped to their minimum values twice during each 11-year cycle.

Solar flare activity is characterized by different classification systems, both in optical and X-ray ranges. The most generally accepted classifications of solar flares describe important parameters of flares such as the maximum of brightness of the flare in the optical range – $H_{\alpha}$ flare class (change from F to B), area of the flare in $H_{\alpha}$ (change from S for areas less than 2 square degrees to 4 for areas more than 24.7 square degrees) and the maximum amplitude of the soft X-ray (SXR)-flux in the band 0.1–0.8 nm ($F^{\rm max}_{0.1−0.8}$) – X-ray flares of classes from C to X. A new classification parameter of solar flares is proposed here – the X-ray index of flare XI, based on GOES measurements of solar radiation in the SXR-range. The XI-index has a clear physical interpretation associated with the total flare energy in the SXR-range. XI is easily calculated for each flare with the use of available GOES data. The XI-index can be used along with other geoeffective parameters of Solaractivity to assess both flares and coronal mass ejections that are connected with them.