Relationships Between Solar Activity Indices in Different Time Intervals

The article presents the results of the analysis of long-term changes in the relationship between solar activity indices for 1953-2023. For this purpose, the annual moving averages of the 10, 30, , and indices were used - the solar radio emission fluxes at wavelengths of 10.7 and 30 cm, the ratio of the central part to the flanks in the magnesium emission band of 276-284 nm, the international sunspot number and the ionospheric index, which is determined from ionospheric data as an analogue of the sunspot number. It has been found that the entire measurement period can be divided into the intervals 1953-1980, 1981-2012 and 2013-2023, in which the relationships between the solar activity indices differ distinctly. In the interval 1953-1980, these relationships are stable, i.e. there is practically no linear time trend in the dependence of one solar activity index on another. In the interval 2013-2023, such trends are usually significant. The boundaries of these intervals (1980 and 2013) approximately correspond to the maxima of the first and last solar cycles in the decreasing activity regime, when the large-scale solar magnetic field and the solar cycle height decrease over time. Therefore, the relationships between the solar activity indices, including the relationships between the ionospheric index and solar indices provide additional information on changes in the solar cycle regimes and can serve as one of the characteristics of changes in these regimes.