Centennial solar EUV irradiance from ionospheric currents: Varying sunspot-EUV irradiance relation and modified spot-facula ratio

Abstract

Sunspots depict large variability during the last 100 years, a period called the Modern Maximum (MM). However, other variables are needed to study the long-term evolution, e.g., of weaker fields and different radiative emissions. Recently, the relation between sunspots and the F10.7 and F30 radio fluxes and the MgII index (proxies of EUV irradiance) was found to vary during the last 70 years so that a relative sunspot dominance over EUV in the 1950s-1960s changed to EUV dominance in the 2000s (Mursula et al., 2024). Here we use data from eight long-operating observatories to calculate the yearly range of daily variation of the geomagnetic Y-component, the rY index, for the last 137 years. The rY index correlates very well with the MgII index and the solar F30 radio flux. These three indices have no trend relative to each other. On the other hand, the F10.7 flux has a significant trend with respect to the three co-varying EUV indices (MgII, F30, rY). Therefore, the rY index replaces F10.7 as the best long-term EUV proxy, and extends the MgII index by 90 years. We verify that all the four EUV proxies (rY, MgII, F30, F10.7) have an increasing trend with respect to sunspots during the last 50-70 years. This is valid both for sunspot numbers and group numbers. We find that the relation between rY index and sunspots has a quadratic evolution over the MM. The Sun has more sunspots relative to EUV irradiance during the growth and maximum of the MM, while the opposite is true during its decay. We estimate that the MgII index increases by 24\% of its solar cycle variation with respect to the sunspot number during the last 70 years. Our results indicate a systematic difference in the evolution between sunspots (photosphere) and plages (chromosphere) with long-term solar activity. The implied varying spot-facula ratio has consequences to the stellar evolution of the Sun and Sun-like stars.