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

Abstract

Sunspots offer a uniquely long view of solar magnetic activity, and depict large variability during the last 100 years, a period known as the Modern Maximum (MM). However, if our view of solar variability was only based on the strongest magnetic fields, it would be incomplete. Therefore, other variables are needed to study the long-term evolution, e.g., of weaker magnetic fields and the different radiative emissions. Recently, the relation between sunspots and several other solar activity proxies like the F10.7 and F30 radio fluxes and the MgII index (all 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. We show that the rY index correlates extremely well with the MgII index and the solar F30 radio flux. These three indices have no trend relative to each other over the respective intervals. 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. Extending this earlier with the rY index, we find that the relation between EUV irradiance and sunspots has a nonlinear, quadratic evolution over the MM. This implies that 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 and minimum. We estimate that the MgII index (solar EUV irradiance) 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 (generally: photosphere) and plages (generally: 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.