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

TL;DR

This study constructs a centennial proxy for solar EUV irradiance using the daily range of geomagnetic Y-component, rY, derived from eight long-running observatories (1887–2023). By comparing rY with MgII, F30, and F10.7, and correlating both short- and long-term rY series with sunspot numbers and group sunspot numbers, the authors demonstrate that rY tracks EUV proxies extremely well and extends the MgII timeline by ~90 years, while F10.7 exhibits a distinct long-term trend. A key finding is a nonlinear, approximately quadratic, evolution in the relation between rY (chromosphere) and sunspots (photosphere) over the Modern Maximum, implying a varying spot-facula ratio that changes with long-term solar activity. These results have implications for interpreting solar and stellar brightness evolution and suggest that long-term proxies based on chromospheric/plage indicators (MgII, F30, rY) provide more reliable EUV records than traditional radio proxies like F10.7, with potential applicability to Sun-like stars undergoing similar activity transitions.

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.

Figures (10)

• Figure 1: The daily variation of the Y-component at the eight stations, each in its own panel. Top two rows: Variation in the UT time. Bottom two rows: Variation in the local time.
• Figure 2: Secular variation of the Y-component at the eight stations at hourly resolution (grey line), and yearly means of the daily maxima (red curve) and minima (blue curve) of the Y-component of each station. Unit of the y-axis is nanotesla (nT).
• Figure 3: Yearly rY indices for the 8 stations over their full time range (colored lines). Scaled sunspot numbers are included for comparison (grey line).
• Figure 4: Yearly rY values of the three multi-station means (black line) and the contributing station rY values (colored lines) constructed from the 8 stations. Top: The 7-station mean and the contributing stations in 1933-2023. Middle: The 5-station mean and the contributing stations in 1913-2023. Bottom: The 2-station mean and the contributing stations in 1890-1925.
• Figure 5: Correlating the three multi-station mean rY indices with sunspot number. Top row: 7-station mean rY; Middle row: 5-station mean rY; Bottom row: 2-station mean rY. Left column: Multi-station mean rY and sunspot number time series; Middle column: Their scatterplot with best-fit line; Right column: Difference (residual) between the multi-station mean rY indices and the correlated sunspot number, together with the best-fit 2nd-order polynomial and the dynamic linear model.