Relation between sunspots and solar EUV irradiance changes during the Gleissberg cycle

TL;DR

The paper addresses the long-standing question of how sunspots relate to solar EUV irradiance over century-scale Gleissberg-cycle variability, where direct spectral measurements are scarce. It uses the daily range of the geomagnetic Y-component (rY) from six long-running stations as a centennial EUV proxy and compares it to sunspot indices and MgII-based EUV proxies. The analysis reveals that sunspot activity dominates EUV irradiance during the growth and peak of the Modern Maximum, while EUV irradiance dominates during the decay, indicating a changing spot–faculae balance across the cycle. These findings imply a temporally varying sunspot–faculae coupling that has implications for interpreting the long-term brightness evolution of the Sun and Sun-like stars.

Abstract

Sunspots are the standard measure of solar magnetic activity, which are also used to estimate solar spectral irradiance over centennial time scales. However, because of the lack of homogeneous, century-long spectral measurements, the long-term relation of sunspots and spectral irradiance has not been independently validated. Here we aim to study the relation between sunspots and solar extreme ultra-violet (EUV) irradiance during the last 130 years, over the latest Gleissberg cycle, also called the Modern Maximum, when sunspot cycle heights varied by a factor of 2.5. We calculate the daily variation of the geomagnetic declination at six reliable, long-running stations, whose amplitude (or range) can be used as a centennial proxy of solar EUV irradiance. We also compare this geomagnetic proxy to the solar MgII index of EUV irradiance over the 40-year interval of overlap. We find that sunspot activity dominated over EUV irradiance when cycle heights increased in the early 20th century during the growth and maximum of the Modern Maximum, but EUV irradiance dominated over sunspots during the decay of the MM, when cycle heights decreased in the late 1900s. Our results suggest that the spot-facula ratio varies during Gleissberg cycle -type large oscillations of solar/stellar activity. This modifies the estimated stellar evolution of the relation between brightness and chromospheric activity of the Sun and Sun-like stars.

Figures (7)

• Figure 1: Top: Yearly rY indices of the six stations over their respective time intervals. Cycle numbers are indicated at the bottom of the panel. Bottom: standardized yearly rY indices of the six stations and their mean for the common time 1932-2020. Sunspots (out of scale) are depicted as grey thick line in both panels for comparison.
• Figure 2: a) standardized yearly 6-station mean rY index (black) and sunspot number (red). b) Scatterplot of standardized 6-st mean rY index and SSN (blue stars), the best-fit line (black) and its correlation coefficient and p-value. c) Difference of standardized 6-st mean rY and correlated SSN. Best-fit line (solid) with correlation coefficient and p-value, as well as lines with slopes that are two standard deviations above or below the best-fit line slope (dotted). d) Same difference of standardized 6-st mean rY and correlated SSN as in panel c, with best-fit second-order polynomial fit (black) and the dynamic linear model curve (blue) with 95% errors (grey area).
• Figure 3: a) standardized yearly 4-station rY index (black) and sunspot number (red). b) Scatterplot of standardized 4-st rY index and SSN (blue stars), the best-fit line (black) and its correlation coefficient and p-value. c) Difference of standardized 4-st rY and correlated SSN (red). Best-fit second-order polynomial (black) with correlation coefficient and p-value, and the dlm curve (blue) with 95% errors around it (grey area). d) - f) Same as panels a-c but for NGK rY index.
• Figure 4: Left panel: Sketch of the $S_q$-current system (blue lines with arrows). Red thick arrows denote the magnetic effect of the $S_q$-current on the Y-component, whose mean daily curves (thin red line) for NGK and HER are depicted in the two insets of the panel. Right panel: Daily curves of the Y-component for the other four stations.
• Figure 5: Secular variation of the Y-component of the six stations at hourly resolution (grey line), as well as the yearly means of the daily maxima (red curve) and minima (blue curve) of the Y-component of each station. The unit of the y-axis is nanotesla (nT).