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

TL;DR

This study investigates the long-term relationship between sunspots and solar EUV irradiance over the last 130 years, revealing that their dominance varies during different phases of the Gleissberg cycle, impacting stellar activity models.

Contribution

It provides the first independent validation of the sunspot-EUV irradiance relation over a centennial scale using geomagnetic proxies and compares it with the MgII index.

Findings

Sunspot activity dominated during the early 20th century peak.

EUV irradiance became dominant during the late 20th century decay.

The spot-facula ratio varies with the Gleissberg cycle.

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.