Speaker
Description
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, since weaker magnetic elements dominate solar surface magnetism, our view of solar magnetic variability would be incomplete if it was only based on the strongest magnetic fields of sunspots. Luckily, there are several long-term series of observations that can yield direct and indirect centennial proxies of solar EUV flux. Since solar EUV emissions mainly come from the chromosphere, both the solar EUV observations and their proxies can give information about the long-term evolution of moderately-strong magnetic elements of plages and magnetic network. Independent measurements of solar magnetic fields of different strength can give interesting information about the long-term evolution of the structure of solar magnetic fields during the MM when solar activity was dramatically changing.
We have recently developed a centennial EUV proxy from the daily variation of the geomagnetic Y-component measured at eight long-operating observatories. This proxy, the rY index, was used as a solar activity proxy already in the 1860s by R. Wolf. We found that the rY index has a temporally changing relation with sunspots over the MM, indicating that the strongest and moderate magnetic field elements evolve differently over the MM. Here we study several direct and indirect long-term proxies of solar EUV activity and compare their mutual relation over the MM, as well as their relation with different sunspot-related photospheric variables. Our results further verify the different evolution of magnetic field elements of different strength over the Modern Maximum, indicating a systematic difference in the magnetic evolution between the photosphere and the chromosphere with long-term solar activity. We also discuss the consequences of our results to solar driving of the ionosphere and to the implied long-term variation of the spot-facula ratio and the stellar evolution of the Sun and Sun-like stars.