What on Earth can the ionosphere tell about the stellar evolution of the Sun?

Solar EUV irradiance ionizes the Earth's dayside atmosphere and, together with the Earth's rotation, forms ionospheric currents called the solar regular (S_R) current system. The S_R current system consists of two vortices whose turning points deflect the magnetic Y (east-west)-component, forming a systematic daily variation in declination, with maximum in the local morning and a minimum in the afternoon (in the northern hemisphere and oppositely in the south). The daily amplitude (range, rY) of this variation depends on the intensity of ionization and, thereby, on the intensity of solar EUV irradiance. This variation was found by Graham Greene already in 1722, and in 1850s Rudolf Wolf used the yearly rY values to fill in gaps in early sunspot observations when continuing his series of relative sunspot numbers to the 18th century.

 

Here we use the yearly rY values from six long-running magnetic stations as a long-term proxy of solar EUV irradiance to study the relation between sunspots and solar EUV irradiance during the last 130 years. This period contains one full cycle of the centennial Gleissberg cyclicity (GC) from low cycles at the turn of the 19th and 20th century to a maximum during the highest cycle 19 with a decay to a low cycle 24.

 

We find that sunspots increase relatively more than EUV irradiance during the GC growth phase (when solar activity is increasing) but also decrease relatively more than EUV irradiance during the GC decay phase (when solar activity is decreasing). Since EUV irradiance mainly originates from solar plages that are chromospheric counterparts of photospheric faculae, this long-term change between sunspots and EUV irradiance implies a variation between sunspots and faculae over the Gleissberg cycle, which gives interesting information about the stellar evolution of the Sun.