Reconstruction of Solar Extreme Ultraviolet Flux 1740-2015

TL;DR

This paper reconstructs the historical solar EUV flux from geomagnetic data spanning over 275 years, revealing its correlation with solar activity indicators and suggesting a consistent minimum flux level during solar minima.

Contribution

It introduces a method to reliably reconstruct solar EUV flux from geomagnetic records dating back to the 1830s, extending understanding of solar magnetic activity over centuries.

Findings

Reconstructed EUV flux matches F10.7 flux accurately.

EUV flux reaches a consistent low during minima, indicating an invariant solar magnetic ground state.

Current sunspot number may not fully represent solar magnetic activity.

Abstract

Solar Extreme Ultraviolet (EUV) radiation creates the conducting E-layer of the ionosphere, mainly by photo ionization of molecular Oxygen. Solar heating of the ionosphere creates thermal winds which by dynamo action induce an electric field driving an electric current having a magnetic effect observable on the ground, as was discovered by G. Graham in 1722. The current rises and sets with the Sun and thus causes a readily observable diurnal variation of the geomagnetic field, allowing us the deduce the conductivity and thus the EUV flux as far back as reliable magnetic data reach. High-quality data go back to the 'Magnetic Crusade' of the 1830s and less reliable, but still usable, data are available for portions of the hundred years before that. J.R. Wolf and, independently, J.-A. Gautier discovered the dependence of the diurnal variation on solar activity, and today we understand and can invert that relationship to construct a reliable record of the EUV flux from the geomagnetic record. We compare that to the F10.7 flux and the sunspot number, and find that the reconstructed EUV flux reproduces the F10.7 flux with great accuracy. On the other hand, it appears that the Relative Sunspot Number as currently defined is beginning to no longer be a faithful representation of solar magnetic activity, at least as measured by the EUV and related indices. The reconstruction suggests that the EUV flux reaches the same low (but non-zero) value at every sunspot minimum (possibly including Grand Minima), representing an invariant 'solar magnetic ground state'.