  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'.
