Solar Physics

, Volume 291, Issue 9–10, pp 2981–3010 | Cite as

Reconstruction of Solar Extreme Ultraviolet Flux 1740 – 2015

  • Leif SvalgaardEmail author
Sunspot Number Recalibration


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 falls with the Sun, and thus causes a readily observable diurnal variation of the geomagnetic field, allowing us to 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 100 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 \(F_{10.7}\) flux and the sunspot number, and we find that the reconstructed EUV flux reproduces the \(F_{10.7}\) 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”.


Solar EUV flux Geo-magnetic diurnal variation Ionospheric E-layer Long-term variation of solar activity 



We acknowledge the use of data from the following sources: i) CELIAS/SEM experiment on the Solar and Heliospheric Observatory (SOHO) spacecraft, a joint European Space Agency (ESA), United States National Aeronautics and Space Administration (NASA) mission. ii) The Laboratory for Atmospheric and Space Physics (University of Colorado) TIMED Mission. iii) The Solar Radio Monitoring Programme at the Dominion Radio Astrophysical Observatory operated jointly by National Research Council, Canada and Natural Resources, Canada. iv) The Nobeyama Radio Observatory, NAOJ, Japan. v) World Data Centers for Geomagnetism in Kyoto and Edinburgh. vi) Data collected at geomagnetic observatories by national institutes according to the high standards of magnetic-observatory practice promoted by intermagnet ( ). vii) Data collected by Wolf and Wolfer in Mittheilungen. viii) Yearbooks from the British Geological Survey ( ). ix) World Data Center for the production, preservation and dissemination of the international sunspot number ( ). x) Wasserfall (1948). xi) The SONNE Network . xii) The Bremen composite Mg ii index ( ). xiii) The several World Data Centers for Geomagnetism ( , , ).

We have benefited from comments by Ingrid Cnossen and Ed Cliver. We thank Vladimir Papitashvilli for the corrgeom program to compute the geomagnetic-field elements for the years 1590 – 1995. We thank a referee for insightful comments. This research has made use of NASA’s Astrophysics Data System. Leif Svalgaard thanks Stanford University for support.

Statement of Conflict of Interest

The author declares that he has no conflict of interest.

Supplementary material

11207_2016_921_MOESM1_ESM.xls (854 kb)
(XLS 855 kB)


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Authors and Affiliations

  1. 1.W.W. Hansen Experimental Physics LaboratoryStanford UniversityStanfordUSA

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