The X-ray class of the Carrington flare

The geomagnetic ‘solar flare effect’ (SFE) results from excess ionization produced by solar X-ray and EUV radiation in the Earth’s upper atmosphere. The simultaneous detection of an SFE at the time of the Carrington flare in 1859 allows us to calibrate this event on the (revised) modern GOES X-ray scale. For this purpose, we make a basic correlation analysis of SFEs recorded at 1-min averaging at a single geomagnetic observatory, the Hartland station, as the site closest to London and the two original observing stations (Kew and Greenwich). We find the equivalent GOES magnitude of the Carrington event to have been X105$^{+41,-24}$ based on a sample of 39 X-class flare events observable as SFEs, with the estimate limited by systematic errors in the interpretation of the modern data. This estimate agrees with estimates based on the visual observations of the flare. This suggests that the Carrington flare was not a ‘superflare’ in the sense that it required physics beyond that needed for ordinary flares.