Magnitude Estimates for the Carrington Flare in 1859 September: As Seen from the Original Records

The Carrington flare in 1859 September is a benchmark, as the earliest reported solar flare and as an event with one of the greatest terrestrial impacts. To date, no rigorous estimate of the energy of this flare has been made on the basis of the only direct observation available, its white-light emission. Here, we exploit the historical observations to obtain a magnitude estimate and express it in terms of its GOES soft X-ray class. From Carrington’s original drawings, we estimated the area of the white-light flaring region to be 116 ± 25 msh. Carrington’s account allows us to estimate the flare blackbody brightness temperature as ≈8800–10,900 K, given the most plausible interpretation of the reported flare brightness. This leads to an unprecedented class estimate of ≈X80 (X46–X126), on the modern revised GOES scale (a factor 1.43 higher than the traditional one). This substantially exceeds earlier estimates but is based on an explicit interpretation of Carrington’s description. We also describe an alternative but less plausible estimation of the flare brightness, as adopted previously, to obtain a class estimate of ≈X14 (X9–X19). This now-deprecated scenario gives an estimate similar to that of with those of directly observed modern great flares. Approximations with “equivalent area,” based on the Hinode observations, lead to comparable magnitudes and approve our estimates, though with a larger uncertainty range. We note that our preferred estimate is higher than the currently used value of X64.4 ± 7.2 (revised) based on indirect geomagnetic measurements.