In his study, Riley looked carefully at a parameter called Dst, short for "disturbance – storm time." This is a number calculated from magnetometer readings around the equator. Essentially, it measures how hard Earth's magnetic field shakes when a CME hits. The more negative Dst becomes, the worse the storm. Ordinary geomagnetic storms, which produce Northern Lights around the Arctic Circle, but otherwise do no harm, register Dst=-50 nT (nanoTesla). The worst geomagnetic storm of the Space Age, which knocked out power across Quebec in March 1989, registered Dst=-600 nT. Modern estimates of Dst for the Carrington Event itself range from -800 nT to a staggering -1750 nT.
In their Dec. 2013 paper, Baker et al. estimated Dst for the July 2012 storm. "If that CME had hit Earth, the resulting geomagnetic storm would have registered a Dst of -1200, comparable to the Carrington Event and twice as bad as the March 1989 Quebec blackout."