[freebeard]

Via S0: link.springer.com/article/10.1007: A history of solar activity over millennia

5 Solar energetic particles in the past
In addition to galactic cosmic rays, which are always present in the Earth’s vicinity, solar energetic-particle (SEP) events with a greatly enhanced flux of less energetic particles in the interplanetary medium also occur sporadically...
....
The reconstructed energy spectrum of the 774/5 AD ESPE is shown in Fig. 29 along with other major ESPE and three strong GLE events with different spectra—from soft to hard. One can see that the 774/5 AD event (as well as other ESPEs) was characterized by the energy spectrum typical for hard GLE events, but several orders of magnitude stronger. Roughly, the 774/5 AD was a factor 70±30 stronger than the largest directly recorded GLE #5 making it really extreme .

That's the Miyake or Charlemagne Event.

In other news [citation needed], fossil fuel use saved Earth from the extinction of plant life due to insufficient atmospheric CO2. It was a squeaker.