I’d like to know how long we have been tracking and measuring solar flares. There was the Carrington event, but how strong was that, and others before and after that hit countries that didn’t have telegraph wires strung for many miles. I don’t think we have had the technology for that long, which makes me wonder just how strong a CME can be. A few decades doesn’t give much data at all.
https://en.m.wikipedia.org/wiki/Carrington_Event
They mention some extrapolations of tree rings and ice core that indicate event an order of magnitude larger than Carrington in the past.
Not really that long. Prior to the 20th century, we really had very little idea about solar flares, or even sunspots because the evidence of them was indirect at best. Chinese records of sunspots going back quite a way. Hundreds of years of records of sunspots IIRC.
We really have no idea how often really big flares happen. The best evidence would be very indirect, such as noting how far south aurora are visible.
For those interested in the phenomena, the best source on the net IMO is solarham.com, as they don't tend to start running around with their hair on fire about it.