Anyone know of an estimate of the cost of hardening the US grid so that there wouldn't be an enormous death count, and likely societal collapse? The danger seems to be very real. See the paper discussed here:
The energetic particles trapped in Earth's magnetosphere produce what's called a ring current. That ring current affects Earth's geomagnetic shield because its polarity is inverted to the geomagnetic field created by the Earth's built-in dynamo (the swirling molten metal in the planet's core).
The stronger the ring current, the greater its impact on our planetary geomagnetic field...the worse a geomagnetic storm...and the more likely geomagnetic induced currents can happen. Because the ring current magnetic field polarity is inverted to our geomagnetic field, the lower the ring current's value (measured in nanoteslas, or nT), the more it weakens the geomagnetic field.
I'm taking some poetic license up there, but it's a good mental model. I'll take some more poetic license below for brevity's sake (HA!).
Now, let's get to the meat of it.
The measurement of this ring current, as applied to solar storms, is called the Disturbance Storm Time (Dst) index. The Dst index normally hovers around between 0 nT and -20 nT in calm space weather. And it varies depending on the location of the earth-bound magnetometer measuring it.
Of note: the "Carrington Event" had an estimated Dst index of anywhere from -850 nT to -1,750 nT. There are so many papers that aim to model the likely Dst index that, for the purposes of this reply, -1,000 nT is a nice round figure for simple comparisons. Not a scientifically rigorous one, mind, but a simple one.
The estimated max Dst of this weekend's geomagnetic storm was around -500 nT. Using our simplified round figure, that's half the strength of the weakest estimated Dst of the Carrington Event (or about one quarter of its strongest estimated Dst).
So, eliding a lot of nuance around how Dst isn't quite linear depending on your geomagnetic latitude and the magnetic properties of the ground beneath an observer's feet: this geomagnetic storm was much weaker than the Carrington Event.
Now: another proxy scale is used to signify the "strength" of a geomagnetic storm and its negative impact on terrestrial systems. It's called the K-index. That K-index, contrary to the Dst it proxies, is arbitrarily bounded. It goes from 0–9.
Each observatory's magnetometer has their own set of thresholds to convert Dst to K, because the further away you are from the nearest geomagnetic pole, the more fluctuation in the geomagnetic field is needed to see aurorae, or to suffer from other terrestrial issues like geomagnetically-induced currents.
For the US Space Weather Prediction Center (SWPC), that magnetometer is in Boulder, CO. Their Dst thresholds for each K-index, from 0–9, are:
nT: 0 -5 -10 -20 -40 -70 -120 -200 -330 -500
K : 0....1....2....3....4....5....6....7....8....9
Still with me? For public alerts and ease of understanding, that Kp-index is converted (AGAIN) to another scale that NOAA/SWPC use to categorize a geomagnetic storm: the G scale. Basically, if the Kp index is 5 or more, subtract 4 from it, put a G before it, and there you go. G1=Kp 5, up to G5=Kp 9.
Now go back to the proxy scale. G5, an "extreme" geomagnetic storm, is any storm with a Dst, as measured in Boulder, CO, of -500 nT or more (assuming most other observatories end up with their own K-index of 9 as well).
Was Carrington an extreme (G5) event? Of course, a Dst of -850 nT (or -1,750 nT) is, indeed, less than or equal to -500 nT.
Was this weekend's geomagnetic storm an extreme (G5) event? Also, yes.
Because the G scale (the K- and Kp-indices) is bounded, it ends up being somewhat arbitrary/subjective. Just like a Category 5 hurricane or an EF5 tornado, when the scale is bounded, there will always be some outliers that defy simple categorization.
TL;DR: it was (technically) anywhere from 25-59% as powerful as the Carrington Event.
How does the recent CME compare to that of the Carrington Event?