The one in 1859 was classified as G5 (extreme), and the one the other day was the very first time G4 warning has been issued. Not as in "it'll definitely be G4", but more along the lines of "it'll be G3, possibly increasing to G4 for a brief period of time".
There's also a more "scientific" scale called the K-index (https://en.wikipedia.org/wiki/K-index), but I don't understand it well enough to be able to explain it.
May 2024 Event: Dst -250 from multiple up to X3.98 flares (and CMEs)
Dst is the disturbance to Earth's magnetic field, Xs are the W/m^2 of X-rays emitted from the solar explosion (flare). It's also important to consider the size and direction of the CME (if any) associated with the flare.
Some more illuminating measurements are:
Carrington: Dst -1000 from an X80 flare and CME
May 2024 Event: Dst -250 from multiple up to X3.98 flares (and CMEs)
Apparently more cautious operators have been busy installing them since at least 1989.
https://assets.lloyds.com/assets/pdf-solar-storm-risk-to-the...
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.
>TL;DR: it was (technically) anywhere from 25-59% as powerful as the Carrington Event.
This feels strange to me. If nobody had told me about the CME yesterday, I probably wouldn't have even noticed it. But a CME 10 times as powerful would have caused a global blackout? That makes me feel like the line between "nobody notices it" and "everything is destroyed" is remarkably thin.
I'm currently trying to educate myself more on this subject. I'm reading these:
https://assets.lloyds.com/assets/pdf-solar-storm-risk-to-the...
https://irp.fas.org/congress/2008_hr/emp.pdf
Anything else you would recommend that I read?
Two other factors are important: firstly, the flare and the CME are inter-related but separate. You can get a big flare, but not a big CME, or the CME can not be Earth-directed, so no hit.
Secondly, Earth's magnetic field has weakened significiantly since 1859 (its strength is cyclical, and related to how the true poles wander and drift over time). This means that smaller events cause larger disruptions to the electromagnetics of Earth.
So we have Dst -1000 for Carrington, and -250 for the storm over the weekend. Due to our impaired EM immune system right now, we could get Carrington level EM disturbance on Earth from smaller events on the sun.