The Active Sun

The flare alphabet, and what a CME is

A solar flare is a flash of X-rays from the Sun's surface, sorted into classes A, B, C, M and X, where each letter is ten times the last and the number is the strength within it. The X-rays arrive in eight minutes and disturb radio and GPS on the daylight side, but they do not cause storms on the ground.

• Flare classes: A and B are background, C is moderate, M is strong, X is extreme. This window's strongest was an X1.0 on 3 June, alongside an M9.3 from the same active region.
• CME: a coronal mass ejection is a cloud of magnetised plasma thrown off the Sun. Unlike the flare's light, it takes one to three days to cross to Earth, and it is the CME, not the flare, that drives a geomagnetic storm.
• Active region: flares and CMEs erupt from sunspot groups. Most of this window's largest events came from a single region, numbered 4455.

The Kp index, the G scale and the ground

When a CME sweeps past, it shakes Earth's magnetic field. The planetary Kp index measures that shaking every three hours on a 0 to 9 scale, and NOAA maps Kp onto the G storm scale: Kp 5 is a G1, Kp 6 a G2, up to Kp 9 for a G5. This window peaked at Kp 6.3, a G2.

A moving magnetic field induces electric currents in long conductors: power lines, pipelines and rail. These geomagnetically induced currents are the mechanism by which a storm in space becomes a problem on the ground, pushing grid transformers towards saturation and, in a severe storm, towards damage. That is how a CME becomes a grid operator's problem rather than an astronomer's.

Why grids and insurers watch the Sun

Space weather is the one natural peril with no geography. A Carrington-class storm does not strike a country; it couples into the whole sunlit and night side at once, so exposures that are normally uncorrelated fail together.

• Simultaneous exposure: satellites, aviation on polar routes, GPS-dependent farming and finance, and continental power grids are all hit in the same hours.
• The tail: moderate storms like this one arrive most months; the systemic events, Carrington 1859 or a near-miss, are decadal. The record's value is the scale it gives that tail.
• The models: Lloyd's and others price extreme space weather at hundreds of billions to trillions of dollars in a severe scenario, which is why it now sits on board risk registers.