A coronal mass ejection that may have swallowed a second, slower eruption on its way to Earth drove a strong G3 geomagnetic storm on Saturday, Oct. 10, a level above the G2 watch NOAA had posted for the event. The NOAA Space Weather Prediction Center said in a G3 warning that storm conditions reached G3 at 17:58 UTC (1:58 p.m. EDT), about three and a half hours after the eruption's arrival.

The timing explains why Europe saw a spectacular show, and most Americans did not. The strongest activity arrived during daylight across the continental United States, while evening skies were already dark over the United Kingdom, Germany and Scandinavia. For U.S. skywatchers, the practical question now is whether lingering activity and a newer eruption bring another chance this week.

A Denser Blow Than the Watch Suggested

NOAA said the solar wind became strongly enhanced after a CME arrived at about 14:34 UTC Saturday. The agency's 3-day forecast, issued Sunday, confirmed that the greatest observed three-hour Kp index over the previous 24 hours was 7, which corresponds to a G3 (strong) storm on NOAA's scale.

The UK Met Office reported two G3 intervals and a single G2 interval after 15:00 UTC Saturday. Some media reports described the event as G4, but NOAA's official observation tops out at G3, and that is the level Nature World News is reporting.

The likely reason the storm ran hotter than the G2 watch is the structure of what hit Earth. Spaceweather.com, written by Tony Phillips, reported that plasma density around Earth jumped tenfold to more than 100 protons per cubic centimeter when the CME struck. "It was so dense, it might have actually been two CMEs," Phillips wrote, noting that two closely spaced eruptions had left the Sun the previous week and may have merged en route.

That remains a suspicion, not a confirmed finding. NOAA's warning attributes the storm to a CME from Oct. 8 and does not describe a merger, and no formal analysis of the event's structure has been published. Spaceweather.com said the storm lasted almost six hours before fading as night fell across North America, leaving U.S. observers with only a faint echo of the initial outburst.

Cannibal CMEs Explained

A cannibal CME forms — in NASA's description — when a faster eruption overtakes a slower one launched earlier from the same region of the Sun and absorbs it. A NASA visualization explains that the combined cloud can carry a larger and more complex structure than a typical CME.

That matters for forecasting. NASA notes that researchers believe cannibal CMEs may be a source of so-called complex ejecta, which can trigger protracted magnetic storms when they envelop Earth. A merged cloud can arrive denser and with a magnetic field arrangement that differs from what forecasters modeled for either eruption alone.

Forecasters face a related challenge with timing and strength. Storm intensity depends heavily on the orientation of the CME's magnetic field, which cannot be measured until the cloud reaches spacecraft about an hour upstream of Earth. A watch issued days ahead is therefore a best estimate, and stronger or weaker outcomes are both possible.

None of this posed a risk to people on the ground. Under NOAA's space weather scales, G3 storms can require voltage corrections on some power systems, cause intermittent satellite navigation and low-frequency radio problems, and affect satellite operations. Grid operators and satellite companies monitor SWPC alerts to manage those effects.

Aurora Odds for the Northern U.S. This Week

NOAA expected activity to ease. Its Sunday forecast called for G1 to G2 storms on Oct. 11 and 12, with the highest predicted Kp of 5.67, a G2 level, between 03:00 and 06:00 UTC Monday, or 11 p.m. Sunday to 2 a.m. Monday EDT, a window that has now passed. For Monday night in the U.S., NOAA's forecast stays below storm levels, so any aurora would likely be confined to Alaska and the far northern border unless conditions strengthen. NOAA attributed the outlook to the continued influence of several CMEs and a coronal hole high-speed stream.

The UK Met Office sees a slightly stronger chance on Monday. Its forecast said that "due to possible combined arrivals of the 10 Oct CME and the HSS," G1 storms are likely on Oct. 12, with a chance of G2 intervals and a slight chance of an isolated G3 interval. NOAA's forecast for Oct. 13 does not reach storm levels.

The newer eruption adds uncertainty. Sunspot region 4549 produced an M6.4 flare on Oct. 10 that launched a CME, according to Spaceweather.com, which said a preliminary NASA model suggests a near miss or glancing blow on Oct. 13. The Met Office expects a possible glancing blow on Oct. 12, so the models do not yet agree on timing.

At G1 levels, the aurora is most likely visible low on the northern horizon from Alaska and northern-tier states such as Washington, Montana, North Dakota, Minnesota, Michigan and Maine. A G2 interval could extend photographable displays farther south. Viewers should find a dark site with a clear northern view, allow eyes about 20 minutes to adjust, and try a phone camera's night mode.

Next Forecast Updates and Open Questions

Nature World News previously reported that NOAA kept a G2 storm watch for Oct. 9 as a CME approached Earth, with the forecast peak expected in U.S. daylight. The arrival came later than that forecast and produced a stronger storm than the watch level, though the timing again favored Europe over North America.

NOAA issues updated 3-day forecasts daily and posts alerts when storm thresholds are reached. Nature World News will update this story if NOAA issues a new watch for the Oct. 10 eruption.

The bottom line is that a suspected merger of two solar eruptions likely helped push Saturday's storm above its forecast level. Northern U.S. skywatchers have modest odds of faint aurora through early this week, and NOAA's official alerts remain the best guide to timing.

What Readers Want to Know

How strong was Saturday's geomagnetic storm?

NOAA observed a maximum Kp index of 7, a G3 (strong) storm, beginning at 17:58 UTC on Saturday, Oct. 10. That exceeded the G2 watch NOAA had posted for the eruption.

What is a cannibal CME?

It is a coronal mass ejection that overtakes and absorbs a slower eruption launched earlier from the same area of the sun. The merged cloud can be denser and more complex, which can lead to longer or stronger storms.

Was Saturday's storm definitely caused by a cannibal CME?

No. Spaceweather.com suggested it based on a tenfold jump in plasma density, but NOAA has not described a merger and no formal analysis has been published.

Why did Europe see the aurora but most of the U.S. did not?

The strongest activity occurred during the U.S. afternoon. By the time night fell across North America, the storm was fading.

Will the northern lights be visible this week?

NOAA's best window, 11 p.m. Sunday to 2 a.m. Monday EDT, has passed, and its forecast for Monday night stays below storm levels. The Met Office sees a chance of G1 storms Monday, so faint aurora remains possible mainly in Alaska and northern-tier states.

Is a G3 storm dangerous?

Not to people on the ground. Strong storms can affect some power systems, satellite navigation and radio communications, which operators manage using NOAA alerts.