A geomagnetic storm is forecast to hit Earth today, pushing the northern lights unusually far south

Space weather forecasters spent much of this week watching a chunk of solar material race toward Earth, warning that its arrival could spark a minor geomagnetic storm and push the northern lights farther from the poles than usual. By early Thursday, though, the anticipated punch had mostly fizzled, with the storm watch called off even as forecasters left the door open for scattered aurora sightings at high latitudes. The episode is a reminder of how uncertain space weather forecasting remains, even a few days out from a predicted impact.

A Filament Eruption Sends a Coronal Mass Ejection Toward Earth

The chain of events began with a violent filament eruption on the sun’s southwest side, associated with a moderate solar flare, that hurled a burst of solar plasma known as a coronal mass ejection, or CME, into space. Forecasters confirmed the CME carried an Earth-directed component and projected it would arrive by September 17, a forecast that, combined with an ongoing stream of fast solar wind escaping from a hole in the sun’s corona, raised the odds of reaching G1, or minor, geomagnetic storm levels. At its most optimistic, the forecast even carried a slight chance of reaching G2 or G3 storm levels, which would have been strong enough to push visible aurora into mid-latitude states and countries well south of their usual range near the Arctic Circle.

From a G1 Watch to a Quiet Morning

That optimism faded as the CME’s actual effects arrived. By the early hours of September 17, the rolling solar-activity log maintained by EarthSky reported that no new Earth-directed CMEs had been detected and that the ejections from several days earlier “either haven’t arrived or delivered only a weak impact.” As a result, NOAA’s Space Weather Prediction Center canceled its watch for a G1 storm, and the planetary Kp index, the standard measure of geomagnetic disturbance, sat just above its lowest level rather than climbing into storm territory. Solar wind speeds, which had been elevated, eased to moderate levels sustained mainly by the ongoing coronal hole stream rather than the CME itself.

How NOAA Grades a Geomagnetic Storm

NOAA’s Space Weather Prediction Center issues alerts, watches and warnings much the way local National Weather Service offices do for terrestrial storms, ranking events on scales that describe expected severity before, during and after they occur, according to the agency’s own alerts and warnings page. Geomagnetic storms are graded from G1, the mildest tier, up to G5, an extreme storm capable of causing widespread power grid problems and satellite disruptions. A G1 watch, the level that had been in play for September 17, typically corresponds only to minor fluctuations in the magnetic field and a modest chance of aurora at far northern latitudes, well short of the kind of event that would threaten infrastructure. NOAA runs parallel scales for two other space weather hazards graded the same way, from R1 to R5 for radio blackouts caused by solar flares and S1 to S5 for solar radiation storms that can affect satellites and high-altitude flights, so a single active region on the sun can sometimes trigger warnings on more than one scale at once even when, as with this week’s activity, none of them reaches a severe level.

What Actually Makes the Sky Glow

An aurora forms when charged particles carried on the solar wind and inside a CME collide with Earth’s magnetic field and get funneled down toward the poles, where they strike oxygen and nitrogen atoms high in the atmosphere. Those collisions excite the atoms and cause them to release energy as light, with oxygen typically producing the familiar green glow and, at higher altitudes, a rarer red, while nitrogen contributes blue and purple hues near the lower edge of the display. The stronger the geomagnetic disturbance, the farther from the poles that funnel of particles reaches, which is why forecasters watch the Kp index so closely: a low Kp keeps the aurora confined to the Arctic Circle, while a high Kp during a strong storm can push visible aurora as far south as the mid-latitude United States.

Where the Aurora Might Still Show

Even with the formal storm watch lifted, EarthSky’s forecast for the rest of Thursday still allowed for isolated active geomagnetic periods, driven by the lingering coronal hole wind stream rather than the underwhelming CME, with a slight uptick possible again on Friday before conditions were expected to quiet further into the weekend. That leaves a narrow but real chance for skywatchers at the highest latitudes, in places like far northern Canada, Alaska and Scandinavia, to catch some aurora activity overnight, though the wide-scale, unusually far-south sightings that a full G1 to G2 storm might have produced are unlikely to materialize on the scale originally floated earlier in the week.

A Sun Running Low on Sunspots

The muted storm arrived alongside another sign of a quiet stretch on the sun itself: as of September 17, only a single numbered sunspot region remained visible on the Earth-facing solar disk, and it was expected to rotate out of view within a day. If no new active region appears in the meantime, the sun could go spotless for the first time since February, a marker solar physicists watch as an indicator of overall activity levels. Flare activity has likewise stayed at very low levels for days, with only weak, so-called B-class flares recorded, reinforcing the sense that the geomagnetic scare tied to this week’s CME was always a modest one rather than a major space weather event in the making.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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