Geomagnetic storms: G1 to G5, and what you see in Norway

A geomagnetic storm is a major disturbance of Earth's magnetic field, usually triggered by a coronal mass ejection (CME) from the Sun or by a long stream of fast solar wind. During a storm the auroral oval widens and shifts south, so the aurora can be seen across all of Norway and far into Europe. The US space weather service NOAA ranks storms from G1 (minor) to G5 (extreme). The two strongest classes are rare: NOAA expects G4 conditions on about 60 days and G5 on about four days in a solar cycle.

What triggers a storm

NOAA describes a geomagnetic storm as a situation where energy from the solar wind is transferred very efficiently into the space around Earth. What it takes is several hours of fast solar wind and, most importantly, a magnetic field in the solar wind pointing south, opposite to Earth's own field (see solar wind, Bz and speed). There are two main sources:

Coronal mass ejections (CMEs). A CME is a vast cloud of plasma and magnetic field, often around a billion tonnes, hurled out from the Sun in an eruption. The strongest storms come from such clouds. A CME typically takes several days to reach Earth, but the fastest have arrived in about 18 hours. CMEs often carry a strong magnetic field, and if it points south when the cloud arrives, the result can be spectacular.

Fast solar wind from coronal holes. When fast solar wind from a coronal hole catches up with slower wind, a compression region forms. According to NOAA these usually cause weaker storms than CMEs, but they can last longer and deliver a lot of energy overall. Such streams tend to return about every 27 days, which we explain in our article on the solar cycle.

NOAA's G-scale: from G1 to G5

Storms are classified using the Kp index, a measure of how much the magnetic field is disturbed over three hours (see the Kp index explained). NOAA also gives a rough idea of how often each level occurs during a solar cycle of about 11 years:

For aurora hunters in Northern Norway, a G1 storm is already a great start, and often no storm is needed at all. In southern Norway you usually need a clear storm, and the stronger it is, the higher the aurora climbs in the sky. What it takes where you are is covered in our article on the aurora in southern Norway.

Famous storms in history

The Carrington storm, 1859. On 1 September 1859 the English astronomer Richard Carrington saw an intense flash of light from a group of sunspots. According to NOAA, the sky lit up about 17 hours later, with aurora as far south as Panama. Telegraph lines received power surges and stopped working. NOAA estimates that storms of this size come only about once every 500 years.

March 1989. An eruption on the Sun on 10 March sent a plasma cloud towards Earth, which struck on the evening of 12 March. According to NASA, the aurora was seen as far south as Florida and Cuba. Currents induced in the ground knocked out the entire power grid of Quebec in Canada in less than two minutes in the early hours of 13 March, leaving millions of people without power for about twelve hours.

May 2024. On 10 May 2024 the storm reached level G5, according to NASA the first G5 storm in more than two decades. In Norway, the night of 10–11 May was when the sky truly lit up. The broadcaster NRK received photos of colourful aurora from places such as Kristiansand, Mandal, Tvedestrand, Modum and Sande in the south. There was unusually much pink and purple, and Pål Brekke of the Norwegian Space Agency described the aurora as pillars rising from the horizon as seen from southern Norway. It was also seen in Switzerland and Italy.

October 2024. Just five months later another powerful storm arrived, strongest on the night of 10–11 October. NRK received photos of intense red aurora from Sortland in Vesterålen, and people in Oslo and nearby Asker watched red, pink and green aurora above their gardens. The aurora was reported as far south as Italy and Spain.

November 2025. More than a year after solar maximum, a new solar storm produced strong aurora across the whole country on the night of 11–12 November 2025. In Hammerfest, in the far north, the sky switched from green to intense red at around two in the morning, and NRK warned that some people might experience GPS disruption.

What a storm looks like from Norway

During a storm the auroral oval expands and becomes more active. What you see depends on where you stand:

• In Northern Norway the aurora can stand high in the sky or cover all of it, often arriving in several powerful bursts during the night. When the oval shifts far south, the best aurora may actually lie to the south of you.
• In central and southern Norway you usually see it low over the northern horizon, as a green arc or red and pink pillars. During the strongest storms it can climb much higher.
• Red often dominates far south. Red light comes from oxygen 200–300 km up and can therefore be seen above the horizon from a long way off. Read more about the colours of the aurora.

Remember that a camera captures much more than the eye. Many of the most spectacular photos from southern Norway show colours that were faint to the naked eye.

More than aurora: storms and technology

Storms are not only beautiful. NOAA's scale describes possible impacts at the highest levels: problems in power grids and disruption to satellites, radio communication and satellite navigation. The 1989 Quebec blackout is the classic example. For an aurora trip, the most practical thing to know is that GPS can become less accurate during strong storms. Keep an offline map and a plan for where you are driving.

How to follow a storm

1. CME alerts: A CME usually takes several days to reach Earth, so an alert often comes well in advance. But both arrival time and strength remain uncertain until the cloud actually reaches the spacecraft at L1.
2. Real-time data: When the cloud hits the spacecraft at L1, you see it as a jump in speed and density. That gives you 15–60 minutes.
3. Kp and weather: Combine the activity level with the cloud forecast. Even the strongest storm is little use under thick cloud. See how to read an aurora forecast.

In short

• Geomagnetic storms are mostly triggered by CMEs and fast solar wind, and they need a southward magnetic field.
• NOAA's scale runs from G1 (Kp 5) to G5 (Kp 9). G4 and G5 are rare.
• Big storms such as May 2024, October 2024 and November 2025 have brought aurora far south in Norway.
• Follow the alerts and real-time data, and check tonight's verdict for your location.