Geomagnetic storms from G1 to G5: what each level means for the aurora

The five levels, as NOAA writes them

The scale was introduced so that the public could read space weather the way it reads hurricane categories. Its physical measure is Kp alone. That is why SWPC's three-day forecast prints a G label next to any block forecast at Kp 5- or higher: the label is simply the Kp, translated. The NOAA Space Weather Scales also cover radiation storms (S) and radio blackouts (R), which are different phenomena with no direct link to the aurora.

Reading a storm level for your own town

The latitudes in the table describe where the aurora has been seen, low on the northern horizon, not where it was overhead. They are also the backbone of this site's view line for the stronger storms, from about Kp 6.3 to 9. Between Kp 6 and Kp 9 the line drops 15 degrees of magnetic latitude, about 5 per level, so each extra level adds a band of states. Caribou, at the top of Maine, needs about Kp 6.1, a G2 (Moderate) storm. The mini-simulator above lists what each level brings, and the Kp index page turns any town into its threshold.

Watches, warnings and alerts

SWPC uses three kinds of messages, and they are easy to confuse. A watch is issued when the highest Kp forecast for a coming day reaches 5, 6, 7 or 8 and above, and it is expressed as a G level: a G3 watch for tomorrow means SWPC expects at least one three-hour block at Kp 7. A warning is shorter term: it is issued when Kp values of 4, 5, 6 or 7 and above are expected soon, typically when the solar wind monitors already see the disturbance coming. An alert says the level has been reached, from Kp 4 up to 9, according to the Planetary K-index page.

For someone planning a night out, the sequence matters more than any single message. The watch, a day or two ahead, is the time to check the cloud forecast and the dark hours for your town; the warning, an hour or less ahead, is the time to leave; the alert confirms that the storm is real, but by the time it is issued the best part may already be under way. The tonight page reads the same Kp feed that these messages are built on, so its hour strip moves with them.

What each level does on the ground

The scale was built for the power grid and satellite operators as much as for sky watchers, and its descriptions show it. At G1, NOAA lists weak power grid fluctuations and minor effects on satellite operations, and notes that migratory animals are affected from this level up. At G2, high-latitude power systems may see voltage alarms and long storms can damage transformers. At G3, voltage corrections may be needed, satellites may suffer surface charging, and radio navigation and satellite positioning can be intermittent. At G4, the scale warns of possible widespread voltage control problems and of protective systems tripping key assets off the grid. At G5, it describes possible grid collapse or blackouts, pipeline currents of hundreds of amps and high-frequency radio out for one to two days.

None of this is a reason for concern on an ordinary storm night: G1 and G2 storms come by the hundreds in a solar cycle and pass unnoticed by almost everyone except those who look up. The higher levels are the ones that make the news, for both reasons at once.

How often, over a solar cycle

The frequencies in the table are averages over an eleven-year cycle, and storms do not arrive evenly. They cluster in the years around and after solar maximum, and the strongest often come in bursts when a large, complex sunspot group launches several coronal mass ejections in a row, as happened in May 2024. In the quiet years near solar minimum, months can pass without a G2. Translated into days, NOAA's figures give roughly 900 days of G1 conditions per cycle, 360 of G2, 130 of G3, 60 of G4 and 4 of G5. For a place that needs a G3 storm, that is on the order of a dozen opportunities a year on average, before darkness and clouds take their share.

Where storms come from

G3 and stronger storms are almost always caused by coronal mass ejections, clouds of magnetized plasma that SWPC describes as a billion tons leaving the Sun at up to a million miles per hour. SWPC notes that the fastest Earth-directed ones arrive in as little as 15 to 18 hours, while slower ones take several days. Moderate storms, G1 and G2, also come from high-speed streams out of coronal holes, which return with the Sun's 27-day rotation. Both are most frequent in the years around solar maximum, and big storms keep coming during the decline, as the solar cycle guide shows.

The same level, a different show

Two G3 nights can look nothing alike. One may bring a steady red glow low in the north for hours; the other a single burst of rays that fills half the sky for fifteen minutes and fades. The level describes the disturbance of the magnetic field over three hours, not the shape or the brightness of the aurora at a given minute. What decides the show is how much energy the solar wind keeps pouring in, which depends above all on the north-south direction of its magnetic field, explained in the Bz guide. That is also why a storm forecast for your town should be followed through the night rather than checked once at dusk.

What the storm level does not tell you

A G level is a three-hour value for the whole planet. It says nothing about the hour of the peak, which may fall in your daytime, nor about the clouds. It also hides the finer structure: a G2 block can contain a twenty-minute substorm brighter than a whole G3 night. For an evening decision, the useful sequence is the watch from SWPC a day or two ahead, then the three-night forecast for your town, then the OVATION map once the storm has started.