What Kp is a G4 storm?

Kp = 8, including a 9-, on the NOAA Space Weather Scales. SWPC's oval-edge rule puts the average edge of the oval at 50 degrees geomagnetic at Kp 8.

How many G4 storms happen per year?

SWPC lists 100 per 11-year cycle on 60 days, about 5.5 storm days a year on average.

Which Glow towns does a G4 storm reach?

10 of 10 sit at or poleward of the Kp 8 oval edge: Reykjavik, Yellowknife, Tromsø, Fairbanks, Anchorage, Duluth, Minneapolis, Seattle, Portland, Maine, Buffalo.

Where has G4 aurora been seen?

NOAA's scale says: "aurora has been seen as low as Alabama and northern California (typically 45° geomagnetic lat.)". Aurora is often seen hundreds of kilometers equatorward of where it actually sits, per SWPC.

NOAA G-scale · Kp = 8, including a 9-

G4 severe geomagnetic storm and the aurora

Answer

G4 Severe is Kp = 8, including a 9- on NOAA's scale. At this level, SWPC says, "aurora has been seen as low as Alabama and northern California (typically 45° geomagnetic lat.)". It occurs about 100 times per 11-year cycle.

Severe, Kp = 8, including a 9-

NOAA Space Weather Scales

What SWPC says about G4

G4 is a severe storm. NOAA's scale defines it as Kp 8, including a 9-, so it spans the upper steps of the index below the maximum. SWPC's tips page puts Kp 8 to 9 in the band where the aurora becomes very bright and very active.

"aurora has been seen as low as Alabama and northern California (typically 45° geomagnetic lat.)"
NOAA Space Weather Scales, G4 Severe

The scale lists G4 aurora as seen as low as Alabama and northern California, typically 45 degrees geomagnetic. That is at least 7 degrees south of every Glow town. The oval-edge rule puts the Kp 8 edge at 50 degrees, south of all 10 towns, so at a G4 the lower 48 towns can have aurora overhead rather than on the horizon.

G4 storms average 60 days per 11-year cycle, about 5 a year. With the oval's average edge at 50 degrees, south of every Glow town, the question for lower 48 readers shifts from whether aurora reaches them to where in the sky to look. SWPC's aurora page describes arcs that can expand to fill the whole sky at the peak of a substorm.

"It should be noted that the aurora can often be observed hundreds of kilometers (miles) equatorward of the actual aurora so these figures do not indicate where the aurora may be but rather the point from which it may be observed."
NOAA SWPC, Tips on viewing the aurora

The 10 Glow towns at G4

10 of the 10 Glow towns sit at or poleward of SWPC's typical G4 latitude of 45 degrees. The Kp 8 edge at 50 degrees reaches all 10. Buffalo sits closest to that edge, 226 km north of it.

Each Glow town against the G4 typical latitude and the Kp 8 oval edge (50 degrees)
Town	Dipole	vs typical latitude	Kp 8 edge
Reykjavik	68.8°	+23.8°	Reaches the town
Yellowknife	68.5°	+23.5°	Reaches the town
Tromsø	67.5°	+22.5°	Reaches the town
Fairbanks	65.7°	+20.7°	Reaches the town
Anchorage	61.9°	+16.9°	Reaches the town
Duluth	55.4°	+10.4°	Reaches the town
Minneapolis	53.5°	+8.5°	Reaches the town
Seattle	53.0°	+8.0°	Reaches the town
Portland, Maine	52.9°	+7.9°	Reaches the town
Buffalo	52.0°	+7.0°	Reaches the town

Dipole latitude from IGRF-14. Typical latitude from the NOAA scale; positive means poleward of it. Edge from SWPC's 66 minus 2 per Kp rule, at 111.2 km per degree. All approximate.

How often G4 storms happen

SWPC lists 100 G4 storms per 11-year solar cycle, falling on 60 days. Spread evenly over the cycle, that is about 9.1 storms and 5.5 storm days a year. That is 15.0 times as many storm days as G5, the next level up.

NOAA G-scale average frequency, per 11-year cycle and per year
Level	Kp	Storms per cycle	Days per cycle	Days per year
G1 Minor	Kp = 5	1,700	900	82
G2 Moderate	Kp = 6	600	360	33
G3 Strong	Kp = 7	200	130	12
G4 Severe	Kp = 8, including a 9-	100	60	5.5
G5 Extreme	Kp = 9	4	4	0.4

Per-cycle counts from the NOAA scales page, where 1 cycle is 11 years. Per year: days per cycle divided by 11. Real storms are not spread evenly.

On a G4 night

From the lower 48 towns, look overhead as well as north.
SWPC's tips page says these are the events that create the best aurora for the most people.
Expect the live OVATION grid to show cells far south of the usual oval; the distance tool measures how close the nearest bright cell is.
The original OVATION model was only reliable to Kp 7, per SWPC; the current version adds data for larger storms, but treat any single cell with caution.

Questions

What Kp is a G4 storm?: Kp = 8, including a 9-, on the NOAA Space Weather Scales. SWPC's oval-edge rule puts the average edge of the oval at 50 degrees geomagnetic at Kp 8.
How many G4 storms happen per year?: SWPC lists 100 per 11-year cycle on 60 days, about 5.5 storm days a year on average.
Which Glow towns does a G4 storm reach?: 10 of 10 sit at or poleward of the Kp 8 oval edge: Reykjavik, Yellowknife, Tromsø, Fairbanks, Anchorage, Duluth, Minneapolis, Seattle, Portland, Maine, Buffalo.
Where has G4 aurora been seen?: NOAA's scale says: "aurora has been seen as low as Alabama and northern California (typically 45° geomagnetic lat.)". Aurora is often seen hundreds of kilometers equatorward of where it actually sits, per SWPC.
How far south does a G4 aurora reach?: SWPC lists sightings as low as Alabama and northern California, typically 45 degrees geomagnetic.

Sources and verification

NOAA Space Weather Scaleshttps://www.spaceweather.gov/noaa-scales-explanation
NOAA SWPC, Tips on viewing the aurorahttps://www.spaceweather.gov/content/tips-viewing-aurora
NOAA SWPC, Planetary K-indexhttps://www.swpc.noaa.gov/products/planetary-k-index

Quoted figures last checked against their sources on 2026-09-30; next review 2026-12-31. Computed figures come from SunCalc and the IGRF-14 dipole as described in the methodology. Every quote is listed in the sources ledger.