Kp to latitude chart for the northern lights

Edge latitude equals 66 minus 2 times Kp, in degrees of geomagnetic latitude. Kp 3 gives 60 degrees. Kp 5 gives 56. Kp 7 gives 52. Kp 9 gives 48. Every figure in that line comes from the SWPC tips page, and SWPC states it holds in geomagnetic latitude, not geographic.

Run it backward to find the Kp you need. Take your geomagnetic latitude, subtract it from 66, halve the result and round up to the next whole step. A town at 55 degrees gives 11 divided by 2, which is 5.5, so the edge reaches it at Kp 6. A town above 66 degrees sits under or inside the quiet-time oval and needs Kp 0.

The table converts each Glow place to IGRF-14 dipole latitude and applies the rule. It also shows how far each town sits from the Kp 0 edge, and the first NOAA G-scale level whose typical latitude reaches it. Those are two separate SWPC yardsticks, and they do not always agree.

Three towns in the table sit poleward of 66 degrees on the dipole: Reykjavik at 68.8, Yellowknife at 68.5 and Tromsø at 67.5. By the rule they can sit under the oval even at Kp 0. Fairbanks, at 65.7, falls just short and needs Kp 1.

The lower 48 towns cluster between 52 and 56 degrees. Duluth reads 55.4 and needs Kp 6. Minneapolis reads 53.5 and needs Kp 7. Buffalo reads 52.0 and needs Kp 7. That is storm territory: Kp 6 is G2 and Kp 7 is G3 on the NOAA scale.

SWPC's storm scale lists where aurora has been seen at each level, with a typical geomagnetic latitude: 55 degrees for G2, 50 for G3, 45 for G4 and 40 for G5. Those numbers sit well south of the oval edge from the Kp rule. At Kp 9 the rule gives 48 degrees for the edge; G5 lists 40 degrees for sightings.

The gap is viewing distance. The Kp rule describes where the aurora is. The G-scale column describes where people saw it from. SWPC notes on the same tips page that aurora can often be observed hundreds of kilometers equatorward of the actual aurora.

Eight degrees of latitude is about 890 km. SWPC's forecast page gives 1000 km as the distance from which bright aurora can be seen when conditions are right. The two yardsticks line up once you allow for that reach. Glow's kp needed tool shows both for your coordinates.

Treat the chart as a threshold, not a promise. SWPC calls the Kp to latitude relationship approximate and says it represents averages. A night at the right Kp can still show little if the storm peaks in your daylight hours, and a lower Kp can surprise.

• Look up your geomagnetic latitude with the geomagnetic latitude calculator.
• Find the Kp where the edge reaches you, and the Kp where it sits within about 9 degrees, the 1000 km viewing distance.
• Watch the SWPC forecast Kp for your dark hours, then switch to the live oval map once the night starts.
• Check cloud cover and the moon before driving anywhere.

Kp is a 3-hour planetary figure, so the live OVATION grid is the better guide once you are outside. It shows the oval's actual shape at that moment, including bulges the Kp rule smooths over.