Geomagnetic latitude, explained for aurora watchers

SWPC describes the aurora as two ovals approximately centered on the magnetic poles. Electrons follow Earth's field lines down into the upper atmosphere, and the field is tilted against the spin axis. So a ring of aurora that looks centered from space sits off center on a globe drawn around the North Pole.

Every SWPC rule of thumb about where aurora reaches uses geomagnetic latitude. The Kp rule on the tips page says so directly, and the G-scale lists typical sighting latitudes in geomagnetic degrees. If you plug your map latitude into those rules you can be off by a full storm level.

The simplest model of Earth's field is a bar magnet at the planet's center, tilted from the spin axis. NOAA NCEI publishes the International Geomagnetic Reference Field, IGRF-14, whose first three coefficients describe that tilted dipole. For epoch 2025.0 they are g10 = -29350.0, g11 = -1410.3 and h11 = 4545.5 nanotesla.

From those three numbers Glow places the north dipole pole at 80.79N, 72.76W, in the Canadian Arctic. It then rotates each place into a frame where that pole sits on top. The new latitude is the dipole, or geomagnetic, latitude. The same rotation gives a geomagnetic longitude, which the oval map uses to draw rings.

The dipole pole is a different point from the north magnetic pole. SWPC's tips page places the magnetic pole about 400 km from the geographic pole, in the islands of northeast Canada. That is where a compass needle points straight down. The dipole pole is where the best-fit bar magnet's axis comes out. Glow uses the dipole pole for every figure because it comes straight from published coefficients and gives one consistent frame.

The chart plots map latitude against dipole latitude. Points above the diagonal sit closer to the oval than a globe suggests.

North American towns all gain. Minneapolis moves from 45.0 to 53.5 degrees, and Buffalo from 42.9 to 52.0. Seattle gains less, from 47.6 to 53.0, because it sits farther west of the pole's meridian. That is why Minneapolis, 2.6 degrees south of Seattle on a map, ends up slightly higher on the dipole.

Europe runs the other way. Tromsø sits at 69.6 on the map and 67.5 on the dipole, a loss. Reykjavik, about 51 degrees of longitude from the pole's meridian against Tromsø's 92, gains: 64.1 becomes 68.8. Yellowknife, at 62.5 on the map, reads 68.5 and ranks among the highest of the 10.

A centered dipole is a first approximation. The real field has higher-order terms, and other sources may use coordinate systems that include them. Glow therefore labels every G-scale match approximate. Differences of a degree or so between Glow and other sources are expected.

The field also changes over time. IGRF-14 is valid through 2030. Glow stores the coefficients with their epoch and will add IGRF-15 when NCEI publishes it.

To get your own value, enter coordinates in the geomagnetic latitude calculator. The Kp needed tool then turns it into a storm level, and the Kp to latitude chart shows the rule behind it.