What is my geomagnetic latitude?

Minneapolis, worked by hand. The point is 44.98N 93.26W and the dipole pole is 80.79N 72.76W. The longitude difference is -20.50 degrees.

sin(mlat) = sin(lat) sin(pole lat) + cos(lat) cos(pole lat) cos(lon - pole lon)

That is 0.7069 x 0.9871 + 0.7074 x 0.1601 x 0.9367 = 0.8038, and the arcsine of 0.8038 is 53.49 degrees. Minneapolis sits 2.6 degrees south of Seattle on the globe (45.0N against 47.6N), yet reads 53.5 against 53.0 on the dipole, because it is closer to the pole's meridian.

The aurora forms around Earth's magnetic poles, so the useful latitude for aurora is measured from the magnetic pole, not the geographic one. Two towns at the same geographic latitude can sit at very different geomagnetic latitudes. The north geomagnetic pole lies in the Canadian Arctic, which pulls North America closer to it than Europe or Asia at the same latitude.

SWPC's geomagnetic storm scale describes how far south aurora has been seen for each storm level in geomagnetic latitude. Knowing your own value lets you read that scale directly.

The International Geomagnetic Reference Field describes Earth's main field as a series of spherical harmonics. The first three terms, g10, g11 and h11, describe the best-fitting bar magnet at Earth's center: the centered dipole. Glow reads them from the IGRF-14 coefficient file for epoch 2025.0.

The field strength of that dipole is B0, the square root of the sum of the three squares. The north geomagnetic pole sits at colatitude arccos(-g10 / B0) and longitude atan2(-h11, -g11). With the 2025.0 values that gives 80.79N 72.76W.

Your geomagnetic latitude is then your angular distance from that pole, subtracted from 90 degrees. Glow computes it by rotating your coordinates so the pole sits at the top of the sphere. The same rotation gives geomagnetic longitude, which is 0 on the meridian through the geographic south pole.

A centered dipole is an approximation. The real field has higher-order terms, and space physicists often use corrected geomagnetic coordinates that follow field lines. SWPC does not state which system its typical latitudes use, so matching a dipole value to a G-scale row is approximate. Glow labels every match that way.

IGRF-14 is valid through 2030. The pole drifts, and the coefficients file includes secular variation for that reason. Glow uses the 2025.0 values and will update the constants when the next generation is published.