Aurora: how far south and when

The aurora is light from Earth's upper atmosphere, glowing about 80 to 500 km above the ground.^[1] It is usually confined to high latitudes, but during a strong geomagnetic storm it can be seen far closer to the equator. How far depends on the strength of the storm, which NOAA's Space Weather Prediction Center measures with the Kp index and rates on its G scale.

What causes it

Electrons, energized in Earth's magnetic field on the night side of the planet, are guided down toward the polar regions, where they collide with oxygen and nitrogen in the upper atmosphere and make them give off light.^[1] Because Earth's magnetic field steers the electrons, the aurora forms two ovals roughly centered on the magnetic poles. During major geomagnetic storms these ovals expand away from the poles.^[1] The best place to watch is under the ovals, between about 60 and 75 degrees of latitude, where aurora can be seen on more than half the nights of the year.^[1]

Kp and the G scale

The Kp index measures disturbance of Earth's magnetic field on a scale from 0 to 9, from magnetometer readings taken over three-hour periods; values of 5 or more mean a geomagnetic storm.^[2] NOAA rates storms on its G scale, and each level comes with how far from the poles the aurora has typically been seen, in geomagnetic latitude:^[3]

• G1, minor (Kp 5): aurora commonly visible at high latitudes, such as northern Michigan and Maine; about 1,700 such storms in an 11-year solar cycle.^[3]
• G2, moderate (Kp 6): typically down to 55°, seen as far south as New York and Idaho; about 600 per cycle.^[3]
• G3, strong (Kp 7): typically down to 50°, seen as far south as Illinois and Oregon; about 200 per cycle.^[3]
• G4, severe (Kp 8): typically down to 45°, seen as far south as Alabama and northern California; about 100 per cycle.^[3]
• G5, extreme (Kp 9): typically down to 40°, seen as far south as Florida and southern Texas; about 4 per cycle.^[3]

During the G5 storm of May 2024, the first since 2003, aurora was reported as low as 26 degrees magnetic latitude, which may rank among the lowest sightings in five centuries.^[4]

Why the line is approximate

These latitudes are typical values, not a boundary. NOAA notes that the link between Kp and the aurora's position is approximate and describes averages that do not always hold.^[5] Aurora can also be seen from a long way off: when it is bright, it can be visible from up to 1,000 km away, low on the horizon toward the pole.^[6]

The latitudes are also geomagnetic, measured from the geomagnetic pole rather than the geographic one. That pole is the point where the axis of Earth's magnetic field, modeled as a simple dipole, meets the surface; in 2025 it lay at about 80.9°N, 72.8°W, and the axis is tilted about 9° from Earth's rotation axis.^[7] North America lies on the side nearer that pole, so the same storm reaches farther south in geographic terms over North America than over Europe or Asia.

The reach lines on Asteroid Alerts maps are drawn this way. They take NOAA's geomagnetic latitudes for each G level and convert them to map positions using the dipole defined by the International Geomagnetic Reference Field, the standard model of Earth's main magnetic field, at its 2025 epoch.^[8][7] The real field is not a perfect dipole, so the line is a forecast of approximate reach, not a map of the aurora itself.

How to give yourself the best chance

• Watch the forecast. NOAA's short-term aurora forecast covers the next 30 to 90 minutes, the time it takes the solar wind to travel from its monitoring point in space to Earth.^[6]
• Get dark and look poleward. Go out at night, away from city lights, and find a clear view toward the north (south in the southern hemisphere); a full Moon makes the aurora look fainter.^[5]
• Choose the hours around midnight. The most active aurora usually comes within an hour or two of local midnight, between about 10 p.m. and 2 a.m.^[5]
• Remember the seasons. Larger storms tend to come around the spring and autumn equinoxes, while at high latitudes summer nights are too bright for aurora.^[5]
• Use a camera. Even standard phone cameras are now much more sensitive to the aurora's colors than they once were.^[4]

What drives the storms themselves is covered in Solar storms.