Northern lights tonight

Kp is one number for the whole planet, worked out every three hours from magnetometers at thirteen observatories between 44 and 60 degrees geomagnetic latitude. It runs from 0 to 9 in thirds, and it measures how much the Earth’s magnetic field is being shaken by the solar wind. A higher Kp means the auroral oval, the ring of light around the magnetic pole, has swollen and pushed toward the equator, so it can be seen from further south.

Kp is not a brightness and not a probability. It says where the aurora is, not whether you will see it: that also needs a dark sky, a clear sky, a view to the north with nothing in the way, and eyes given twenty minutes to adjust. At the line for a given Kp the aurora is a faint arc low on the northern horizon, often gray to the eye and green in a photograph; far poleward of the line it is overhead and unmistakable.

Two more things worth knowing. The aurora keeps to the night side of the Earth and swings furthest toward the equator around local midnight, so the hours either side of midnight are the ones to be out in. And the equinoxes, March and September, bring more geomagnetic storms than the solstices, which is why aurora sightings cluster in spring and autumn.

Each place is placed by its geomagnetic latitude, measured from the geomagnetic dipole pole, and compared with the reach NOAA states for each storm level on its space weather scales: G1 (Kp 5) about 60 degrees geomagnetic, G2 55, G3 50, G4 45, G5 (Kp 9) 40. Reading that backwards gives the Kp a place needs. Below Kp 5 the oval contracts more slowly and the line runs from 66 degrees at Kp 0 to 60 at Kp 5.

The forecasts have different horizons and the pages keep them apart: OVATION is the only thing that knows whether an aurora is out now, and it reaches 30 to 90 minutes ahead; the planetary Kp forecast runs three days; the 27-day outlook names likely active days and is not a forecast of any one night. None of them knows the clouds, which is why a town page reads its own forecast for those.