Forecasts of Northern Lights displays have been standard in the past few weeks, and the reason for that is last weekend's fall equinox. In its wake, there could be a brief and spectacular aurora season, with geomagnetic storms generally becoming more intense than usual in the weeks surrounding the equinox. Around the fall equinox, the magnetic field of Earth and the solar wind align, a phenomenon known as the “Russell-McPherron Effect.”
The Northern Lights occur when charged particles from the sun accelerate down Earth’s magnetic field lines and collide with molecules high in Earth’s atmosphere. That interaction produces the green and red lights.
Scientists noticed long ago that geomagnetic activity tends to be stronger twice a year, around the equinoxes in March and September.
This uptick in Northern Lights is explained by the Russell-McPherron Effect, first described in a paper in 1973.
It means that any forecasts of geomagnetic activity over the next few weeks should be taken seriously because they may, in practice, turn out to be significantly more intense than predicted.
The Russell-McPherron effect shows how Earth’s magnetosphere and the solar wind align during the equinoxes. The sun and Earth's magnetic fields are lined up with their equators, but since Earth rotates on a tilted axis, they are not aligned. Twice a year, near the equinoxes, the south-pointing part of the sun's magnetic field more easily connects with Earth’s magnetic field. When that happens, charged particles from the sun penetrate Earth’s defenses more easily, driving more and more intense geomagnetic storms and auroras.
The sun is currently near the peak of its 11-year activity cycle. Although NASA and NOAA dated the moment of solar maximum as around October 2024, it's difficult to tell for sure until years after. They may even be a double peak, according to experts, some of whom think we may be in for the best Northern Lights season until 2035. During the sun’s solar maximum period, solar flares and coronal mass ejections are more frequent. It's the latter that launches massive waves of charged particles toward Earth that, a few days later, can spark geomagnetic storms and displays of aurora.
There is no true “season” for geomagnetic activity — auroras occur year-round — but visibility depends on darkness. If skies are bright, they cannot be seen. For Northern Hemisphere observers, the September equinox marks a sweet spot for viewing because darkness lasts for about 12 hours, providing longer and lengthening windows to catch the lights. Around the Arctic Circle — over where the auroral oval hangs, and where the Northern Lights are very common — it’s not until the September equinox that skies are dark enough to regularly spot them because summer twilight prevents viewing, even though auroras continue to occur.