Aurora Season: Why the Northern Lights Shine Brightest in Spring and Fall

There is a special time for seeing the Northern Lights, called aurora season. This season happens twice a year, around the spring and fall equinoxes. During these times, auroras are more common and often brighter. We know that storms on the sun cause disturbances in Earth's magnetic field. These are called geomagnetic storms. The sun has an 11-year cycle of activity. This cycle reached its peak in late 2024, so the sun is still very active.

But an 11-year cycle does not explain why auroras increase twice every year. Why do more geomagnetic storms happen during the equinoxes? The answer involves magnetism, the positions of Earth and the sun, and something called the Russell-McPherron effect.

Scientists have been studying this seasonal pattern for over 100 years. In 1912, an astronomer named Aloysius Cortie published the first major paper linking the equinoxes to more frequent auroras.

Later, in 1940, mathematician Sydney Chapman and his colleague Julius Bartels wrote about this twice-yearly aurora season in their important book, Geomagnetism. This book was the main reference on Earth's magnetism for many years. More recently, a NASA solar physicist named David Hathaway created a new chart showing the same strong seasonal pattern.

For years, scientists tried to explain the twice-yearly change in geomagnetic storms. A strong explanation came in 1973 from researchers Christopher Russell and Robert McPherron. Their model showed a physical link between the shape of Earth's magnetic field and the magnetic field that comes from the sun with the solar wind. This connection is now called the Russell-McPherron effect.

The key to this effect is a part of the magnetic field called Bz. Just like a bar magnet has a north and south pole, the magnetic fields from the sun have polarity too. Russell and McPherron showed that the north-south part of the sun's magnetic field changes over the year. This change matches the tilt and wobble of Earth's axis. These changes are strongest during the equinoxes.

Geomagnetic storms—and the auroras they cause—are most likely when the north-south part of the solar wind points the opposite way from Earth's own north-south magnetic field. Think of it like two bar magnets with opposite poles facing each other: they pull together. This pull opens a temporary gap in Earth's magnetic shield. This gap lets charged particles from the solar wind flow more easily toward the magnetic poles. When these particles hit gases in the atmosphere, they create the beautiful light shows we call auroras.

Another factor, called the equinoctial effect, also helps create more auroras during the equinoxes. This effect is about the changing positions of Earth and the solar wind. During the equinoxes, Earth's magnetic poles are at a right angle to the flow of the solar wind twice each day. At these specific times, the solar wind connects more powerfully with Earth's magnetic field. This increases the chance for magnetic storms. As Earth's position changes with the seasons, the poles point more toward or away from the sun. This reduces the geometric advantage.

In short, there is a clear physical reason why we see more auroras around the equinoxes. Researchers have studied this for more than a century and are still learning more. While some details of the models may differ, scientists agree the cause is the combined power of the sun's and Earth's magnetic fields. This works together with the specific positions of the sun and Earth in its orbit.

As meteorologist Curtis Grevenitz once wrote:

It is not just a coincidence that these two beautiful phenomena have a relationship.

There is a defined aurora season each year, centered on the March and September equinoxes. This season is mainly driven by the Russell-McPherron effect. During the equinoxes, the alignment of the sun's and Earth's magnetic fields is best for transferring energy. A second factor, the equinoctial effect, gives an extra boost. Together, these principles of magnetism and orbital geometry explain why skywatchers can expect more frequent and vibrant Northern Lights during these specific times of the year.