The fall equinox arrived this week, and that means Earth is entering a roughly two-month window when geomagnetic storms, and the auroras they can produce, become more common.

That means, starting now, and for the rest of the season, you'll have a greater chance of spotting the northern lights. 

The reason comes down to some pretty simple (and cool) geometry and physics. 

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How it works

The Sun is constantly blowing a stream of charged particles known as "solar wind" toward Earth. Luckily, our planet's magnetic field acts as a giant shield, deflecting much of that material.

But around the equinoxes, Earth's magnetic field is oriented in a way that can make it easier for energy from the solar wind to get into our atmosphere, which eventually helps power those spectacular auroral displays.

(MORE: The best northern lights photos from across the U.S.)

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Scientists call this seasonal effect the Russell-McPherron effect. NASA describes the geometry as especially favorable around the spring and fall equinoxes.

Why that translates to more northern lights

More solar energy in our atmosphere (specifically the magnetosphere) means geomagnetic storms are more likely when the right solar wind conditions arrive.

The British Geological Survey notes that magnetic storms generally occur more often around the equinoxes, particularly in March-April and September-October.

And research into major geomagnetic storms has found the same seasonal pattern: activity tends to rise around the equinoxes and dip around the solstices.

That doesn't mean an aurora is guaranteed every night. Clouds, darkness, location and the Sun's activity still matter.

But the odds get that seasonal nudge.

(MORE: Best settings to shoot the night sky on your phone)

What to do to increase your chances

Stay up to date on NOAA's Space Weather Prediction Center, which posts a three-day outlook and aurora forecast.

Once the index climbs to 5 or higher, the northern tier of the U.S. gets a real shot at spotting an aurora.

Check your cloud forecast, find a dark spot away from city lights and look north in the hours around midnight.

Happy skywatching!

(MORE: Don’t miss this week’s celestial events)