Why are the auroras seasonal?

Auroras: Must-see Shows in Spring and Fall
This NASA illustration shows the sun-Earth connection and how the solar wind travels between the two.
This NASA illustration shows the sun-Earth connection and how the solar wind travels between the two.
Image courtesy NASA

So why is it that during the autumn and spring we have a much more robust aurora season? Geomagnetism doesn't follow a seasonal pattern, nor does solar activity (although it does have that 11-year sunspot cycle), so it doesn't seem to make any sense. Scientists acknowledge that they're not entirely sure why autumn and spring seem to be the best times for auroras. But they have a few ideas, thanks to sources like NASA's THEMIS mission, short for Timed History of Events and Macroscale Interactions during Substorms.

Some of the seasonality seems to point to geometry. The Earth's magnetic field points north, and there are times when the sun's wide-ranging magnetic field (called the interplanetary magnetic field, or IMF) points south. That allows for some serious alignment; Earth's magnetic field line can point directly into the solar wind. The sun's own north-south magnetic field line is called Bz (bee-sub-zee). When Bz points south, the IMF aligns with Earth's magnetic field and diminishes it, so it's easier for the solar wind to rush in and for its energy to get into our inner magnetosphere [sources: NASA, NASA]. Bz vacillates between north and south, but in the spring and fall it can take big swings south. What we get is a deluge of disco lights in the sky.

There's another geometric reason for autumn-spring favored auroras. The sun's rotation axis is tilted a bit, and the solar wind is strongest at the poles. So every six months, when Earth is at its highest latitude with respect to the sun, we'll be most in contact with the sun's poles and thus its wind [source: NASA].

So there you have it. Auroras happen seasonally because of geometry and Bz. Well ... not quite. In 2001, a paper was published that argued that all these known factors of seasonal auroras only accounted for about a third of geomagnetic storms [source: NASA]. The rest? Heaven only knows.

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