If you've ever wandered outside to see a night sky splashed with green or red billows of color, either you have access to interesting drugs, or you've seen an aurora firsthand.
If you're in the latter category, it's safe to say that your sky-gazing took place during spring or autumn. Long, dark nights that aren't miserably cold make a nice atmosphere for auroras to live. Although there are some ideas as to why auroras make their strongest showing in the time surrounding the fall and spring equinoxes (roughly Sept. 23 and March 21), no one knows the full reason why geomagnetic storms spike during that period.
And with that casual mention of geomagnetism, we should all get on the same page about what an aurora is. At their heart, the wondrous auroras we know and love originate from solar storms interacting with Earth's magnetic field (which can cause -- you guessed it -- geomagnetic storms). The solar wind contains energized particles that enter our magnetic field. That influx of solar energy is eventually transferred to atmospheric ions, mostly nitrogen and oxygen ions. Those excited ions give off that extra bit of energy as light, and an aurora is born. It's actually a lot like the way we get neon light [source: Taylor]. But, of course, an aurora can be 60 miles (100 kilometers) or more high, and we can view it in the chilly night sky [source: Taylor].
While many who haven't seen an aurora think of it as a rare phenomenon, there's actually one occurring somewhere on Earth at all times [source: Lummerzhein]. Now granted, that aurora isn't necessarily the ghostly, painted pattern that we all typically think of. When the solar wind is calm, only those in extremely high latitudes would be able to see it -- in fact, they might not be able to see it at all, as it could be terribly faint.
Auroras: Must-see Shows in Spring and Fall
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.
Author's Note: Why are the auroras seasonal?
One of the coolest things about studying science in space is that for every answer you find, you'll come upon another question. The study of auroras is a super cool example of that; sure, they know a couple things about why they're seasonal. But that's only a really small part that doesn't explain the whole. To get more information about auroras in general, NASA launched the THEMIS mission, and you can read about the agency's findings here.
- Lummerzhein, Dirk. "Frequently asked questions about auroras and answers." University of Alaska Fairbanks. July 1, 2012. (Nov. 15, 2012) http://odin.gi.alaska.edu/FAQ/#cause
- NASA. "'Tis the season for auroras." United States Government. Oct. 26, 2001. (Nov. 15, 2012) http://science.nasa.gov/science-news/science-at-nasa/2001/ast26oct_1/
- NASA. "Aurora Season Begins." United States Government. Sept. 23, 2002. (Nov. 15, 2012) http://science.nasa.gov/science-news/science-at-nasa/2002/23sep_auroraseason/
- NASA. "Spring is aurora season." United States Government. March 20, 2008. (Nov. 15, 2012) http://science.nasa.gov/science-news/science-at-nasa/2008/20mar_spring/
- NASA. "The Mystery of Auroras." United States Government. April 25, 2006. (Nov. 15, 2012) http://www.nasa.gov/mission_pages/themis/auroras/aurora_feature2.html
- Phillips, Tony. "Spring is aurora season." Astronomy. March 5, 2008. (Nov. 15, 2012) http://www.astronomy.com/en/sitecore/content/Home/News-Observing/News/2008/03/Spring%20is%20aurora%20season.aspx
- Tate, Kari. "Season to Season: Earth's equinoxes and solstices." OurAmazingPlanet.com. March 16, 2012. (Nov. 15, 2012) http://www.ouramazingplanet.com/2629-season-season-earth-equinoxes-solstices-infographic.html
- Taylor, Kenny. "Auroras -- Heavenly Lights." National Geographic. (Nov. 15, 2012) http://science.nationalgeographic.com/science/space/universe/auroras-heavenly-lights/#page=1