Aurora Borealis

There have been so many amazing and infrequent celestial events lately.  First, the eclipse (see my June blog post).  Then in May, a spectacular display of the aurora borealis so far south that many in the U.S. could see it without much travel.  My camera was very good at capturing this display.  Usually, the aurora is faint enough this far south that exposures must be very long.  In May, however, it was so strong that I was able to use a one-second exposure and, with a little push from Lightroom, have some wonderful images which are included in the United States/Minnesota gallery (photo 1, photo 2, photo 3, photo 4 and photo 5).  Why can my camera see the aurora so much better than I can?

Because I know so little about this subject, I’ve researched a bit for this post, including this NatGeo Aurora article. Auroras are light shows in the sky—“borealis” if you’re near the North Pole, “australis” if you’re near the South Pole.  The Sun shoots out electrically charged particles called ions, creating the solar wind.  When the solar wind meets Earth’s magnetic field, it is mostly blocked by Earth’s magnetosphere and forced around the planet to travel farther into the solar system.  However, some of the solar wind is briefly trapped in the Earth’s ionosphere, centered around Earth’s geomagnetic poles, which mark the axis of Earth’s magnetic field (close to, but not the same as, the geographic poles).  This made me wonder:  Do other planets have auroras?  And they do!   See this article.  

Simply put, in the Earth’s ionosphere, the solar wind collides with the oxygen and nitrogen in Earth’s atmosphere, and the energy released causes an aurora, usually from 60-620 miles above the Earth’s surface.  I am not a scientist, and I have not attempted to explain this phenomenon satisfactorily.  I urge you to look it up for yourself.   

The colors of the aurora depend on altitude and the kind of atoms struck by the energy particles accelerated by the solar wind.  The most common color is green, produced when the particles strike oxygen at lower altitudes.  Also, green is the color the human eye can see best.  Particles striking oxygen at high altitude produce a red glow.  Blue and purple light is produced by particles striking nitrogen at lower altitudes.  Yellow and pink auroras are rare and result from a mixture of red auroras with green or blue auroras.  All these are part of the electromagnetic spectrum, much of which our eyes cannot detect.  The visible light for humans are wavelengths from violet to red and all the colors between, especially green.  (As an aside, different animals all see color differently.  Chickens see blue and red light better than we do.  Pigs see less of the red spectrum than we do.  Mantis shrimp [see my photo here] have the most complex eyes on the planet.  They perceive the world through 12-16 visual pigments compared with three for humans.)

This brings me to the question of why my camera can easily detect aurora/electromagnetic colors that my eye cannot see.  When looking for an aurora, I’ve learned first to use my phone to take a photo.  If an aurora is present, the photo will reveal the colorful aurora even though my eye may detect almost no color shift.  Why is that?  Inside the human eye there are rods and cones.  The rods are sensitive to low light but cannot distinguish color.  The cones are sensitive to color but need lots of light to function.  Thus, in low light, the cones barely function and your eyes will not see the color that is there.  The camera does not have the limitations of rods and cones.  In low light, the camera compensates with a longer exposure, collecting light and color for as long as its settings allow. 

Even with its exposure flexibility, the camera will not be able to register the aurora or the stars if you are somewhere with “light pollution” or if it’s daylight.  You need to be somewhere dark to have the best viewing experience so you and your camera will both see more color.  It’s worth the effort.