If you live in a subtropical or temperate part of the globe, or if you live in a light-polluted northern or southern metropolis, you may have gone a long time without seeing a live show of the aurorae borealis or australis. So for your viewing pleasure, I present to you in the above video a real-time view of a recent auroral display that shows a very close approximation what of this famous and mesmerizing upper-atmospheric phenomenon looks like when you see it with your own eye.
My astronomy colleague Alan Dyer braved -30ºC temperatures to capture this display from Churchill, Manitoba on the shore of Hudson’s Bay in February 2016. At a latitude of 58°N, Churchill lies under the region of the Earth’s magnetic field where aurorae borealis are most prominent and frequent. (Churchill is also a good place to see polar bears in the winter).
The aurorae are caused by blasts of incoming electrons ejected by the Sun. The particles are diverted from directly hitting Earth’s surface by our planet’s magnetic field which directs the particles into the upper atmosphere in an oval band around the north and south poles. The electrons excite atoms of oxygen and nitrogen in the rarefied upper reaches of the atmosphere some 60 km to 100 km above the ground. The green light of the aurorae comes from excited oxygen atoms relaxing to a lower energy state. Red light also comes from oxygen further up in the atmosphere, and a slightly different wavelength of red light as well as blue light come from nitrogen atoms.
Alan’s video is so striking because it accurately shows how aurorae appear to the eye– the color and brightness, yes, but also the quick and unpredictable motion as the auroral curtains quickly propagate across the sky like a precipitating crystal. You also see the fine filamentary structure within the large auroral curtains, detail which is often lost in longer-exposure images and in time lapse video.
These images are a testament to the sensitivity of modern digital cameras. Alan used a relatively new camera, a Nikon D750, which has a full-frame sensor and a large pixel size of 5.9 microns. Large pixels capture more light with lower noise than smaller pixels. By comparison, the Nikon D5500, a DSLR camera with a cropped sensor, has a pixel size of 3.9 microns, and the camera in the iPhone 6 has a pixel size of just 1.5 microns. The D750 and most DSLR cameras of the past few years can also amplify the signal from the sensor without generating excessive noise, even at the ISO speeds of 12,800 to 51,200 used to capture this video. You still see noise here, to be sure, but the D750 is by no means optimized for low light. The venerable Nikon D4 can show similar displays at lower ISO with lower noise. You can see an example here.
A fast lens is also a must-have for real-time imaging and video in low light. Alan used a 20 mm f/1.4 Sigma Art lens for this work.
This is not a Nikon-boosting article. Canon also has counterparts for low-light imaging in the Canon 6D DSLR, a longtime favorite of nightscape imagers, and the upscale Canon 1D series.
Cameras with even larger pixels, such as the Sony a7s which has 8-micron-wide pixels, yield even better results in low light. In fact, the a7s is so sensitive, photographers can take real-time video of the Milky Way and meteor showers using a fast lens and an ISO of up to 409,600. You can see what this camera can do with the Milky Way in this video review by Ian Norman at LonelySpeck.com.