New Northern Lights Camera Finds 'Ripples' in the Sky

NORUSCA II is the mother of all fisheye whole-sky cameras. The device can rapidly tune through the spectrum from 430 nm to 750 nm, using a 3.5 mm focal length, f/1.1 lens with a 180-degree field of view, to capture images on a high-resolution Princeton Instruments electron multiplying charge coupled device (EMCCD) camera.

Fred Sigernes of the University Centre on Svalbard (with collaborators from the Ukraine National Academy of Sciences, the Murmansk Region Polar Geophysical Institute, and Calgary’s Keo Scientific) built NORUSCA II to study auroras at the Kjell Henriksen Observatory. The observatory is located on the Norwegian island of Spitsbergen, at 78º N (where the sun sets for the winter before Halloween and doesn’t appear again until Valentine’s Day). The results are described in a new, open-access Optics Express paper.

NORUSCA II all-sky camera, with 12 optical elements, rather than the 19 of a standard auroral telescope: 1) focusing and collimation, 2) filter box, 3) camera lens, 4) camera body.

The camera’s hyperspectral heart is a filter box—an array of electronically controlled wave plates. These tunable Lyot filters—a series of successively thinner liquid crystal screens whose optical properties vary with the voltage across them—can switch from one wavelength to another in about one second.

Sigernes set NORUSCA II up under a clear observatory dome at the Spitsbergen observatory on 7 November 2011, to watch the sky during the nearly four-month-long night.

Over that period, the camera captured one full spectral scan a minute--cycling through 15 wavelengths (set to 13 key nitrogen, oxygen, hydrogen, and sodium transitions plus two background control settings) with time for a half-minute nap in between cycles--to produce stunning time-lapse videos of the Northern Lights...and the northern sky, generally.

Back on 24 January 2012, NORUSCA II caught the earthly aftereffects of a storm on the Sun, a strong coronal mass ejection of particles blasted out of an M8.7-force solar flare. While the flare was not big enough to make an historic Top-Ten list, it was impressive enough to generate NASA press releases and prompt space-weather storm warnings from IEEE Spectrum, among others..

The result was a mesmerizing video of the aurora borealis undulating in the solar wind—followed by a phenomenon that had never been documented before: glowing ripples running across the heavens, for all the world as though someone had dropped a pebble into the pond of the sky. (The red arrow in the photo at left indicates the ripples; the blue arrow shows faint emissions from the Milky Way.) Sigernes says he is still analyzing the ripple spectrum (which also appeared on the observatory’s near-infrared hydroxide imager), so we will have to wait to see what all of the (electron) excitement was about.

Images: Optics Express / Fred Sigernes