Cool scientific instruments: the NORUSCA II hyperspectral camera

Thumbnail image of two NORUSCA II 1st Generation
hyperspectral cameras. For full-size image see Optics InfoBase.

Up north in Svalbard, at the Kjell Henriksen Observatory, something very cool recently happened: the first hyperspectral images of the Northern Lights were taken. The NORUSCA II is the instrument developed to take them.

The Aurora Borealis and Aurora Australis—the Northern and Southern Lights respectively—occur because of the interaction between the solar wind and the Earth's magnetic field.

The Earth has an iron core, the inner part of which is under such pressure that, despite having a temperature of 5430 °C, it's a solid ball. The outer part, meanwhile, is cooler but under less pressure, so it remains liquid. Thanks to a combination of convection currents in the molten iron, the Coriolis force, and induced magnetic and electric fields that isn't totally understood (see Wikipedia or Gary A. Glatzmaier's page for more detailed descriptions), the geomagnetic field of the Earth is created and sustained.

Hundreds of kilometres from the Earth's surface, this field is met by the solar wind, charged particles that stream away from the Sun, through its own magnetic field. The solar wind distorts the Earth's magnetic field into the shape shown below:

Artist's rendition of Earth's magnetosphere, from Wikimedia Commons.

The charged particles, as well as ultraviolet radiation from the Sun, ionise the outer layer of the atmosphere to produce a plasma shell known as the ionosphere. (The ionosphere also has a dynamo region of its own, producing electric fields and magnetic effects. There are a lot of interrelated feedback systems at work here!) Accelerated along the lines of the distorted magnetic field, charged particles from the ionosphere and the solar wind are funnelled towards the poles.

When they get there, they can ionise nitrogen atoms by kicking out one of the atom's electrons, and excite both oxygen and nitrogen atoms that absorb their extra energy, moving an electron up into a higher-energy state. These states can't last, though—the atoms naturally tend to the lowest-energy state possible. When the nitrogen atom regains an electron, or the excited electrons drop back down to their ground states, photons are emitted—that is, light.

The wavelengths of the light emitted depend on the exact events that generated it. For the human observer, that means breathtaking displays in green, red and blue. For scientists, measuring the spectra of the aurora additionally makes it possible to discover a lot about the interplay of the upper atmosphere with the magnetic fields of the Earth and the Sun and the solar wind.

Northern lights over Nybyen, Svalbard, by Torbjørn Taskjelle,
shared on Flickr  under a Creative Commons licence.

Finally, this brings us to the NORUSCA II!

The ideal scientific image of the aurora would show us the whole sky at a high resolution, and for every pixel we could see each different wavelength of electromagnetic radiation that came from that part of the sky. To get this, some way of separating out the spectrum into bands is necessary. This can be done using prisms, diffraction gratings or, most usefully for our purposes, filters. The type used in the NORUSCA II is a kind of Lyot filter, a piece of birefringent material. The two perpendicular components of the electromagnetic radiation travel at different rates through the material and usually, when they recombine, their intensity will be reduced. Only for specific wavelengths will the intensity be preserved. This effect is increased by directing the light through a polariser before it gets to the
camera, thus picking out only the intended wavelength.

It's possible to scan through multiple bands of wavelengths with a Lyot filter, by switching out plates, but this takes too long to give really good results. Instead, the NORUSCA II uses a tunable liquid crystal filter. The birefringence of the liquid crystals can be tuned by adjusting the electric field across the filter, allowing NORUSCA II to scan through 41 wavelength bands in a matter of microseconds and with no moving parts. A fisheye lens allows the whole of the night sky to be seen at once and an EMCCD (electron-multiplying charge-coupled device) captures the image.

Thumbnail image of the lens mechanics and
optical diagram of the NORUSCA II all-sky lens:
(1) focusing mechanism and collimator lenses,
(2) filter box - chamber,
(3) camera lens, and (4) camera head.
For full-size image see Optics InfoBase.

Tunable liquid crystal filters have poor transmission of blue light, which is why they haven't been used for this type of application before, but the combination of high-throughput lenses and the EMCCD in NORUSCA II compensate for this well.

In the 2011-12 aurora season at Svalbard, not only did the Norwegian/Russian team successfully take the images they wanted, but they might already have discovered a new phenomenon.

Part A – The aurora as seen as a color composite image from the NORUSCA II camera. Three bands were combined to make the image. Each band was assigned a different color -- red, green, and blue – to enhance the features of the aurora for analysis.

Part B - The red arrow points to the unidentified low-intensity wave pattern, which the researchers suspect is an auroral-generated wave interaction with airglow. For contrast, the blue arrow points to the faint emission of the Milky Way. (Credit: Optics Express - image taken from UNIS). 

The red arrow on the image to the right shows a wave pattern picked up during a coronal mass ejection, when a particularly strong burst of high-energy particles hit the Earth's atmosphere. The researchers believe this could show an interaction between the aurora and airglow, normal light emission from the upper atmosphere (shown beautifully here).

You can read more about NORUSCA II here, and the full paper on its design and early results is available here. I'm very excited to see what it can discover in future aurora seasons.

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Bonus material:

• The great solar storm of 1859 produced huge and splendid auroral displays, as well as widespread disruption of telegraph lines. Some lines, however, were just the right length and in just the correct position that they functioned better under auroral power than with their batteries!
• I'm currently reading Thomas Keneally's novel Victim of the Aurora, a murder mystery set in an imagined British Antarctic expedition in 1909. The aurora so far hasn't played much of a role but there is plenty of discussion of ice phyiscs, Arctic marine biology and Edwardian scandal. I thoroughly recommend it.
• Here you can see the students at Kjell Henriksen Observatory demonstrate new ways of removing the observatory's dome covers: both scared and safe turtle methods.