The Real Aurora: New Scientific Instrument Reveals True Colours Of The Northern Lights

The journal Earth, Planets, and Space published a technical report by Japanese researchers on the results of developing and testing a new instrument for observing the northern lights on 2 August. The ultra-sensitive hyperspectral camera for auroral imaging (HySCAI) provides a full-spectrum, two-dimensional image of the aurora borealis. It will help researchers better understand the interaction processes between plasma and the Earth’s magnetic field.

What is the real colour of the Northern Lights?

Polar auroras are natural light phenomena resulting from the interaction between electrons and the upper layers of the atmosphere. These light shows are caused by collisions between electrons, atoms, and molecules in the atmosphere, which become excited and emit light when they return to their ground state.

So far, observing the northern lights has only used a system of filtering light through specific wavelengths (images of a particular colour). The disadvantage of using an optical filter is its limited wavelength acquisition and low resolution. Hyperspectral cameras, on the other hand, have the advantage of being able to obtain a spatial distribution of spectra with high wavelength resolution.

A hyperspectral image is a two-dimensional image broken down by wavelength or colour, which allows scientists to study the aurora borealis in great detail. These images can measure the energy of the incoming electrons that cause the aurora borealis and make specific colours light up the night sky. Other forms of pictures of the aurora borealis are filtered by wavelength but need to give such a comprehensive view.

 A New Tool For Observing The Real Colours of Aurora Borealis

Engineers from Japan’s National Institute for Thermonuclear Research developed the new technology over five years. In May 2023, the team installed HySCAI on the KEOPS optical platform at the Swedish Space Corporation’s Esrange Space Centre in Kiruna. KEOPS is located just below the auroral belt and is an excellent location for auroral displays. Observations began in September 2023. It wasn’t until early this August that the results of the extensive study became available to experts and the general public.

Using data from observations of the aurora borealis that occurred on 20 October 2023, the researchers estimated the energy of electrons from the light intensity ratio at different wavelengths.

They found a difference in the colour of the northern lights when electrons reach low energies and speeds and when they reach high energies and speeds. When the electrons are slow, they emit red solid light at high altitudes, mainly due to transitions in oxygen atoms at 630 nm wavelength.

 On the other hand, when electrons are fast, they penetrate to lower altitudes and emit intense green or violet light, mainly due to transitions in nitrogen molecules) at wavelengths of 557.7 nm and 427.8 nm, respectively.

Using HySCAI, the researchers obtained a detailed spatial distribution of the colour of the aurora borealis. The different colour distribution was observed because the elements that emit light differ depending on the altitude at which it is generated.

From the intensity ratio of red and violet light, they can determine the energy of the incoming electrons that caused the aurora borealis. In the case of the auroras observed at this time, the power of the incoming electrons was estimated to be 1600 electron volts.

 What’s Inside The Box?

 HySCAI consists of a lens spectrometer, an EMCCD camera and an optical image unwrapping system using galvanometric mirrors. This combination allows the camera to produce hyperspectral images with high wavelength resolution and high sensitivity capable of measuring auroras at the 1kR (1 kiloRel) level. (A Rayleigh is a measure of the brightness of the aurora borealis, equivalent to a radiation rate of 1010 photons per square metre per second.)

The system can capture a wide range of wavelengths from 380 nm to 1000 nm, which covers most of the emission lines of neutral or ionised nitrogen and oxygen atoms and the molecular emission bands associated with the aurorae.

HySCAI used technology previously developed for the world’s second-largest experimental facility for high-temperature plasma research, the Large Helical Device (LHD) fusion stellarator in Japan. The LHD uses various optical systems to analyse the plasma’s spectral features in magnetic fields, which was helpful for the HySCAI “camera.”

Prospects For Future Research

HySCAI will continue to collect data on the aurora borealis over the next few years. With the new hyperspectral camera, scientists plan to gain a broader knowledge of energy transfer through the interaction between charged particles and waves in the magnetic field, as well as the distribution of electrons, their relationship to the colour of the aurora borealis and the mechanism for creating the atmospheric phenomenon.

The system will also be used to study other atmospheric phenomena, such as nighttime glow and mesospheric clouds, which will help better understand the processes occurring in the Earth’s upper atmosphere.

This interdisciplinary research is expected to be promoted in cooperation with universities and research institutes worldwide.

Early Predecessors

In 2012, another international team of space weather researchers designed and built the NORUSCA II camera, which could simultaneously image multiple spectral bands, essentially different wavelengths or colours of light. The camera was tested at the Kjell Henriksen Observatory (KHO) in Svalbard, Norway, where it acquired the first-ever hyperspectral images of the aurora borealis.

On 24 January 2012, during the first NORUSCA II survey campaign, a large solar flare ejected an explosion of high-energy particles known as a coronal mass ejection (CME). The coronal ejection eventually crashed into Earth’s magnetic field, causing magnificent auroras and an opportunity to test the new camera thoroughly.

Researchers were able to image the aurora with unprecedented clarity through a layer of clouds at a low altitude that would have prevented earlier-generation instruments from doing so.

Details of the camera and the results of its first images have been published in the Optical Society’s (OSA) open-access journal Optics Express.