A Hyperspectral View of the Northern Lights

The natural nighttime light shows known as auroras have long astonished stargazers, their dazzling and sometimes eerie curtains, arcs and streamers inspiring myths and superstitions. They have also captured the attention of scientists. In 1950, Aden Meinel used diffraction gratings to capture the first modern photographic spectrographs of an aurora. Research on auroras has continued in the intervening decades, but questions remain about the emission process and the causes behind the colors observed.

Now, a team in Japan has developed a hyperspectral camera that it reports can measure the full spectrum of 2D images of auroras (Earth Planets Space, doi: 10.1186/s40623-024-02039-y). The researchers say the instrument can provide new insights into the mechanism of auroral emission and the characteristics of the particles that cause the spectacular displays.

A closer look at auroras

Auroras—the northern lights (aurora borealis) in the Northern Hemisphere and the southern lights (aurora australis) in the Southern Hemisphere—arise when charged particles from the Sun that are carried by the solar wind collide with Earth’s magnetosphere. Most of the particles are deflected away, but some are captured in the magnetic field and accelerated toward the polar regions, where they strike the upper atmosphere and excite atoms (mainly oxygen or nitrogen).

Most observed light in an aurora consists of emission lines of neutral or ionized nitrogen and oxygen atoms and molecular emission bands. The color is determined by the energy levels and transitions of its constituents, as well as vibration and rotational temperatures for molecules and hydrogen lines from protons.

To get a deeper understanding of the process behind the light show, scientists need comprehensive temporal and spatial spectral observations.

Scientists have used interferometric bandpass filters to obtain 2D images of specific colors in an aurora, but this method provides only a limited acquisition wavelength with low resolution. Diffraction-grating spectrometers provide the full spectrum in ground-based aurora observation, but they offer only one spatial resolution and do not provide the spectrum of 2D images. To get a deeper understanding of the process behind the light show, scientists need comprehensive temporal and spatial spectral observations.

HySCAI heads to Sweden

In its quest to improve auroral knowledge, the team in Japan drew on tools and technologies from another, seemingly very different research effort. At the National Institute for Fusion Science (NIFS), Japan, scientists had developed imaging systems to measure the spectrum of light emitted from plasma in a magnetic field in the Large Helical Device, the world’s largest superconducting plasma device. In 2018, NIFS researchers came up with the idea to adapt those systems to build a hyperspectral camera capable of observing auroras.

Five years of planning and development later, in May 2023, the team installed HySCAI (hyperspectral camera for auroral imaging) at the KEOPS (Kiruna Esrange Optical Platform Site) of the SSC (Swedish Space Corporation) in Kiruna, Sweden. KEOPS is located just below the auroral belt, and thus has a great seat for aurora displays.

HySCAI is capable of measuring auroras at 1 kilorayleigh. (A rayleigh is a measure of auroral brightness equivalent to an emission rate of 10¹⁰ photons per square meter per second.) The instrument consists of an all-sky lens, monitor camera, galvanometer scanner, grating spectrograph and electron multiplying charge-coupled device (EM-CCD). The galvanometer scanner can scan a slit image of the spectrograph on the all-sky image plane in the direction perpendicular to the slit. HySCAI has two gratings; one is 500 grooves/mm for a wide spectral coverage of 400 to 800 nm with a spectral resolution (FWHM) of 2.1 nm, and the other is 1500 grooves/mm for a higher spectral resolution of 0.73 nm with a narrower spectral coverage of 123 nm.

2D hyperspectral snapshot

HySCAI began operations in September 2023, with the team collecting data remotely from Japan. During an aurora substorm on 20 and 21 October 2023, the researchers successfully captured, for the first time, an aurora’s detailed spatial distribution of color—a 2D hyperspectral image. It gave them the chance to confirm what kind of information the system could gather, including estimating the energy of electrons from the intensity ratio of light at different wavelengths.

The team found that, when the electrons arrive at low speed, they emit strong red light at high altitudes. When the electrons are fast, they penetrate to lower altitudes and emit a strong green or purple light. The different color distribution was observed because the elements that produce the light differ based on the height in the atmosphere at which the light is generated. From the ratio of the intensity of the red light (630 nm) to that of the purple light (427.8 nm), the team estimated that the energy of the incoming electrons that caused the aurora was 1600 eV.

Using HySCAI, the researchers hope to “contribute to solving important auroral issues such as the distribution of precipitating electrons, their relationship to auroral color and the mechanism of auroral emission,” according to a press release. They also note that the system will provide insight into energy transport due to the interaction between charged particles and waves in a magnetic field, an area that is attracting attention in fusion plasmas.