ASIS : the Auroral Spectrograph In Skibotn, 1.5 years of observations

2023-2024
ASIS (Auroral Spectrograph In Skibotn) is an auroral spectrograph installed at the Skibotn observatory in Norway, as a collaboration between BIRA-IASB, IPAG and UiT. Using a small astronomical lens and an optical fibre, auroral light is captured in a 4° field-of-view producing an auroral spectrum every 30 seconds.

Since October 2023, ASIS has run continuously in Arctic conditions, fully automated and remote. With only a few similar instruments worldwide, and being just 2km from the future EISCAT_3D radar facility, it provides valuable and complementary data enabling many collaborations.

Auroras are the most spectacular display of the complex interactions between the solar wind and the Earth’s magnetosphere and ionosphere.

They are usually studied using radar and/or optical observations. Most of the optical instruments use cameras and narrow or RGB filters providing an accurate view of the phenomenon but at specific wavelengths (or for specific emission lines).

There are very few auroral spectrographs worldwide, able to simultaneously cover many auroral emission ones in a large wavelength range. There are:

  • the MISS2 (The Meridian Imaging Svalbard Spectrograph) and ASG (Auroral SpectroGraph) instruments in Svalbard (located inside the polar cap),
  • two TREx (TRansition Region Explorer) spectrographs in Canada (one in the auroral oval and one equatorward), and
  • an additional spectrograph at the ZhongChen Chinese station in Antarctica.

There was therefore a need for such an auroral spectrograph inside the auroral oval in Scandinavia. Moreover, such an instrument can also help shed light on the recently discovered mysterious continuum auroral emissions associated with strong dynamical aurora.

Set up

For all these reasons, we have set up in October 2023 the ASIS (Auroral Spectrograph In Skibotn) instrument at the Skibotn observatory, as a collaboration between BIRA-IASB (Belgium) and IPAG (France), which funded the instrument, and UiT (Norway) who is hosting it. 

ASIS is using a small astronomical lens pointing along the local magnetic field line to capture auroral light within a circular field-of-view of 4 degrees (see Figure 1). The light is directed inside an optical fibre and focused on the adjustable slit of a spectrograph equipped with a turret containing 3 gratings. The selected grating splits the incoming light and produces an auroral spectra onto a CCD camera cooled with a Peltier.

Observations

Since October 2023, ASIS has continuously acquired data without any failure despite the harsh Arctic conditions. It is fully automated, starting and ending observations respectively after sunset and before sunrise. It is also controllable remotely which allows to modify e.g. the choice of grating, the exposure time or even the slit width.

So far, data have been obtained with a 300 lines/mm grating and a slit width of 100 mm providing a spectral range from roughly 400 to 700 nm with a spectral resolution of ~ 0.3 nm. ASIS has been calibrated on-site in February and October 2024 using lamps calibrated at the B.RCLab. Data are automatically transferred every day and archived at BIRA-IASB. After calibration, they become available on a dedicated ASIS website.

An example of spectrum obtained with ASIS during a geomagnetic substorm is shown in Figure 2. It demonstrates the richness of the spectrum in addition to the strongest usual auroral emission lines at 427.8 nm (blue), 557.7 nm (green), and 630.0 / 636.4 nm (red).  A zoom on the intensity (vertical) scale was made to made the additional emission lines / bands clearly visible. 

ASIS is currently the only spectrograph located in the auroral oval in Scandinavia. Moreover, the Skibotn observatory is located only 2 km from the main site of the future EISCAT_3D radar facility.  ASIS will therefore provide extremely valuable and complementary data to the future radar observations and to other optical instruments located nearby, opening the possibility for many collaborations.

Aurora above astronomical dome Skibotn Norway
Aurora captured above the astronomical dome of the Skibotn observatory, in Norway (Crédit : Gaël Cessateur)
Small astronomical lens used to capture auroral light
Figure 1: the small astronomical lens used to capture auroral light and inject it into our spectrograph and CCD camera is located at the Skibotn observatory, Norway. Here, the view from inside the dome.
Example of spectrum obtained with ASIS
Figure 2 : Example of spectrum obtained with ASIS on 5th November 2023 during a geomagnetic storm. The intensity scale has been adapted such that all the smaller emission lines / bands are clearly visible and some of them have been annotated.