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Aurora Borealis brought to light

Research Topic Chapter
News flash intro
At the poles of the Earth the magnetic field curves down to the ground. At these locations energetic particles create a strong interaction with the atmosphere leading to a spectacle of amazing colours in the sky: the so called Aurora Borealis (North Pole) and Aurora Australis (South Pole). These phenomena can be studied by investigating the polarisation of the observed light. To this end, the Engineering department developed, in collaboration with the Limb sounding group, the polarimeter instrument ASPA based on passive optical filtering.
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The mission to Norway

In order to observe the Aurora Borealis or Northern Lights an observation campaign was organised in Skibotn (Norway), in collaboration with the University of Tromsø, who kindly put an observation platform at our disposal.

In February 2020 all the material had to be ready for shipment.  Before leaving, however, the instrument had to be ‘up and running’ and equipped to withstand the extremely cold environment. Temperatures as low as -20°C are not unusual in this northern region. The Engineering department assembled the instrument, tested it extensively so that everything was fully operational.

ASPA from the inside

The instrument that was developed in a close collaboration between the Limb sounding research group and the Engineering department was called ASPA (AOTF-based Spectro-Polarimeter for Aurorae) and its optical design is strongly based on the ALTIUS instrument. A passive filtering system allows the selection of the requested optical wavelengths. Mainly the green, red and purple/blue bands appear in aurorae, all other colours have to be filtered out.

This can be done using an AOTF (Acousto-Optical Tunable Filter), a Tellurium Dioxide-crystal in which generated sound waves produce a “double separation” of the incoming light, leading to two light beams at the AOTF exit. By applying an appropriate radio-frequent (RF) signal to the AOTF, the correct wavelength is selected for the outgoing light beams. These beams contain information about the polarisation strength of the aurora.

In order to retrieve the different polarisations, a double channel instrument is required, containing two AOTFs, each with their unique RF driving system. Such a system consists of:

  • an RF generator
  • a matching RF amplifier

The design and manufacturing of the RF generators was done by the BIRA-IASB Engineering department. They also developed the firmware of an FPGA module, used to control the RF generators and to read out the temperature sensors and incliniometer. The box was isolated thermally and equipped with a heat regulation system to make the instrument resistant against the extreme cold temperatures at the measuring site.

The full assembly of the different parts and the establishment of a Graphical User Interface (GUI) was realized under the responsibility of the Engineering department. During the campaign in Norway the instrument performed as expected. As soon as the COVID-19 situation allows, the instrument will be reused for further validation.



Vanhamel, J., Dekemper, E., Berkenbosch, S., and Clairquin, R. (2021). Novel acousto-optical tunable filter (AOTF) based spectropolarimeter for the characterization of auroral emission, Instrumentation Science & Technology.

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Preparation of the ASPA-instrument
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Internal lay-out of the ASPA instrument
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ASPA instrument optics with two channels, each consisting of a unique RF generator and matching amplifier
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