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Multi-wavelength observations and modelling of aurora

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Despite different atmospheres and magnetospheres, aurora are observed on all magnetized bodies in the solar system. Within the BRAIN-Be project MOMA (Multi-wavelength Observations and Modelling of Aurora) we study the UV, visible, and radio emissions of aurora on Earth, through both observations and modelling, to shed some light on the physical processes at play in the formation of aurora. More generally, this project allows to better understand the near-Earth space within the space weather context.
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Auroral emissions

The phenomenon of aurora, or polar lights, can occur on any planet with an atmosphere and a magnetic field. Energetic particles from the magnetosphere are accelerated along the planetary magnetic field lines and interact with the neutral constituents of the upper atmosphere. Photo-emissions from the deactivation of the excited atoms/molecules give rise to the polar lights.

The colors of the emissions depend on which atoms/molecules are excited. On Earth, green and red emissions, at 557.7 and 630 nm respectively, are due to the atomic oxygen at altitudes of ~110 and 220 km. Some energetic particles can reach altitudes lower than 100 km, and interact with molecular nitrogen, giving blue (427.8 nm) and purple emissions.

Auroras are not limited to the visible spectral range:

  • UV emissions also happen through the same electrons excitations mechanisms.
  • The interaction between the precipitating electrons and some electromagnetic waves at high altitude can also produce radio emissions, the Auroral Kilometric Radiation (AKR).

Observations, techniques and models

Using visible observations of auroral arcs from ground-based optical ALIS (Auroral Large Imaging System) stations located in Scandinavia, the three-dimensional volume emission rate (VER) of the arc can be reconstructed using tomographic techniques.

The blue emission is directly proportional to the energy deposited by the precipitating electrons, with no involvement of chemistry or secondary processes. This property allows us to infer the differential energy flux of precipitating electrons from the blue VER reconstructed between ~ 100 and 260 km altitude, using a second inversion. This method provides a two-dimensional map of the precipitating electron fluxes while in-situ measurements with spacecraft only gives one-dimensional solutions along its trajectory.

The precipitating fluxes at 260 km are also used as boundary conditions in a magnetosphere-ionosphere coupling model developed at BIRA-IASB. The plasma properties of the magnetospheric generator, the source of the precipitating electrons, can then be indirectly retrieved using an optimization procedure.

The methodology was successfully tested and is able to retrieve the properties of a distant magnetospheric interface (at ~24 000 km altitude) feeding the energy necessary to ignite the polar lights observed by ALIS. These analyses will be extremely useful to reconstruct the precipitating electron fluxes in near-real time with the new ALIS_4D network and, to produce synthetic UV emissions useful for future observations onboard the ESA/CAS mission SMILE (Solar Wind Magnetosphere Ionosphere Link Explorer).



This research was carried out in the frame of MOMA, a project funded by the research program BRAIN-Be during the period 2016-2020.

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Figure 2 caption (legend)
Volume Emission Rate (VER) reconstruction for the blue emission at 427.8 nm using a Tomography Technique with the ALIS data for an auroral arc. Altitude projection at 110 km altitude in the longitudinal-latitudinal plan (left) and latitudinal projection versus altitude in the plane passing through the UHF EISCAT radar located nearby (right).
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Figure 3 caption (legend)
The method selects from an ensemble of theoretical auroral arc configurations the one that fits best the observations (e.g. from ALIS or in-situ from spacecraft). Then, using the magnetosphere – ionosphere coupling model developed at BIRA-IASB, the type of magnetospheric generator able to produce that particular arc is produced. The figure shows several examples from the collection of theoretical auroral arc configurations illustrating, from top to bottom, the energy flux carried by the precipitating auroral electrons, the accelerating electric potential, the current carried by the electrons and the ionospheric perturbation of the auroral electric conductivity. Also shown are in-situ measurement by DMSP F15 satellite.
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