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