Research Article

Synthetic spectra of the aurora: N₂, N₂⁺, N, N⁺, O₂⁺ and O emissions

¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
² Univ. Grenoble Alpes, CSUG, 38000 Grenoble, France
³ Royal Belgian Institute for Space Aeronomy, Avenue Circulaire 3, 1180 Brussels, Belgium

^* Correspondinag author: mathieu.barthelemy@univ-grenoble-alpes.fr

Received: 4 July 2024
Accepted: 2 April 2025

Abstract

Studying the auroral emissions is of great importance since they are created in an atmospheric layer (80–300 km) where in-situ measurements are complicated and they represent a good proxy of the particle precipitations into the atmosphere. The emission spectrum of the aurora is complex, made of both atomic and molecular lines. The intensities of these emissions vary with the solar and geomagnetic activities, mostly due to the particle precipitations. In this paper, we simulate auroral emission spectra for a given distribution of precipitating electrons using Transsolo, a kinetic code solving the transport equation of the electrons along the local magnetic field line. It allows calculations of the particle fluxes for different energies, pitch-angles and altitudes, from which the related emissions are computed. The modules to compute the emissions have recently been updated by including the vibrational structures of the molecular bands and several atomic lines. Only the O₂ emissions and the hydrogen emissions due to proton precipitations are not considered in the model. The code also permits to relate auroral spectra with characteristics of the precipitating electrons such as their mean energy and total fluxes at the top of the atmosphere. Such simulations also provide the auroral spectrum at any altitude of the upper atmosphere, which is important in the perspective of volume emission reconstructions as done using tomographic-like techniques. Moreover, many auroral monitoring instruments are equipped with filters of variable widths. Such synthetic spectra can help to identify possible contamination of the measurements due to overlap of several emission lines. For example, the green line at 557.7 nm overlaps with several O₂⁺, N I and N₂ lines or bands in a ±5 nm range. Calculating the relative ratio of these lines in different conditions is therefore crucial for measurements using a filter of 5 nm width. For example, we discuss if these overlapped lines could possibly be at the origin of the linear polarization measured in the green line despite the fact that the theory of impact polarization predicts zero polarization.