Quantifying the Electron Energy of Mars Aurorae Through the Oxygen Emission Brightness Ratio at 130.4 and 135.6 nm

Abstract

Mars discrete aurorae are caused by accelerated electrons precipitating into the atmosphere and interacting with species such as atomic oxygen. However, the energy of the electrons causing these aurorae remains currently unclear: no simultaneous and concurrent measurements of electron analyzers and spectrometers have been performed so far, preventing from assessing the exact energy of the downgoing auroral electrons. Several auroral emissions have been observed so far on Mars, among which are two oxygen emissions in the far ultraviolet at 130.4 and 135.6 nm. In this study, we simulate the vertical distribution of these auroral oxygen emissions with an electron transport calculation coupled with a radiative transfer model to account for the optical thickness of the atmosphere for the 130.4‐nm triplet. We show that the brightness ratio of these oxygen emissions is independent of the downward electron energy flux and only slightly depends on the atomic oxygen atmospheric composition. In contrast, the brightness ratio is strongly related to the initial energy of the auroral electrons. Measuring the brightness ratio is therefore a unique tool to remotely estimate the energy of the electrons causing the Mars discrete aurorae. We compare our model results with observations from the Emirates Mars Ultraviolet Spectrometer on board the Emirates Mars Mission and find that electrons with typical energies of 250–300 eV are compatible with the observed ratio of 5.

EMM EMUS Observations of FUV Aurora on Mars: Dependence on Magnetic Topology, Local Time, and Season

“…The oxygen emission lines at 130.4 and 135.6 nm among others, and carbon monoxide (CO) emission lines including the Fourth Positive Group (CO 4PG) band emission are prominent among the detected extreme/far ultraviolet (EUV/FUV) auroral emission features. This is expected for electron impact on O, CO 2 , and CO resulting in a range of excited states of O and CO (Soret et al, 2024). Lillis et al (2022) classified the auroral features captured by EMUS as crustal field aurora in regions of strong crustal magnetic fields (e.g., Figure 1a), non-crustal field sinuous aurora which are elongated, filamentary emissions usually formed away from strong crustal fields (e.g., Figure 1b), and non-crustal field patchy aurora in spatially extended weak crustal field regions often with less defined edges (e.g., Figure 1c).…”

“…Non-crustal field aurorae additionally shows a secondary peak near L s 30°. This is related to the solar EUV photoionizing irradiance and CO 2 photoionization frequency (proxies for solar activity and photoelectron production) peaking near perihelion, as well as the seasonal variability of thermospheric atomic oxygen abundance (which is the main source of OI 130.4 nm auroral emission) (Sánchez-Cano et al, 2018;Soret et al, 2024). Auroral occurrence also shows an increase with increasing solar activity due to the rise of Solar Cycle 25.…”

“…This further strengthens the assumption that ionospheric photoelectrons and thermospheric atomic O density are important in determining the auroral occurrence in non-crustal field regions. Recent modeling study by Soret et al (2024) found that the main source of both OI 130.4 nm and OI 135.6 nm oxygen auroral emissions in the Martian atmosphere is electron impact on atomic oxygen over a wide range of initial electron energies. They used MCD atomic oxygen profiles from different seasons for the simulations, and the highest density was obtained for L s 45°, matching with our observations (Figure 12c), as well as the previous study by Sánchez-Cano et al (2018).…”

“…(Adams et al, 2018;Brain et al, 2020;Fang et al, 2022;Weber et al, 2017Weber et al, , 2020Xu et al, 2017). The auroral electron energy distribution can be better constrained using the ratio of oxygen auroral emissions OI 130.4 nm and OI 135.6 nm (Fox & Stewart, 1991;Gérard et al, 2008;Soret et al, 2024). Future detailed studies are required to understand the effect of atomic O abundance, O/CO 2 ratio, O volume mixing ratio, solar EUV flux and CO 2 ionization frequency on the FUV auroral occurrence as well as the auroral brightness.…”

“…Mars' nightside aurora (a.k.a. electron discrete aurora), first identified by the SPICAM (SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars) ultraviolet spectrometer on Mars Express (Bertaux et al, 2005(Bertaux et al, , 2006Leblanc et al, 2006;Soret et al, 2016), are spatially confined photon emissions in Mars' nightside upper atmosphere, generated by the decay of electronic states in atoms and molecules primarily excited by suprathermal electrons (energies greater than ∼9 eV; Soret et al (2024)). These discrete aurorae (a total of 19 detections by SPICAM: 16 in nadir mode and 3 in limb mode), distinguishable by their small spatial scales and association with strong vertical crustal magnetic fields, differ significantly from other Mars auroral types (González-Galindo et al, 2024;Haider et al, 2022): diffuse aurora, caused by the global precipitation of solar energetic particles (Gérard et al, 2017;Nakamura et al, 2022;Schneider et al, 2015Schneider et al, , 2018, and proton aurora, resulting from solar wind protons directly entering the upper atmosphere after charge exchange with neutral hydrogen in the corona (Chaffin et al, 2022;Deighan et al, 2018;Gérard et al, 2019;Hughes et al, 2019;Ritter et al, 2018).…”