Discrete aurora and the nightside ionosphere of Mars: an EMM–MEX conjunction of FUV imaging, ionospheric radar sounding, and suprathermal electron measurements

Abstract

Since 2021, a new surge in discrete aurora detections at Mars has been observed by the Emirates Mars Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission (EMM) Hope Orbiter as EMUS started to regularly obtain synoptic auroral images with a high sensitivity. Here we report on a fortuitous conjunction between EMM and Mars Express (MEX) using far ultraviolet (FUV) imaging of discrete aurora by EMM EMUS, in situ measurements of suprathermal electrons by the MEX Analyzer of Space Plasma and Energetic Atoms Electron Spectrometer (ELS), and topside radar sounding of the nightside ionosphere by the MEX Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS). In this event, EMM EMUS imaged a clear discrete aurora signature around moderately strong crustal magnetic fields on the nightside near the dusk terminator, 11 min before which MEX MARSIS measured a prominent local enhancement of the peak electron density in the nightside ionosphere and MEX ELS observed an in situ enhancement of suprathermal electrons at the corresponding location. A remarkable geographic agreement is found between the enhancements of the aurora, ionosphere, and suprathermal electrons, suggesting that the enhanced ionization and auroral emission are caused concurrently by precipitating suprathermal electrons. Subsequent images indicate that the discrete aurora slightly changed its shape in 15 min and mostly disappeared in a few hours. The MEX MARSIS measurements of the auroral ionosphere display overlapping ionospheric and surface echoes indicative of horizontal gradients of the peak electron density. Analysis of the overlapping echoes implies that the auroral ionosphere and electron precipitation could be highly structured with horizontal spatial scales on the order of several tens of km. MEX MARSIS also observed a non-auroral ionospheric enhancement with a wider spatial extent than the local auroral enhancement, suggesting alternative sources of the enhanced nightside ionosphere such as plasma transport. The comparison between the ionospheric structures measured by MEX MARSIS, suprathermal electron flux measured by MEX ELS, and discrete auroral emission imaged by EMM EMUS underscores the complexity of the auroral and non-auroral nightside ionospheres. This motivates further investigations of their sources, transport, and connections to the magnetotail dynamics of Mars.

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

“…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). Note that all three types of aurora can appear simultaneously at different locations on the nightside (Atri et al, 2022;Harada et al, 2024;Lillis et al, 2022;Pacios et al, 2024). However, these studies used limited data sets and did not provide in-depth analyses of Mars' FUV aurora, its variability, and drivers.…”

“…Quasi-static field-aligned potential could be one mechanism that accelerate electrons and ions (Brain & Halekas, 2012;Lundin, Winningham, Barabash, Frahm, Brain, et al, 2006). These accelerated down-going electrons encounter the atmosphere and lead to both enhanced ionization and auroral emission (Fillingim et al, 2007;Harada et al, 2024;Lillis & Brain, 2013;Lillis et al, 2011Lillis et al, , 2018. Leblanc et al (2006) simulated the emission rate of this aurora due to electron precipitation, and suggested that the measured emissions might have been produced by electrons of a few tens of eV rather than by the electrons of above 100 eV.…”

“…Furthermore, the mechanisms for particle precipitation and aurora formation require detailed exploration (Bowers et al, 2023;Brain & Halekas, 2012;Haider et al, 2022). The consequences of aurora, such as enhanced ionization (creating a "patchy" ionosphere), Joule heating of the upper atmosphere, changes in chemical composition, and enhanced ion escape via auroral flux tubes, need further investigation (Brain & Halekas, 2012;Fillingim et al, 2007;Harada et al, 2024;Shematovich et al, 2017).…”