What do aurorae look like on Mars and Venus? (Invited)

Mars and Venus are our closest neighbors, yet their environments are completely different from Earth’s: their atmospheres are mainly composed of CO2, and they both lack a global internal magnetic field. As a result, aurorae were not expected to occur on these planets. Yet, diffuse aurorae were observed on Venus and a multitude of auroral phenomena have been discovered on Mars.
In 1986, the Pioneer Venus UltraViolet Spectrometer (PVO-UVS) detected ultraviolet auroras on the Venus nightside. The OI 130 and 136 nm emissions were observed but did not show any structure (diffuse aurora). A correlation with interplanetary shocks was observed. Following periods of solar wind intensifications, the [OI] green line at 557.7 nm was also observed in ground-based observations.

The first auroral detection in the Mars atmosphere was an ultraviolet discrete aurora observed with the Mars Express orbiter in a region of strong crustal field in the southern hemisphere. The strongest observed emissions were the CO Cameron bands and the CO2+ UV doublet. The MAVEN/IUVS spectrometer showed that discrete aurorae occur almost everywhere on the planet but the brightest are located near strong crustal field regions and discovered global diffuse aurorae as well as proton aurorae on the dayside of the planet. Recently, EMM/EMUS has acquired global images of the Mars nightside, showing a wide variety of aurorae, including discrete, diffuse, patchy and sinuous aurorae. Each type of aurora bears the signature of the interaction between the solar wind, the induced (or crustal) magnetic field and the atmosphere. Finally, the Perseverance rover acquired the first signature of a visible aurora at 557.7 nm in the Mars nightside.

Observational results have to be coupled with 1D and 3D numerical simulations adapted to the conditions of the observations on Mars and Venus to thoroughly constrain the interconnected physical and chemical processes leading to auroral emissions.

Finally, we discuss the possibility to detect the auroral oxygen green line with TGO/NOMAD/UVIS on Mars and show that global observations with the M-AC visible camera on board the M-MATISSE orbiters will generate considerable progress in our understanding of the morphology, time variations and energetics of the Martian aurora.