Characteristics of the Martian discrete auroral emissions observed with MAVEN-IUVS

Three types of aurorae have been observed in the Martian atmosphere: the discrete aurora, the diffuse aurora (Schneider et al., 2015) and, more recently, the proton aurora (Deighan et al., 2018, Ritter et al., 2018). We focus here on the discrete aurora, which was discovered with the SPICAM instrument on board the ESA Mars Express spacecraft (Bertaux et al., 2005).

Discrete aurorae are generated by the precipitation of energetic electrons. They have been shown to be very localized in space, time and altitude (Leblanc et al., 2008, Gérard et al., 2015, Soret et al., 2016).

Their signature is characterized by the presence of several emissions in its spectrum: the CO (a³Π–X¹Σ) Cameron bands between 190 and 270 nm, the CO (A¹Π–X¹Σ⁺) Fourth Positive system (CO 4P) between 135 and 170 nm, the (B²Σ_u⁺–X²Π_g) doublet at 289 nm, the OI at 297.2 nm and the 130.4 nm OI triplet emissions (see figure 1).

 

Figure 1: Spectrum observed during a discrete auroral event with the SPICAM instrument on board the ESA Mars Express spacecraft (Bertaux et al., 2005).

 

More recently, the discrete aurora was also observed by the MAVEN-IUVS ultraviolet spectrograph. Schneider et al. (2019) confirmed that most of the discrete auroral observations occurred in the southern hemisphere, near the open-closed magnetic field line boundary in cusp-like structures, as shown by Leblanc et al. (2008) and Gérard et al. (2015).

In this study, the auroral emissions have been quantified and their altitudes have been estimated using limb data from the IUVS instrument. More than 10 000 orbits from 2014 to 2020 have been analyzed.

Each individual spectrum acquired by MAVEN-IUVS was automatically compared to a reference auroral spectrum. Only the spectra with a high correlation coefficient were retained for a manual confirmation.

Auroral signatures were thus found in more than 50 different orbits.

Synthetic spectra of the emissions produced during an aurora have then been scaled to the observed spectra in order to quantify their intensities. The altitudes of tangent points were also retrieved from the limb observations.

We confirm previous finding from Mars Express (Bertaux et al., 2005, Soret et al., 2016): in discrete aurorae, the CO Cameron bands emission layer is located between 105 and 165 km.

We also find that the CO Cameron bands intensity varies linearly with the CO₂⁺ UVD intensity.

These results are of a great importance to model and understand the production mechanisms of a discrete aurora on Mars. The MAVEN Solar Wind Electron Analyzer (SWEA) measurements can be used to estimate the electron energy needed to produce a discrete auroral event.

 

References:

 

Bertaux J.-L. et al., 2005, Discovery of an aurora on Mars, Nature 435, 790–794, https://doi.org/10.1038/nature03603

 

Deighan J. et al., 2018, Discovery of a proton aurora at Mars, Nature Astronomy, vol. 2, 802-807, https://doi.org/10.1038/s41550-018-0538-5

 

Gérard J.-C. et al., 2015, Concurrent observations of ultraviolet aurora and energetic electron precipitation with Mars Express, J. Geophys. Res. Space Physics, 120,6749–6765, https://doi.org/10.1002/2015JA021150

 

Leblanc F. et al., 2008, Observations of aurorae by SPICAM ultraviolet spectrograph on board Mars Express: Simultaneous ASPERA-3 and MARSIS measurements, J. Geophys. Res., 113, A08311, http://dx.doi.org/10.1029/2008JA013033

 

Ritter B. et al., 2018, Observations of the proton aurora on Mars with SPICAM on board Mars Express, Geophysical Research Letters, 45, 612–619, https://doi.org/10.1002/2017GL076235

 

Schneider N. et al., 2015, Discovery of diffuse aurora on Mars, Science, 350, 1-5, https://doi.org/10.1126/science.aad0313

 

Schneider N. et al., 2019, MAVEN Remote Sensing and In Situ Observations of Discrete Aurora on Mars, AGU Fall meeting, SM42B-03, https://agu.confex.com/agu/fm19/meetingapp.cgi/Paper/506680

 

Soret L. et al., SPICAM observations and modeling of Mars aurorae, 2016, Icarus, 264, 398-406, https://doi.org/10.1016/j.icarus.2015.09.023