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Discovery of a proton aurora at Mars


Proton aurorae are a distinct class of auroral phenomena caused by energetic protons precipitating into a planetary atmosphere. The defining observational signature is atomic hydrogen emissions from the precipitating particles after they obtain an electron from the neutral atmospheric gas, a process known as charge exchange. Until now, proton aurorae have been observed at Earth only. Here, we present evidence of auroral activity driven by precipitating protons at Mars, using observations by the MAVEN spacecraft. We observed transient brightening of upper atmospheric hydrogen Lyman-α emission across the Martian dayside correlated with solar wind activity. The driving mechanism is one not found at Earth and originates from energetic neutral atom production by solar wind protons directly interacting with the extended hydrogen corona surrounding Mars. We characterize this new type of Martian aurora and compare the observed emissions with preliminary modelling guided by simultaneous in situ particle measurements. These observations provide insights into how the solar wind can directly deposit energy into the Martian atmosphere as well as all other planetary objects that are surrounded by a substantial neutral corona exposed to the solar wind.

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Fig. 1: Mechanism for Martian proton aurorae originating from solar wind charge exchange.
Fig. 2: Limb observations showing the signature of a proton aurora in atomic hydrogen emission.
Fig. 3: Timeline of SWIA penetrating protons and IUVS Lyα enhancements in March–April 2015.
Fig. 4: Observed and modelled Lyα brightness profiles.

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The MAVEN mission is supported by NASA through the Mars Exploration Program. J.-Y.C., F.M. and F.L. are funded by the programme ‘Systeme Solaire’ of Centre National d’Etudes Spatiales. A.S. is supported by the Belgian Fund for Scientific Research (FNRS).

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J.D., S.K.J. and M.S.C. performed the analysis. X.F. created and ran the precipitating particle transport model used here. J.S.H. provided SWIA measurements and guidance in using them. J.S.E. provided neutral atmosphere airglow retrievals. All authors contributed to the development of the instrument pipeline and/or data acquisition as well as interpretation and presentation of these results.

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Correspondence to J. Deighan.

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Deighan, J., Jain, S.K., Chaffin, M.S. et al. Discovery of a proton aurora at Mars. Nat Astron 2, 802–807 (2018).

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