Energetic Particle Showers Over Mars from Comet C/2013 A1 Siding Spring

This paper is a phenomenological description of multi‐spacecraft observations of energetic particles caused by the close flyby of comet C/2013 A1 Siding‐Spring with Mars on 19 October 2014. This is the first time that cometary energetic particles have been observed at Mars. The Mars Atmosphere and Volatile EvolutioN (MAVEN) Solar Energetic Particle (SEP) and the Mars Odyssey High Energy Neutron Detector (HEND) instruments recorded evidence of precipitating particles, that are likely O + pick‐up ions, during the ~10 hours that Mars was within the region of the comet's coma. O + pick‐up ions were also detected several hours after, although whether their origin is the comet or space weather is not conclusive. We discuss the possible origin of those particles, and also, the cause of an additional shower of energetic particles that HEND observed between 22 and 35 h after the comet's closest approach, which may be related to dust impacts from the comet's dust tail. An O + pick‐up ion energy flux simulation is performed with representative solar wind and cometary conditions, together with a simulation of their energy deposition profile in the atmosphere of Mars. Results indicate that the O + pick‐up ion fluxes observed by SEP were deposited in the ionosphere around 105‐120 km altitude, and they are compared with pre‐comet flyby estimations of cometary pick‐up ions. The comet's flyby deposited a significant fluence of energetic particles into Mars' upper atmosphere, at a similar level to a large space weather event.