Origin of the extended Mars radar blackout of September 2017

The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on board Mars Express, which operates between 0.1 and 5.5 MHz, suffered from a complete blackout for 10 days in September 2017 when observing on the nightside (a rare occurrence). Moreover, the Shallow Radar (SHARAD) onboard the Mars Reconnaissance Orbiter, which operates at 20 MHz, also suffered a blackout for 3 days when operating on both day and nightsides. We propose that these blackouts are caused by solar energetic particles (SEP) of few tens of keV and above associated with an extreme space weather event between 10 and 22 September 2017, as recorded by the MAVEN mission. Numerical simulations of energetic electron precipitation predict that a lower O2+ nighttime ionospheric layer of magnitude ~1010 m-3 peaking at ~90 km altitude is produced. Consequently, such a layer would absorb radar signals at HF frequencies and explain the blackouts. The peak absorption level is found to be at 70km altitude.