Radar Blackouts at Mars and Solar Energetic Particles: The Low Altitude Ionisation Layer

We present observations of a low-altitude nightside ionisation layer in the Martian ionosphere during a period of Solar Energetic Particle induced radar blackout.  The interval, in December 2014, has an unprecedented coverage of data from the MARSIS Advanced Ionospheric Sounder (AIS) onboard Mars Express.  The orbit of Mars Express is ideal during this interval as it has periapsis close to the terminator and spends much of the AIS observational period on the nightside, where the layer is observed to form.  During the SEP period, the surface return signal is lost as a result of absorption of the radar signal along its path to and from the surface.  This absorption appears to have been caused by a low-altitude layer of ionisation created by the SEP event.  The AIS observed an ionospheric trace, unusual for the nightside, which we interpret as evidence for the low-altitude layer.  We demonstrate the variation of the peak electron density of this layer as a function of time and the SEP electron and ion energy flux.  We conclude that although the ions may have played a role in the creation of this layer, the electrons are most likely the major cause.