Short-wave fadeout on mars: Radio absorption in the dayside martian ionosphere enhanced by solar flares

Solar flares can cause radio absorption in the D region of the Earth's ionosphere and consequently interrupt high-frequency radio communication systems, also known as short-wave fadeout or the Dellinger effect. We present an analogous radio absorption event observed at Mars during a solar flare. In this event, the Mars Express Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument fortuitously operated at low altitudes on the dayside around the terminator in a favorable configuration for surface echo measurements at the flare peak. The surface echo power during the flare is abnormally weak compared to nominal echo powers at corresponding solar zenith angles, suggesting flare-induced radio absorption in the dayside lower ionosphere of Mars. Additionally, long-term MARSIS data statistically demonstrate the radio absorption dependence on solar soft X-ray fluxes. Our results point to the need for Martian space weather prediction including ionospheric effects on radio waves.