Hook structure in MARSIS ionogram and its interpretation

MARSIS/MEX is a low frequency radar used to sound the ionosphere of Mars. In general the echo trace associated with reflections in nadir indicates an ionosphere with a main maximum electron density at typically about 130 km altitude, and a monotonously decreasing density with increasing altitude. Another maximum is sometimes detected at higher altitudes, indicating a double layer in the upper ionosphere. Sometimes a special structure (here called a hook'') is observed at the low frequency end of the trace: as the sounding frequency decreases the delay time of sounding signal increases rapidly. It is here proposed that this signature indicates a secondary maximum with density which is so low that it can be only partly resolved by the radar. An analytical inversion approach coping with the hook'' case is then proposed. It is further suggested that the hook'' is caused by vertical wavelike structures in the Martian topside ionosphere. Citation: Wang, X.-D., J.-S. Wang, E. Nielsen, and H. Zou (2009), Hook'' structure in MARSIS ionogram and its interpretation, Geophys. Res. Lett., 36, L13103, doi:10.1029/2009GL038844.