Transient layers in the topside ionosphere of Mars

Radar soundings from the MARSIS instrument on board the Mars Express spacecraft have shown that distinct layers can occur in the topside ionosphere of Mars, well above the main photo-ionization layer. These layers appear as cusps, or sometimes steps, in plots of the time delay as a function of frequency. Usually only one topside layer is observed, typically at altitudes from 180 to 240 km. However, occasionally an additional layer occurs at even higher altitudes. The layers are transient features and are present about 60% of the time near the subsolar point, decreasing with increasing solar zenith angle to less than 5% at the terminator and the nightside. The transient nature of the layers suggests that they are produced by a dynamical process, most likely involving an interaction with the solar wind in the upper levels of the ionosphere.