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[1] 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. Citation: Kopf,

Broadband whistler‐mode emissions, commonly observed by the Cassini spacecraft at high latitudes in Saturn's magnetosphere at frequencies below about 100 Hz, have characteristics very similar to auroral hiss observed at high latitudes in Earth's magnetosphere. In contrast to terrestrial auroral hiss, which shows no obvious rotational modulation, Saturnian auroral hiss shows a very pronounced rotational modulation. We show that the rotation period of the auroral hiss is different in the northern and southern hemispheres, with a period of about 10.6 hours in the northern hemisphere and about 10.8 hours in the southern hemisphere. To within experimental error the rotation periods in the two hemispheres match the rotation periods of Saturn Kilometric Radiation, an intense radio emission generated along the auroral field lines at frequencies from about 20 to 500 kHz. These north–south asymmetries have potentially important implications on how angular momentum is transferred from the planet to the magnetospheric plasma.

Here we report the discovery of a well‐defined plasma density boundary at high latitudes in Saturn's magnetosphere. The boundary separates a region of relatively high density at L less than about 8 to 15 from a region with densities nearly three orders of magnitude lower at higher L values. Magnetic field measurements show that strong field‐aligned currents, probably associated with the aurora, are located just inside the boundary. Analyses of the anisotropy of energetic electrons show that the magnetic field lines are usually closed inside the boundary and open outside the boundary, although exceptions sometimes occur. The location of the boundary is also modulated at the ∼10.6 to 10.8 hr rotational period of the planet. Many of these characteristics are similar to those predicted by Brice and Ioannidis for the plasmapause at a strongly magnetized, rapidly rotating planet such as Saturn.

We report the detection of a dense ionized layer in the upper atmosphere of Mars caused by the impact of dust from comet Siding Spring. The observations were made by the ionospheric radar sounder on the Mars Express spacecraft during two low‐altitude passes approximately 7 h and 14 h after closest approach of the comet to Mars. During these passes an unusual transient layer of ionization was detected at altitudes of about 80 to 100 km with peak electron densities of (1.5 to 2.5) × 105 cm−3, much higher than normally observed in the Martian ionosphere. From comparisons to previously observed ionization produced by meteors at Earth and Mars, we conclude that the layer was produced by dust from the comet impacting and ionizing the upper atmosphere of Mars.