Santolík, O., Kolmašová, I., Pickett, J. S., & Gurnett, D. A. (2021). Multi-point Observation of Hiss Emerging from Lightning Whistlers. Journal of Geophysical Research: Space Physics, 126, e2021JA029524. https://doi.org/10.1029/2021JA029524

Due to the lower ionospheric thermal pressure and existence of the crustal magnetism at Mars, the Martian ionopause is expected to behave differently from the ionopause at Venus. We study the solar wind interaction and pressure balance at the ionopause of Mars using both in situ and remote sounding measurements from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on the Mars Express orbiter. We show that the magnetic pressure usually dominates the thermal pressure to hold off the solar wind in the ionopause at Mars, with only 13% unmagnetized ionopauses observed over a 11-year period. We also find that the ionopause altitude decreases as the normal component of the solar wind dynamic pressure increases. Moreover, our results show that the ionopause thickness at Mars is mainly determined by the ion gyromotion and equivalent to about 5.7 ion gyroradii.

Cassini plasma wave and charged particle observations are combined with magnetometer measurements to investigate Titan’s induced magnetosphere. Electric field emissions close to Titan are identified as upper hybrid resonance emissions, which provide a density estimate of Titan’s cold plasma. These observations have been combined with electron spectrometer measurements to build an integrated map of electron density in Titan’s near environment using observations from TA to T82 flybys, ie which includes flybys from the Cassini prime, equinox and part of the soltice mission. We identify a dense ionospheric region and an extended plasma wake with values ranging between 10-2 and 103 cm-3. Upstream of the induced magnetosphere, the presence of pickup ions in the positive hemisphere of the kronian plasma convective electric field are detected. The mass of the observed pickup corresponds to methane group ions, N2+ and HCNH+ ions as well as Titan’s protons and molecular hydrogen ions. These ions are progressively accelerated by the kronian background electric field and we estimate its intensity by reconstructing the energization of this population. We find values on the order of 0.7 mV/m , consistent with an average estimate of 0.61 mV/m deduced from ∼ |VxB| computation.

In October 2014, the close encounter between Mars and comet Siding Spring produced a transient ionized layer in the upper atmosphere composed primarily of Mg⁺ ions. The layer was detected by instruments on three spacecraft, including the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on Mars Express. Analyses of the MARSIS data indicated the transient layer persisted up to ~19 hours after the comet’s closest approach. We report MARSIS observations that suggest the transient layer lasted at least 7 days – and potentially as long as 32 days – after closest approach. During this period, the transient layer was mostly confined to a narrow latitude range between 20°N-60°N and a longitude range spanning 275°E to 95°E. Since this period coincided with a highly active Sun, we discuss how solar flares may have contributed to the layer’s prolonged lifetime.