[1] Warming of the martian lower thermosphere (100-130 km) at north polar latitudes near the perihelion/winter solstice (Ls = 270) was recently observed. No analogous warming has been observed within the south polar thermosphere during its aphelion/winter season (Ls $ 90). Detailed global model simulations are required to investigate the physical processes driving these seasonal variations. New simulations are conducted for conditions approximating the atmosphere during these Mars Global Surveyor (MGS) and Odyssey (ODY) aerobraking periods. Strong northern winter polar warming features are calculated near 120 km, yielding nightside mean temperatures 10 -15 K warmer than observed ODY values. No southern winter polar warming trend is simulated; however, nightside mean temperatures are 20-30 K warmer than observed by MGS. The stronger interhemispheric circulation during northern winter is clearly driven by stronger insolation and dust heating near perihelion, resulting in subsidence and warmer temperatures in the northern polar night.

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To study the transport of the ionospheric plasma on Mars, we have included a 3-D multifluid dynamical core in a Martian general circulation model. Vertical transport modifies the ion density abovẽ 160 km on the dayside, especially the ions produced at high altitudes like O + , N + , and C + . Near the exobase, the dayside to nightside flow velocity reaches few hundreds of m/s, due to a large horizontal pressure gradient. Comparison with Mars Express/Analyzer of Space Plasmas and Energetic Atoms-3 measurements between 290 and 500 km suggests that this flow could account for at least 20% of the flow produced by the solar wind. This flow is not sufficient to populate substantially the nightside ionosphere at high altitudes, in agreement with recent observations, because of a strong nightside downward flow produced by vertical pressure gradient. The O 2 + and NO + ion densities on the nightside at low altitudes (~130 km) are modified by this downward flow, compared to simulated densities without ion dynamics, while other ions are lost by chemical reactions. Variability at different time scales (diurnal, seasonal, and solar cycles) are studied. We simulate diurnal and seasonal variations of the ionospheric composition due to the variability of the neutral atmosphere and solar flux at the top of the atmosphere. The ionospheric dynamics are not strongly affected by seasons and solar cycles, and the retroaction of the ionosphere on the neutral atmosphere temperature and velocity is negligible compared to other physical processes below the exobase.

Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright 'dipole' feature surrounded by a cold 'collar' at its north pole. The polar dipole is a 'double-eye' feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus' south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50 km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.

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