The European Space Agency Venus Express Radio Science experiment (VeRa) obtained 118 radio occultation measurements of the Venusian atmosphere between July 2006 and June 2007. Southern latitudes are uniformly sampled; measurements in the northern hemisphere are concentrated near the pole. Radial profiles of neutral number density derived from the occultations cover the altitude range 40–90 km, which are converted to profiles of temperature (T) and pressure (p) versus height (h). Profiles of static stability are found to be latitude‐dependent and nearly adiabatic in the middle cloud region. Below the clouds the stability decreases at high latitudes. At an altitude of 65 km, the VeRa T[p(h)] profiles generally lie between the Venus International Reference Atmosphere (VIRA) and VIRA‐2 models; the retrieved temperatures at any given pressure level typically are within 5 K of those derived from the Pioneer Venus Orbiter Radio Occultation experiments. A large equator‐to‐pole temperature contrast of ∼30 K is found at the 1‐bar (1000 hPa) level. The VeRa observations reveal a distinct cold collar region in the southern hemisphere, complementing that in the north. At the latitudes of the cold collars, the tropopause altitude increases relative to higher and lower latitudes by ≈7 km while the temperature drops roughly 60 K. The observations indicate the existence of a wave number 2 structure poleward of ±75° latitude at altitudes of 62 km.

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The daytime martian ionosphere has been observed as a two-layer structure with electron densities that peak at altitudes between about 110 and 130 kilometers. The Mars Express Orbiter Radio Science Experiment on the European Mars Express spacecraft observed, in 10 out of 120 electron density profiles, a third ionospheric layer at altitude ranges of 65 to 110 kilometers, where electron densities, on average, peaked at 0.8 x 10(10) per cubic meter. Such a layer has been predicted to be permanent and continuous. Its origin has been attributed to ablation of meteors and charge exchange of magnesium and iron. Our observations imply that this layer is present sporadically and locally.

[1] We have analyzed a brief period of same-day observations of the Martian ionosphere using data obtained in December 2004 from the Mars Global Surveyor (MGS) and Mars Express (MEX) radio occultation experiments. These data were taken shortly after sunrise under solstice conditions in both hemispheres, with MGS in the summer (northern) hemisphere at high latitudes while MEX was in the winter (southern) hemisphere at midlatitudes. Such two-satellite, dual-hemisphere data sets are unique for the modern era of ionospheric observations at Mars and provide good test cases for constraints of key parameters commonly used in models of the Martian ionosphere. Several iterations of a 1-dimensional model are developed in attempts to simulate more successfully the altitudes, absolute magnitudes and shapes of the two photo-chemical layers (M1 and M2) obtained during the joint MGS-MEX observing period. Three basic processes are examined: (1) selection of the optimal model neutral atmospheres, (2) the effects due to departures from thermal equilibrium between electrons, ions and neutrals, (3) methods of handling secondary ionization. While general circulation models fully coupled to plasma transport codes are required for global simulations of the full system, the computational complexity and computer resources needed often result in the use of parameterizations relating electron and ion temperatures to neutral temperatures and secondary ionization to primary photo-ionization profiles. Here we develop such schemes and test them within the framework of same day observations in both hemispheres. The occurrence of same day, separate hemisphere, radio occultation profiles is important because the solar irradiance has to be held constant for modeling both sites, and thus this is the first study of this kind to be done. The overall results stress the dominant influence of solar zenith angle effects on production for the M2-layer via primary solar ionization, its augmentation by ∼30% due to secondary ionization, and further enhancements due to reduced chemical loss when the electron temperature exceeds the neutral temperature. Secondary ionization is the most crucial process for the M1-layer. The influence of very different crustal magnetic field morphologies at the two observing locations did not seem to be a crucial source of differentiation for processes that control the average values of the peak electron densities of the two photo-chemical layers.

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[1] The vertical structure of the nightside ionosphere of Mars and its dependence on solar zenith angle are currently poorly determined, as is the importance of two key sources of nightside plasma, electron precipitation and transport of dayside plasma. We examined 37 electron density profiles of the ionosphere of Mars at solar zenith angles of 101 -123obtained by the Mars Express Radio Science Experiment (MaRS) between 18 August and 1 October 2005. In general, solar activity was low during this period, although several solar energetic particle events did occur. The results show that (1) trends in peak electron density and altitude with solar zenith angle are consistent with transport of dayside plasma as an important plasma source up to 115 , but not higher; (2) peak altitudes of around 150 km observed at larger (>115 ) solar zenith angles are consistent with simulated plasma production by electron precipitation; and (3) peak altitudes observed during solar energetic particle events are at 90 km, consistent with accepted models. Solar energetic particle events can be the main source of nightside plasma. These results challenge current models of the nightside ionosphere, including their implications for plasma sources. The total electron content is correlated with peak electron density, requiring explanation. Due to the geographical distribution of this data set (latitudes poleward of 38 N), we do not explore the influence of crustal field strength and direction on the nightside ionosphere.

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