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[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.

Citation:Fallows, K., P. Withers, and M. Matta Here we introduce an automated and repeatable method for determining properties of the M1 and M2 layers simultaneously in 5600 Mars Global Surveyor radio occultation profiles of dayside electron density. The results support previous findings for M1 and M2 subsolar peak densities and the dependence of peak densities on solar zenith angle. The ratio of M1 peak density to M2 peak density remains constant at 0.4 for 70• , in contrast with previous numerical simulations. The M1 altitude increases with solar zenith angle with a lengthscale, L 1 = 2.5 km, which is about half that of the M2 layer, L 2 = 5.2 km, indicating that the two layers become increasingly separated at high solar zenith angles. The vertical width of the M1 layer, H 1 , decreases from 7 km to 5 km as solar zenith angle increases from 70• to 90• , whereas the vertical width of the M2 layer, H 2 , increases from 10 km to 14 km. The prediction of ideal Chapman theory that both the widths H i and the lengthscales L i equal the neutral scale height is not supported by observations. These findings provide meaningful observational constraints for numerical models, which are known to have trouble reproducing observations and observed trends associated with the M1 layer.

A one‐dimensional model of the Martian ionosphere is used to explore the importance of atomic and molecular hydrogen chemistry in the upper atmosphere and ionosphere. Neutral and ionized H and H2 undergo chemical reactions that lead to the production of the hydrogenated ions: H+, H2+, H3+, OH+, HCO+, ArH+, N2H+, HCO2+, and HOC+. Simulations are conducted for the cases of photochemistry only and photochemistry coupled with transport in order to asses the separate effects of plasma diffusion in the topside ionosphere. For both of these cases, the sensitivity of the ionosphere is tested for (1) molecular hydrogen abundance and (2) reaction rate, k1, for the charge exchange between H+ and H2. Results are reported for midday solar minimum conditions. We find that the ionospheric composition of Mars is sensitive to H2 abundance, but relatively insensitive to the reaction rate, k1. Depending on the conditions simulated, the topside ionosphere can contain appreciable amounts of hydrogenated species such as H3+, OH+, and HCO+. Comparisons are made with Viking ion density measurements as well as with results of other published Mars ionospheric models. Future comparisons with more extensive ion composition will be available when the Mars Atmosphere and Volatile Evolution mission arrives at Mars.

The spatially inhomogeneous, small-scale crustal magnetic fields of Mars influence the escape of planetary atmospheric species and the interaction of the solar wind with the ionosphere. Understanding the plasma response to crustal magnetic field regions can therefore provide insight to ionospheric structure and dynamics. To date, several localized spatial structures in ionospheric properties that have been observed over regions of varying magnetic field at Mars have yet to be explained. In this study, a two-dimensional ionospheric model is used to simulate the effects of field-aligned plasma transport in regions of strong crustal magnetic fields. Resulting spatial and diurnal plasma distributions are analyzed and found to agree with observations from several spacecraft and offer compelling interpretations for many of the anomalous ionospheric behaviors observed at or near regions of strong crustal magnetic fields on Mars.