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We have developed a pseudo two-dimensional model of a planetary atmosphere,which takes into account thermochemical kinetics, photochemistry, verticalmixing, and horizontal transport, the latter being modeled as a uniform zonalwind. We have applied the model to the atmospheres of the hot Jupiters HD209458b and HD 189733b. The adopted eddy diffusion coefficients are calculatedby following the behaviour of passive tracers in three-dimensional generalcirculation models, which results in eddy values significantly below previousestimates. We find that the distribution of molecules with altitude andlongitude in the atmospheres of these two hot Jupiters is complex because ofthe interplay of the various physical and chemical processes at work. Much ofthe distribution of molecules is driven by the strong zonal wind and thelimited extent of vertical transport, resulting in an important homogenisationof the chemical composition with longitude. In general, molecular abundancesare quenched horizontally to values typical of the hottest dayside regions, andthus the composition in the cooler nightside regions is highly contaminated bythat of warmer dayside regions. As a consequence, the abundance of methaneremains low, even below the predictions of previous one-dimensional models,which is likely to be in conflict with the high CH4 content inferred fromobservations of the dayside of HD 209458b. Another consequence of the importantlongitudinal homogenisation of the abundances is that the variability of thechemical composition has little effect on the way the emission spectrum ismodified with phase and on the changes in the transmission spectrum from thetransit ingress to the egress, these variations in the spectra being mainly dueto changes in the temperature rather than in the composition between thedifferent sides of the planet.

Pseudo 2D chemical model of hot Jupiter atmospheres: application to HD 209458b and HD 189733b