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Mesospheric water vapour concentration effects on the ion composition andelectron density in the lower ionosphere under quiet geophysical conditionswere examined. Water vapour is an important compound in the mesosphere andthe lower thermosphere that affects ion composition due to hydrogen radicalproduction and consequently modifies the electron number density. Recentlower-ionosphere investigations have primarily concentrated on thegeomagnetic disturbance periods. Meanwhile, studies on the electron densityunder quiet conditions are quite rare. The goal of this study is tocontribute to a better understanding of the ionospheric parameter responsesto water vapour variability in the quiet lower ionosphere. By applying anumerical D region ion chemistry model, we evaluated efficiencies for thechannels forming hydrated cluster ions from the NO+ and O2+primary ions (i.e. NO+.H2O and O2+.H2O,respectively), and the channel forming H+(H2O)n protonhydrates from water clusters at different altitudes using profiles with lowand high water vapour concentrations. Profiles for positive ions, effectiverecombination coefficients and electrons were modelled for three particularcases using electron density measurements obtained during rocket campaigns.It was found that the water vapour concentration variations in the mesosphereaffect the position of both the Cl2+ proton hydrate layer upperborder, comprising the NO+(H2O)n andO2+(H2O)n hydrated cluster ions, and theCl1+ hydrate cluster layer lower border, comprising theH+(H2O)n pure proton hydrates, as well as the numericalcluster densities. The water variations caused large changes in the effectiverecombination coefficient and electron density between altitudes of 75 and87 km. However, the effective recombination coefficient, αeff, and electron number density did not respond even to largewater vapour concentration variations occurring at other altitudes in themesosphere. We determined the water vapour concentration upper limit ataltitudes between 75 and 87 km, beyond which the water vapour concentrationceases to influence the numerical densities of Cl2+ and Cl1+,the effective recombination coefficient and the electron number densityin the summer ionosphere. This water vapour concentration limit correspondsto values found in the H2O-1 profile that was observed in the summermesosphere by the Upper Atmosphere Research Satellite (UARS). The electron density modelled using theH2O-1 profile agreed well with the electron density measured in thesummer ionosphere when the measured profiles did not have sharp gradients.For sharp gradients in electron and positive ion number densities, a waterprofile that can reproduce the characteristic behaviour of the ionosphericparameters should have an inhomogeneous height distribution of water vapour

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Influence of water vapour on the height distribution of positive ions, effective recombination coefficient and ionisation balance in the quiet lower ionosphere