The PI intends to improve basic understanding of the climatology (long term variations) of the F-region ionosphere. He will perform a detailed analysis of the seasonal and solar cycle variations of the ionospheric peak electron density, primarily to determine the reasons for the density variation as a function of solar activity. Better ionospheric models, important for both military and commercial applications, will improve capabilities of over-the-horizon radar. The PI will study causes of the observed density variation with an innovative theoretical ionosphere model, known as the FLIP model, which is capable of utilizing three key ionospheric measurements as constraints.