Introduction Because of the complicated nature of the ionosphere, there have been numerous approaches for ionospheric modeling. In an effort to achieve simplicity, some of the models have been restricted to certain altitude or latitude domains, while others have been restricted to certain ionospheric parameters.The different types of the ionospheric models can be categorized as follows
Empirical models Empirical models are based on ionospheric measurements. Typically, the data are collected over an extended period of time and then fitted with simple analytical and empirical functions. The Klobuchar model is a simple ionospheric model that gives the vertical ionospheric delay at a given time and location for the GPS single-frequency users in real-time. Another example is the NeQuick model that proposed to use as ionospheric correction model for the future European Galileo.
Numerical maps Numerical maps are in the global and regional scales that provide values of a ionospheric parameter in a grid.
Analytical model Analytical models are based on orthogonal function fits to the output obtained from numerical models.
Physical models Physical models are developed based on typically solution of the continuity equation or the momentum and energy equations for the electrons and ions. The solutions of equations provide for instance the electron density either as a function of altitude or along geomagnetic field lines.