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Effective electron collision frequencies, ν, in the ionosphere’s D-region are usually derived from some kind of radio wave experiment. Hence, ν is not measured, but rather deduced from the effect it has upon radio waves. This interaction between collision frequency and wave propagation is described by a magneto-ionic theory and the complex refractive indices expressed by it. The most general formulation commonly used today is the one derived by Sen and Wyller (1960) which is based on laboratory measurements by Phelps and Pack (1959). They found a linear relationship between electron energy and collision frequency for nitrogen, the most abundant gas in the D-region. This linearity which was later also found for O2, the second most abundant constituent, implies as a further consequence a proportionality of the pressure, p, and the collision frequency, ν.