Gradient Instabilities as Possible Causes of Irregularities in the Ionosphere

In this investigation a theory is proposed to explain the production of certain irregularities in the ionosphere. It is found that the region of the ionosphere below the peak of the electron distribution may become unstable because of the joint effect of the gravitational field and the positive electron density gradient there. The unstable modes are found to correspond to quasi‐transverse modes. For the case when only plasma is considered, the condition for instability to occur is gρ′p/v‐P > ξ, where g is the gravitational acceleration, ρp the plasma density, ρ′p its height gradient, v the ion‐neutral collisional frequency, and ξ the effective attachment coefficient. When the coupling between plasma and neutral particles is taken into account, an additional instability condition may occur when the neutral particle distribution has a positive density gradient. This quasi‐transverse unstable mode is proposed as a possible causative mechanism for the formation of irregularities in the ionosphere. When either one of the instability criteria is satisfied, spontaneous generation of irregularities is possible. Some experimental evidence is presented and discussed.