Irregularities in the upper ionosphere

Abstract

The electron density distribution in ionospheric F-region shows irregularities of dimensions ranging from less than 1 km to more than 1000 km. These have been studied as spread- F, radio star scintillation, auroral structure and other phenomena. A review of the experimental data indicates that at least in the case of spread- F there are two different phenomena with different origins. Low-latitude (⋞20°) spread- F appears to be under the influence of the dynamo ionospheric region. It may be a secondary effect following the formation of a type of night-time sporadic- E. Sporadic- E patches will become electrically polarized in two ways: first as a result of the S Q electric current and second by turbulence in the dynamo region. The polarization field is efficiently transferred along the magnetic field lines to cause differential drifts in F-region. The mechanisms are modifications of those of Martyn and Dagg respectively. High-latitude (⋟40°) spread- F is under the control of the DS electric field, which extends down from magnetospheric motions. These motions originate near the boundary of the geomagnetic field and are transferred to the ionosphere at high latitudes via hydromagnetic “twist” waves. They drive electric currents and drifts, the latter leading to outstanding accumulations of ionization in E-region. These accumulations react violently onto the magnetospheric motions, causing turbulence and creating small-scale irregularities, localized electric fields, precipitation of energetic trapped ions, spread- F and columns of excess electron density at higher levels. A model of a hydromagnetic twist is shown to lead to the formation of a magnetospheric irregularity in the electron density distribution.