LOW FREQUENCY PLASMA INSTABILITIES IN IONOSPHERE

The possibility of plasma instabilities in the ionosphere is investigated. Low frequency instabilities due to a background vertical gradient of ionization are considered, and formal solutions for plane wave irregularities of arbitrary direction are given. The growth of irregularities due to electron and ion drift is investigated and is shown to be a maximum for irregularities which are not quite aligned along the magnetic field. The diffusion and velocity of small scale irregularities are not very dependent on the background gradients, allowing simplification of the analysis. It is shown that diffusion cannot be described in terms of a tensor, and that whilst the velocity of field-aligned irregularities is almost equal to the electron drift velocity, that of irregularities far from field aligned is almost equal to the ion drift velocity. Boundary conditions between layers are discussed. For a region consisting of two exponential layers with a maximum electron density at the boundary, it is shown that thin horizontal irregularities (which could be interpreted as being sporadic-E) cannot be the result of this instability. It is also shown that if the layer of interest (with an exponentially increasing background ionization density) is bounded by a conducting ionosphere, the diffusion rate is increased and would usually prevent growth of the instability. © 1968 Pergamon Press.