Generation of ionospheric irregularities associated with plasma lines induced by an HF wave

The magnetic field‐aligned ionization irregularities or ducts, in which the electrostatic waves exist that are responsible for the plasma lines induced by the HF heater wave, are small in diameter compared to the spatial separation of these ducts. There is reason to doubt that these ducts are formed by the thermal self‐focusing instability. A model is proposed for duct formation, in which a duct is analogous to a receiving antenna. Energy is extracted from the incident HF field over the ‘effective area’ of the duct, which is much greater than the cross‐sectional area of the duct itself. The separation of individual ducts is then determined by this effective area.