Delayed generation of an electromagnetic pulse in the topside ionosphere

High-latitude, topside, spread-F ionograms often show a trace protruding from the spread F of the extraordinary wave trace at frequencies slightly greater than twice the electron cyclotron frequency at the height of the satellite. The characteristics of this trace, called the remote-resonance trace, can be explained by the following model: a transmitted pulse of angular frequency ω propagates downward, along a magnetic field-aligned wave guide, through the height at which ω is equal to twice the local electron cyclotron frequency 2ωH. A gradient in the electric field of the transmitted wave results from the variations in electron concentration across the guide. Owing to this gradient, electrons gyrating in the magnetic field with certain phases can gain energy from the wave, whereas electrons with other phases can lose energy to the wave. The electrons that gain the largest amount of energy from the wave have, immediately following the passage of the downward-propagating wave, a particular phase distribution with height. This distribution changes with time in such a way that at some time τ0 after the passage of the transmitted pulse the distribution corresponds to that which would be produced by an upward-propagating wave. Owing to variations in the electric field of the downward-propagating wave (or in the electron concentration or temperature) electric currents of frequency 2ωH can flow in the medium and, at the time τ0, an upward-propagating pulse can be generated that is responsible for producing the remote-resonance trace on ionograms. This model successfully accounts for the time delay of the remote-resonance traces observed on Alouette ionograms. Energy considerations indicate that variations in the electric field of the downward-propagating wave of several per cent within a distance of one electron cyclotron radius are required to explain the strength of the remote-resonance echoes. Consideration is given to the parameters required for the observation of remote-resonance echoes from laboratory plasmas.