Effect of ionospheric self‐conditioning and preconditioning on the broad upshifted maximum component of stimulated electromagnetic emission

Abstract

Stimulated electromagnetic emissions (SEE) are high‐frequency radio emissions that are generated during high‐power, high‐frequency, ordinary mode (O‐mode), radiowave heating of the ionosphere. These emissions are particularly useful in ionospheric heating experiments because they provide a way of monitoring space plasma processes remotely and passively. In order to utilize these emissions for diagnostic purposes, it is necessary to understand the space plasma processes involved in their generation. The purpose of these experiments was to observe the responses of a particular component of the SEE, the broad upshifted maximum (BUM), to a variety of heating stimuli in an effort to understand the factors involved in its development. Heating experiments were conducted at the Radiophysical Research Institute SURA Ionospheric Modification Facility in Russia. Experiments consisted of single‐pump, two‐pump, and single‐pump power‐stepping experiments. The single‐pump and two‐pump transmissions were organized into groups of pulses of varying widths and spacings to facilitate the investigation of self‐conditioning, preconditioning, and two‐pump‐interaction conditioning effects. The major findings of these experiments are that the action of a pump can have a conditioning effect on the medium that affects the time development of the BUM. The result of the conditioning process is the formation of an overshoot in the temporal development of the BUM. A residual conditioning effect is sustained after the end of a pump pulse for a period of time (∼30 s). The residual conditioning acts as preconditioning for the BUM of a subsequent pump pulse. A second O‐mode pump (pump2), at a frequency a few hundred kilohertz above that of the first pump (pump1), is observed to cause additional suppression of the pump1 BUM, implying an enhanced conditioning effect. Time constants for the buildup and decay of the conditioning effects are estimated. During power‐stepping experiments, the BUM spectrum was observed to evolve from a weak, narrow spectrum at an effective radiated power (ERP) of ≈5 MW, to a strong, broad spectrum with a ramp‐like spectral tail at an ERP of ≈150 MW. Other features noted during power stepping include (1) strong BUM transients at pump power level transitions, (2) BUM amplitude asymmetry between power‐up and power‐down sides of a power stepping cycle, (3) reduction of the BUM spectral peak offset from the pump frequency with increasing pump power, and (4) power law dependence of BUM power on pump power (exponent ∼2). Results of these experiments are used in an attempt to assess the relevance of small‐scale irregularity generation and electron heating mechanisms to the observed conditioning effects.