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

Stimulated electromagnetic emissions (SEE) are high-frequency radio emissio ns that are generated during high-power, high-frequency, ordinary mode (O-m ode), radiowave heating of the ionosphere. These: emissions are particularl y useful in ionospheric heating experiments because they provide a way of m onitoring space plasma processes remotely and passively. In order to utiliz e these emissions for diagnostic purposes, it is necessary to understand th e 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 ex periments were conducted at the Radiophysical Research Institute SURA Ionos pheric Modification Facility in Russia. Experiments consisted of single-pum p, two-pump, and single-pump power-stepping experiments. The single-pump an d two-pump transmissions were organized into groups of pulses of varying wi dths and spacings to facilitate the investigation of self-conditioning, pre conditioning, and two-pump-interaction conditioning effects. The major find ings of these experiments are that the action of a pump can have a conditio ning 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 sustain ed after the end of a pump pulse for a period of time (similar to 30 s). Th e 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 kil ohertz above that of the first pump (pump1), is observed to cause additiona l suppression of the pump1 BUM, implying an enhanced conditioning effect. T ime constants for the buildup and decay of the conditioning effects are est imated. During power-stepping experiments, the BUM spectrum was observed to evolve from a weak, narrow spectrum at an effective radiated power (ERP) o f approximate to 5 MW, to a strong, broad spectrum with a ramp-like spectra l tail at an ERP of approximate to 150 MW. Other features noted during powe r stepping include (1) strong BUM transients at pump power level transition s, (2) BUM amplitude asymmetry between power-up and power-down sides of a p ower 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 similar to 2). Results of these experime nts are used in an attempt to assess the relevance of small-scale irregular ity generation and electron heating mechanisms to the observed conditioning effects.