Optical emissions excited by high-power radio waves in the ionosphere can be used to measure a wide variety of parameters in the thermosphere. Powerful highfrequency (HF) radio waves produce energetic electrons in the region where the waves reflect in the F region. These hot or suprathermal electrons collide with atomic oxygen atoms to produce localized regions of metastable O(1D) and O(1S) atoms. These metastables subsequently radiate 630.0 and 557.7 nm, respectively, to produce clouds of HF pumped artificial airglow (HPAA). The shapes of the HPAA clouds are determined by the structure of large-scale (•10 km) plasma irregularities that occur naturally or that develop during ionospheric heating. When the HF wave is operated continuously, the motion of the airglow clouds follows the E x B drift of the plasma. When the HF wave is turned off, the airglow clouds decay by collisional quenching and radiation, expand by neutral diffusion, and drift in response to neutral winds. Images of HPAA clouds, obtained using both continuous and stepped radio wave transmissions, are processed to yield the electric fields, neutral wind vectors, and diffusion coefficients in the upper atmosphere. This technique is illustrated using data that were obtained in March 1993 and 1995 at the ionospheric modification facility near Nizhny Novgorod, Russia. Analysis of HPAA clouds yields zonal plasma drifts of 70 m s -• eastward at night. On the basis of artificial airglow from energetic electrons generated at 260 km the zonal neutral wind speed was estimated to be 96 m s -• and the O(•D) diffusion coefficient was determined to be between 0.8 and 1.4 x 10 TM cm 2 s -•. The quenched lifetime of the O(•D) was determined to be 29.4 s. The diffusion and quenching rates are directly related to the atomic and molecular concentrations in the thermosphere. Improvements in the remotesensing technique may be obtained if the intensity of the artificial airglow emissions is increased. High-power radio transmissions employing pulse sequences and tuning near electron cyclotron harmonics were attempted to increase the optical emissions. Both of these, however, produced reduced intensity, and consequently, continuous transmission at frequencies away from electron gyro harmonics is the preferred heating regime.
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.
