| Electromagnetic wave-plasma interaction experiments have been extended from the laboratory to the ionospheric plasma regime. In these experiments high power, high frequency radio waves are transmitted into the ionosphere where a wide range of perturbation are generated. This may include the excitation of electron and ion electrostatic waves by the direct conversion or the parametric decay of the em waves, which may be studied using an incoherent radar. When two em pump waves are transmitted, the frequencies and amplitudes of the electrostatic waves depend critically on the frequency separation of the two pumps. Enhancement of the lower frequency electron plasma waves is observed when the frequency separation of the two pumps is f(,i), 2f(,i), or 4f(,i), where f(,i) is the ion acoustic frequency. These experiments were performed using the HF Facility and the 430 MHz radar available at the Arecibo Observatory. These studies have been extended to the high latitude ionosphere using the HIPAS HF Transmitting Facility, which is located about 30 miles east of Fairbanks, Alaska. This environment is of interest because it is within the auroral zone (where electron precipitation may occur), and since the magnetic dip is 76.5(DEGREES), the HF beam may be easily tilted along the earth's magnetic field. New diagnostic techniques, which involve the measurement of the ionospherically-reflected sky wave signals, were developed and have been used to study fast time scale, transient effects which are prevalent at this latitude. If two short em pulses are used, it is possible to determine that the gentle ionospheric density profile contains short scale density inhomogeneities. Another experiment was similar to the two frequency studies performed at Arecibo. By monitoring the sky wave return in this case, it was possible to determine when either scattering or absorption was the dominant mechanism in determining the strength of the sky waves. |
