where 6 is the angle between the wave-propagation direction and the magnetic field. For typical Arecibo experimental conditions. The HF power flux at 250 km is estimated at 30 /zW/m2 for this experiment, yielding a north-south striation width of X± = 1.1 km. In the east-west plane, the striation widths are predicted to be approximately 500 m (13). However, these theories derive only the instability threshold conditions; a saturation theory is being developed. A schematic of a self-focusing experiment conducted at the Arecibo Observatory is shown in Fig. 7. Intense HF electromagnetic radiation incident on an overdense ionospheric plasma excites parametric instabilities, enhancing electron-plasma oscillations observable by incoherent backscatter radar. These instabilities continue to be the subject of intense experimental study. Of importance here is the fact that, above
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