Radar backscattering from artificial field-aligned irregularities

In June of 1992 a NASA sounding rocket was fired into the Arecibo heater be am to provide in situ observations of artificially induced ionospheric irre gularities. In this paper we provide a radar scattering calculation based o n in situ data and compare the same with previous remote sensing experiment s and with theory. The calculated backscatter cross section is in good agre ement with prior observations over the Arecibo heater at 50 MHz. More impor tant, when we scale the observed in situ power spectrum appropriately and c ompare it with multiradar cross-sectional results from the Platteville, Col orado experiments, we find a remarkably similar radar frequency dependence, albeit one shifted to smaller scales over the higher-latitude site. Even t hough the rms fluctuation level is almost the same over the Arecibo and Pla tteville heaters, the shift in scales toward smaller structures over Platte ville explains the much larger VHF radar cross section measured there. Comp arison of our waveform and its power spectrum with similar predictions from a recent theory shows excellent agreement for k values up to about 5 times the breakpoint in the spectrum of the theoretical prediction. Taken togeth er, these results give very strong evidence for the production of needle-li ke solitary structures as the dominant final state when high-power radio wa ves reflect from a magnetized plasma. The organization of these structures by as yet unexplained processes may explain the scales between 10 m and 10 km which occur in the heated volume. Finally, the dominant needle-like fiel d-aligned density depletions seem to support a second source of smaller-sca le irregularities. This creates a second break in the power law slope from its one-dimensional value of k(-4.3) predicted by theory to one more nearly characterized by k(-3). The multiradar results from Platteville show a sim ilar break, and we speculate that a density and/or temperature-gradient-dri ven instability such as the drift wave is operating.