Multiple frequency radar observations of high-latitude E region irregularities in the HF modified ionosphere

In September 1983, experiments were conducted in Scandinavia using the high-power heating facility near Tromsø, Norway. The purpose of the HF ionospheric modification experiments was to investigate the behavior of artificially produced E region irregularities at auroral latitudes. The majority of observations were made with backscatter radars operating at 46.9 and 143.8 MHz, but limited observations were also made at 21.4 and 140.0 MHz. These radars are sensitive to irregularities having scale lengths of between 1 and 7 m across the geomagnetic field lines. The growth and decay of the irregularities are scale length dependent with the shorter lengths growing and dissipating more rapidly than the longer lengths (e-folding growth times=10¹-10² ms; decay times=10²-10³ ms). During periods of full power ordinary mode heating, irregularities having peak cross sections of 10⁴ m² at 46.9 MHz and 10⁵ m² at 143.8 MHz are observed. However, the cross sections normally measured are 1 to 2 orders of magnitude smaller than the peak values. The cross sections are nonlinearly dependent on the HF power and begin to saturate at levels greater than 50-75 percent of full power. Past E and F region data from Arecibo are used in conjunction with the Tromsø measurements to ascertain the relative roles played by various mechanisms in exciting irregularities. In the E region, the results tend to favor those instability processes which operate at the upper hybrid resonance level (e.g., thermal parametric and resonance instabilities) over those that operate at the reflection level (e.g., parametric decay instability). However, it is likely that anyh of the mechanisms studied could at times contribute to irregularity production in the E regions. At F region atlitudes, the findings show support for the parametric decay, four-wave thermal parametric, and resonance instabilities. The aspect sensitivity of the irregularities is measured using the 140.0 and 143.8 MHz radars. The results indicate a reduction in echo strength of 8-14 dB/deg for geometries that do not match those for specular reflection.