Scope and limits

The Arecibo measurements are HF ionospheric-modification experiments at far lower frequency and power flux than an SPS beam; Duncan's own text says SPS-equivalent experiments are conducted at lower heating frequencies precisely because the threshold scales as frequency cubed. So the 5 per cent figure and the striation widths are measured in a scaled experiment, not in an SPS beam. They are also overdense observations, while the paper's threshold expression and the SPS case are underdense; as at 1980 the paper says the underdense observations were still to be made. The prediction for SPS is an extrapolation, which Duncan states as such elsewhere in the same paper (PDF p.7). Two months earlier, the April 1980 programme review (SRC-449 PDF p.428, CLM-1068) reported from Duncan and Gordon preliminary results that large-scale irregularities also developed during nighttime underdense HF heating, which they could not yet assign to HF self-focusing rather than to an HF-triggered natural spread-F condition. A 2026 preprint (SRC-105 PDF p.2), citing two earlier analyses that are not held here, reports that a reference 2.45 GHz SPS beam lies "well below the strong self-focusing regime"; its own result (CLM-0058 (low confidence), CLM-0440 (low confidence)) is on ponderomotive density depletion, and SRC-117 itself (PDF p.7) puts the onset of thermal self-focusing at far lower power flux than the ponderomotive kind. Earlier in the same paper (PDF p.7, printed p.6), at the end of its heating section, the author states that on the preliminary experimental results the instability threshold for SPS excitation of nonlinear ionospheric interactions is believed to be above the 23 mW/cm2 design limit, and puts the threshold for significant nonlinear heating at about 40 mW/cm2; the self-focusing section that follows says no threshold can be set in the usual sense for thermal self-focusing (PDF p.8, CLM-0694).