Nighttime above‐the‐MUF HF propagation on a midlatitude circuit
Bibliographic record
Abstract
HF radio propagation was observed regularly during the night at frequencies above the classical Maximum Usable Frequency (MUF) on the circuit from Fort Collins, Colorado, to Boston, Massachusetts, during 2003 (McNamara et al., 2006). This propagation was attributed to above‐the‐MUF propagation, in the sense of normal refraction by over‐dense regions in the F2 layer of the ionosphere, following Wheeler (1966). Similar propagation was also observed on a regular basis on the Ottawa to Boston circuit, at an operating frequency of 7.335 MHz. We have reanalyzed the nighttime Ottawa to Boston propagation in conjunction with observations of foF2 recorded by the digisonde at Millstone Hill (near Boston). Normal propagation (via refraction) would be supported on this circuit only when foF2 exceeds the equivalent vertical frequency, which is ∼5.7 MHz. The actual propagation extends through the midnight to dawn period even when foF2 drops below 4 MHz, with signal powers decreasing from about −80 dBW to −110 dBW. We deduce that there are three possible modes of propagation that are, in order of appearance: (1) normal refraction when foF2 exceeds 5.7 MHz; (2) “sporadic” reflection from large scale irregularities (tens of kilometers) with plasma frequencies that exceed 5.7 MHz in a lower density background; and (3) a 2‐hop ground side scatter mode with hop lengths greater than the 7.335 MHz skip distance. The second mechanism is the one discussed by Wheeler (1966).
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".