Nighttime above-the-MUF HF propagation on a midlatitude circuit

摘要

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).