Nighttime above‐the‐MUF HF propagation on a midlatitude circuit
Nighttime above‐the‐MUF HF propagation on a midlatitude circuit
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夜间中纬度电路上 MUF 高频传播
DOI：
10.1029/2007rs003742
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发表时间：
2008
期刊：
影响因子：
1.6
通讯作者：
Y. Yampolski
中科院分区：
文献类型：
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作者：
L. McNamara;T. Bullett;E. Mishin;Y. Yampolski
2003年，在从科罗拉多州柯林斯堡到马萨诸塞州波士顿的线路上，以高于经典最大可用频率(MUF)的频率定期观察到高频电波传播(McNamara等人，2006年)。这种传播归因于Wheeler(1966)之后电离层F2层过密区域的正常折射意义上的MUF以上传播。在渥太华到波士顿的电路上，也经常观察到类似的传播，工作频率为7.335兆赫。我们结合米尔斯通山(波士顿附近)数字探空仪记录的foF2观测，重新分析了渥太华到波士顿的夜间传播。只有当foF2超过等效垂直频率(即∼5.7兆赫兹)时，此电路才支持正常传播(通过折射)。即使当fF2降到4兆赫以下时，实际传播也会持续到午夜到黎明期间，信号功率从大约−80dBW下降到−110dBW。我们推导出有三种可能的传播模式，按出现的顺序是：(1)当foF2超过5.7兆赫兹时的正常折射；(2)在较低密度背景中，等离子体频率超过5.7兆赫兹的大尺度不规则性(几十公里)的“零星”反射；(3)跳跃长度大于7.335兆赫跳跃距离的两跳地面侧散射模式。第二种机制是惠勒(1966)讨论的机制。
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).