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Abstract
Observations of the direction of arrival and time of flight of HF signals propagating on a 1400 km path oriented along the mid-latitude trough are presented. At night, the signal commonly arrives from directions offset from the great circle bearing by up to 80° and these events have been categorised into five main types. Statistics indicating how often these categories of propagation were observed in the period August 2006 to September 2007 are presented. The physical mechanisms which result in the off great circle propagation are also discussed.
Highlights
The mid-latitude trough is a region of depleted electron density in the nighttime F-region ionosphere in which the critical frequencies drop by a factor of at least two and the altitude of the electron density peak rises by 100 km or more (Moffett and Quegan, 1983)
During the winter and equinoctial months, the trough takes the form of a band a few degrees wide in latitude to the equatorward side of the auroral oval, stretching in local time from dusk to dawn
The establishment of multi-frequency HF circuits between Uppsala, Helsinki and Leicester has allowed us to observe simultaneously the direction of arrival, time of flight, Doppler shift and Doppler spread of signals reflected from the ionosphere
Summary
The mid-latitude trough is a region of depleted electron density in the nighttime F-region ionosphere in which the critical frequencies drop by a factor of at least two and the altitude of the electron density peak rises by 100 km or more (Moffett and Quegan, 1983). Gradients in electron density associated with the trough walls and embedded ionospheric irregularities often result in propagation in which the signal path is well displaced from the great circle direction, with directions of arrival at the oW
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