Abstract
The presence of the midlatitude trough can severely impact on HF radio systems since the electron density depletion within the trough reduces the maximum frequency which can be reflected by the ionosphere along the great circle path. Furthermore, the associated horizontal gradients in the electron density distribution frequently result in propagation well displaced from the great circle path. The signal characteristics associated with this type of propagation have been investigated for a 1400 km link oriented along the midlatitude trough between Sweden and the UK. As anticipated, the observed delay and Doppler spread characteristics are strongly dependent upon time of day and season since the trough is a nighttime feature which occurs predominantly during the winter. In particular, the Doppler spread is often large when great circle propagation has been suppressed and reflections are from the north of the great circle path (i.e., from the poleward wall of the trough or from gradients and/or irregularities associated with the auroral zone).
Highlights
[2] An important feature of the ionosphere is the midlatitude trough, a region in which the ionospheric electron concentration is depleted by up to an order of magnitude
The trough, which forms at latitudes just below those of the auroral oval, has been investigated by various workers, and a number of statistical models have been developed [e.g., Halcrow and Nisbet, 1977] which give estimates of the position and extent of the trough as a function of geomagnetic conditions, time of year, etc
Measurements of the multipath delay and Doppler spreads observed over several paths within Scandinavia and spanning the auroral zone between Scandinavia and Svalbard, somewhat south of those reported by Warrington et al [1997], have been presented by Angling et al [1998]
Summary
[2] An important feature of the ionosphere is the midlatitude (subauroral) trough, a region in which the ionospheric electron concentration is depleted by up to an order of magnitude. Presence of the midlatitude trough reported by Rogers et al [1997] and [2003] for a 910 km east-west path within Canada Unlike these earlier measurements, the nature of the test signals and the receiving equipment enabled the elevation angles of arrival and propagation delays of the individual modes (and the multipath delay spreads) to be measured. Observations [17] As may be expected, the measured spread characteristics are strongly dependent upon time of day and season since the trough is a nighttime feature which occurs predominantly during the winter This is illustrated for the 11.12 MHz signal in Figures 4 and 5 for summer and winter, respectively. [18] For the winter period, it is interesting to consider the spread parameters separately for signals arriving close
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