A comparison of measured directions of arrival with ray tracing simulations for propagation on a 4490 km path along the mid-latitude ionospheric trough

The mid-latitude ionospheric trough is a region of depleted electron density which forms at night just equatorward of the auroral oval and is more pronounced during the winter and equinoctial months. The latitudinal width of the trough is typically a few degrees and its position and opening time (sometime after sunset) are dependent on geomagnetic activity. Within the trough, critical frequencies are typically reduced to below half the value of those outside of the trough region. A number of models exist which predict the location of the trough. The model proposed by Halcrow and Nisbet (1977) has been used in this study. This model, which is based on the average of a number of satellite observations, provides information on the opening position of the trough, which is dependent on the level of geomagnetic activity, the closing position, which occurs at sunrise, and the location of the trough walls. The observed changes in bearing on a west-east path during a period of relatively low geomagnetic activity have been simulated by means of ray tracing. Two potential modes of propagation have been investigated: (a) a two hop propagation with the ground reflection via nonspecular scatter from the sea surface at locations to the south of the equatorward wall of the trough; and (b) reflection from the electron density gradients present at the equatorward wall of the trough