SEASONAL BEHAVIOUR OF THE AFTERNOON MAXIMUM OF A LONG-DISTANCE 40 KHZ SIGNAL

The afternoon maximum (AMX) observed in 40 kHz signal strength, propagated over a distance of 5100 km in the low latitudes of the Northern Hemisphere, exhibits an appreciable asymmetric decrease in its time of occurrence about June-July. The time of occurrence of the AMX of this low-latitude E-W propagation follows the sunset lime equation at the centre of the propagation path. A significant test shows a high correlation of the absence of the AMX with geomagnetic storms. The extra-ionizations in the ionospheric D region during geomagnetic storms may lead to the absence of the afternoon maximum.