Waiting for the Grayline
May 15, 2004
The photo above showing the onset of evening twilight was taken from Elvange, Luxembourg on March 30, 2004 at 18:45 UTC (6:45 p.m. local time). The Fritzel beam antenna in the foreground is cut for bands at 20, 15 and 10 m (or 14, 21 and 28 MHz) and belongs to Mr. Gunter Meier. At dawn and dusk, high frequency (HF) shortwaves can take advantage of the line of the terminator (approach of Earth's shadow -- seen hugging the horizon) to reach far across the globe. This is because at that time, and for approximately the next hour, while the D and E layers of the ionosphere are vanishing, they're replaced by the F layer, located at an altitude of about 150-200 km (90 to 120 mi).