You must account for the fluctuations in the ionosphere when you’re DX-ing. Depending on the hour, the ionosphere either absorbs a signal or reflects it over the horizon. In the daytime, the 20, 17, 15, 12, and 10-meter bands, called the High Bands, tend to be “open” (support propagation) to DX stations.
Before daylight, signals begin to appear from the east, beginning with 20-meters and progressing to the higher bands over a few hours. After sunset, the signals linger from the south and west for several hours with the highest frequency bands closing first in reverse order. Daytime DXers tend to follow the Maximum Useable Frequency (MUF), the highest signal the ionosphere reflects. These reflections are at a very low angle and so can travel the longest distance for a single reflection (one reflection is called a hop) and have the highest signal strengths.
From 30-meters down in frequency are the nighttime bands of 30, 40, 60, 80, and 160-meters, known as the Low Bands. These bands are throttled during the daytime hours by absorption in the lower layers of the ionosphere. After the sun begins to set, these bands start to come alive. First, 30, 40, and 60-meters may open in late afternoon and stay open somewhat after sunrise. 80 and 160meters, however, make fairly rapid transitions around dawn and dusk. Signals between stations operating on 80 and 160-meters often exhibit a short (15 to 30 minute) peak in signal strength when the easternmost stations are close to sunrise. This is known as the dawn enhancement. This time is good for stations with modest equipment to be on the air and take advantage of the stronger signals on these more difficult DX bands. 160-meters is known as Top Band because it has the longest wavelength of any current amateur band. This long wavelength requires larger antennas. Add in more atmospheric noise than at higher frequencies and you have a challenging situation. That’s why some of the most experienced DXers love Top Band DX-ing. Imagine trying to receive a 1 kilowatt broadcast station halfway around the world. That’s what the Top Band DXer is after! As difficult as this task sounds, many of the top DXers have managed it.