The following is a digest collection of thoughts that I have written in CQ Magazine, Popular Communications, and Monitoring Times. The following is copyright, 2003, by me, Tomas Hood.
WINTER SEASON DX:
A welcomed change takes place as the winter season arrives which can be observed in the Northern Hemisphere as we move away from the long sunlit days of summer into the longer hours of winter’s darkness. The amount and strength of radiation arriving and passing through our atmosphere varies from season to season, as well as from the solar cycle minimum to the solar cycle maximum.
While the chemistry and mechanics of the ionosphere might be complex, a few simple ideas can be used to understand how things work. The temperature of the air in the highest regions of our atmosphere plays a role in how the ionosphere is structured. During the summer, the way the various gases are distributed in these upper layers differs from the wintertime distribution. In addition, during the Northern Hemisphere’s winter months, the Earth is closer to the sun than during any other time of its travel around the sun. This makes the daytime ionization more intense than that of summer daytimes.
To understand the significance of this, think of a wood stove. When you open the front door to add more fuel to the fire, and get very close to the fire, you feel intense heat. When you close the door and back away from the fire, the heat decreases. This is much like the position of the Earth in the winter—closer to the sun than during the summer. But the "door" is only open during the short period of daylight. With the more intense ionization during winter’s daylight hours the radio waves refracted off of the ionosphere are relatively higher in frequency than those of summer. During the longer winter hours of darkness, the ionosphere has more time to lose its electrical charge. These conditions cause a wide daily variation in the maximum frequency that can be refracted by the wintertime ionosphere.
At any given time during the day, a fairly wide range of frequencies will be refracted from the ionosphere. The Ionosphere is made up of ionized particles and electrons in the uppermost portion of the earth's atmosphere that is formed by various influences, including solar radiation. These ions are responsible for the reflection or bending of radio waves occurring between certain critical frequencies with these critical frequencies varying with the degree of ionization. The highest frequency that will still be refracted by the ionosphere is called the critical frequency, or Maximum Usable Frequency (MUF). This critical frequency varies with the amount of ionization at the point where a signal enters the ionosphere, and the angle by which the radio wave arrives. In winter months, the noticeable rise during the day in this critical frequency brings a steady parade of DX signals through the higher shortwave bands during the day. But the winter daytime openings are short. Summer openings last much longer, since the ionization continues as long as the daylight lasts, and can last all night long as well, because recombination times are much shorter during the shorter hours of darkness.
In the summer, the long hours of sunlight keeps the ionosphere from recombining, but because the heating of the gases causes the layers to expand and thin out, the daytime critical frequency is generally lower than during the winter. But, the nighttime critical frequencies of summer are typically higher than nighttime critical frequencies during the winter. This gives us better nighttime DX in the summer, but better daytime DX in the winter over paths that propagate through sunlight regions. In addition, winter nights are far more quiet on lower shortwave bands due to the seasonal low in tropical storms, and because the lower critical frequencies won't propagation as much of the atmospheric and man-made noises.
It is the combination of these conditions that cause many radio enthusiasts to celebrate the arrival of the winter shortwave season. The winter of 2003 and early 2004 is promising, in part because of the seasonal relief we are having from the high geomagnetic storminess we've had this year, and because the latest and second geomagnetic peak of this cycle is slowly declining now, as we move ever closer to the solar cycle minimum. With these improvements, we also experience a relief from the electrical storm and atmospheric noise of summer. This makes it much easier to DX those tropical band broadcasts, medium wave AM broadcast stations, and HF International broadcasters.
(copyright, 2003, Tomas Hood. All rights reserved.)