Technical Topics, June 1998
The Sun And Short-Wave Reception
(Part 1)
The ionosphere refracts short-wave signals back to earth. The sun strongly affects the ionosphere. But you probably knew that.
The sun has finally shown signs of increasing activity as the next 11 year solar activity cycle begins. Changes in short-wave propagation are becoming obvious. Solar flux numbers and maximum usable frequencies are rising. Daytime absorption levels cause weaker signals on daylight paths at lower frequencies. Higher frequencies remain useful long after the sun has set.
This month begins a multipart article to explain the art of predicting short-wave radio propagation.
Solar radiation and eruptions can disturb or enhance ionospheric refraction of short-wave signals. You have probably heard the solar and geomagnetic status reports and forecasts on WWV. You can hear them at 18 minutes past the hour and 45 minutes past the hour on WWVH. After reading this series, you should be able to understand and apply the information to improve your short-wave DXing hobby. You will also learn how scientists predict future short-wave radio propagation conditions.
The National Oceanic and Atmospheric Administration’s Space Environment Services Center (SESC) of the US Department of Commerce gathers data used for the reports you hear on WWV and WWVH. The reports are sent to the transmitter site in message form. Here technicians record voice announcements. The reports are automatically triggered to play at the appropriate time. The reports update every three-hours.
The SESC’s network of observatories monitors the geomagnetic field and the space in between the earth and sun. Both terrestrial and satellite observatories gather the data.
The sun’s electromagnetic spectrum consists of radiation spanning x-rays, ultraviolet, visible, infrared, and radio wavelengths. Sensors on earth and in space continuously observe each of these segments of the sun’s total energy spectrum. The changing energy levels alert scientists when significant events occur. These solar emissions are all electromagnetic so they radiate at the speed of light outward from the sun. Radiation from events on the sun reaches the Earth’s environment in slightly more than 8 minutes. Electromagnetic radiation (ER) travels faster than any other type of emanation. ER provides the earliest warning of solar eruptions and serves as a predictor of future short-wave disturbances.
Have you ever noticed that propagation conditions are often very good just before a major geomagnetic storm? This is no accident. The ER energy from the sun excites the atoms in the upper ionosphere causing higher levels of ionization. The upper layers become more efficient as they refract short-wave signals beck to the earth. When other solar emanations, traveling much slower than the speed of light, finally reach earth the party ends.
In addition to ER, the sun constantly ejects matter such as atomic and subatomic particles. These moving particles form a gaseous solar wind. The solar wind consists of electrons, protons, and helium nuclei hurtling away from the sun out into the solar system. Normally these particles move away from the sun at about 250 miles per second. The particles travel across the space between the sun and earth in about 4 days. When eruptions hurl particles away from the sun at faster speeds, the time will be shorter, more like 3 days. So ER increases can give three days advance notice before moving particles can influence short-wave propagation.
Predictions more than three days ahead are based upon what happened the last time a particular part of the sun was facing toward the earth. Conditions tend to follow a 27 day cycle because the sun rotates on its axis once every 27 earth days. This rotation results in particles flying outward in an expanding spiral. Picture a rotary sprinkler head spewing water in a spiral string of tiny water molecules.
As the arms of the spiral reach out 93,000,000 miles, they sweep across the earth. Normally the geomagnetic field and the earth’s atmosphere prevent the particles from reaching the earth’s surface. At times some of these particles can interact with the upper layers of the magnetosphere. Particles become trapped and spiral along the magnetic field lines toward the earth’s magnetic poles.
As the magnetic lines dip toward the earth’s surface, the energetic particles traverse the ionosphere. The charged particles cause the Aurora Borealis. The aurora can disrupt short-wave transmissions passing through the polar regions. The effect is particularly noticeable on paths which pass through the auroral zones surrounding the magnetic poles. A listener in California, trying to hear European transmitters, will hear these kinds of disturbances.
The geomagnetic field causes the particles to deflect and flow around the earth. Think of this field as a deflector shield as used by the starships in the Star Trek TV show.
The magnetic shield is roughly spherical on the side toward the sun. Scientists call this region the magnetosphere. The solar wind tends to push the magnetosphere toward the earth on the sunlit side. On the side away from the sun, the magnetosphere forms a long tail reaching far into space. This is similar to how the solar wind causes a comet to exhibit a tail pointing away from the sun. Envision the particles moving around the earth’s magnetosphere just as water would flow around a rock in a stream.
When solar disturbances occur, solar particles blast away from the sun at very high speeds. These high speed solar particles hit the earth’s magnetosphere changing its intensity and direction.
Collisions with these particles will whip around the tail of the magnetosphere changing its direction. Do you remember how a boat sail flutters when a gust of wind momentarily disrupts the smooth flow of air? The magnetosphere tail acts similarly. Scientists use sensitive magnetic detectors to monitor these fluctuations. When the direction of the magnetic field changes, scientists know the collision is in progress. If you look carefully you can see really strong disturbances deflecting compass needles.
The magnetosphere interacts with the ionosphere in complex ways when these disturbances occur. Polar paths experience high flutter and absorption as previously noted. People often speak of signals having a "watery" sound as rapid signal fluctuations modulate the received signal. North/South paths are often actually enhanced especially at lower frequencies on dark paths. I often hear South American stations on the broadcast band here in Delaware. A 650 kHz station in Bogota, Columbia often wipes out much closer WSM in Nashville. Both stations are 50 kW but Nashville is more or less west of me. Bogota is almost due south. The disturbance enhances only the north/south path.
Next time we’ll continue with part 2. Until then, stay tuned.