The Sun: Man's Friend & Foe

Listen to an astronaut talking to students using ham radio via Real Audio.

Throughout the ages people have used the light of the Sun to communicate with each other. Even today you can "harness the Sun" to communicate. The Greek historian Herodotus tells us that the Greeks used a polished mirror to reflect sunlight and send signals. This is what is known as heliograph signaling.

Ships used light flashes to communicate with each other long before the radio was invented. Armies employed the same technique to communicate on the battlefield. However light signals do have their disadvantages. Despite the fast speed at which light travels, a message can only travel as far as one can see. Light signals work best on a level plain and at sea and obviously will not work well in mountainous regions unless the people trying to communicate do it from the tops of peaks. Even in situations at sea or in level areas the light will eventually be cut off by the curve of the earth.

Heliography is the use of the Sun's light to transmit messages. Greeks and Romans used mirrors or polished combat shields. In the 1880's U.S. Army Troops used mirrors to communicate between mountain ranges in the western part of the United States. Today radio signals are used a great deal to communicate and solar activity is constantly bombarding the ionosphere of Earth and positively effecting communications. This has far reaching repercussions on anyone or anything that is dependent on the transmission of signals.

Radio-wave Propagation

Effects on the Ionosphere

The solar activity of the Sun influences radio-wave propagation by effecting the Earth's ionosphere. The ionosphere consists of four layers of charged particles which are ionized by ultraviolet radiation from the Sun. The layers have been given letter names starting with the D layer which is the lowest layer (about 30 to 50 miles up) that effects radio-wave propagation. The layers extend out to the E, F1, and F2 layers with the F2 layer (about 100 to 260 miles up) most responsible for long-distance amateur communication. There are no A, B, or C layers. The ionosphere refracts or bends radio waves back to the earth. A one-hop radio transmission can travel about 2500 miles using the F2 layer.

This chart shows the Sunspot cycle from 1850 to 1970. Notice that the Sunspot cycle is every eleven years. This cycle has been recorded since 1749.

Listen to a Morse Code transmission in Real Audio.

Day-to-day and yearly solar conditions will effect radio signals which makes radio-wave propagation difficult to predict. Maximum ionization occurs during peak sunspot activity. Sunspots occur in cycles with inconsistent patterns with the 1755 cycle designated number one. Cycle 22 occurred in September of 1986 and Cycle 23 began 1996. High sunspot numbers at the cycle peak normally produce the best wave propagation conditions. The Sun must be visible to obtain the sunspot number.

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Solar flux or radio energy coming from the Sun produces greater ionization in the ionosphere. Large antennas pointed at the Sun measure solar flux which may be taken under any weather conditions. The solar flux index number along with the sunspot number which are both directly related with the activity of the Sun is used to predict radio-wave propagation. Flux values in the 60s and 70s mean fair to poor propagation.

What is the ionosphere?

What is the lowest ionospheric layer?