PROPAGATION
Those who follow this column know that radio communication is directly affected by the variable Sun and “space weather.” Sunspots, coronal mass ejections, the Earth’s geomagnetic field, the ionosphere, and even terrestrial weather all affect how our radio signals get from transmitter to receiver. Looking at the Sun is helpful in gauging its activity, and therefore helpful in figuring out what sort of conditions we might experience while trying to get our radio signals from our station, through the atmosphere via the ionosphere, to a distant station.
Frequently, this column includes images of the Sun in a single color. However, when viewing the Sun (you should never directly look at the Sun with your eyes — instead, view the Sun indirectly by way of an electronic viewer on a digital camera, for instance), we see a bright, featureless disk that is yellow, or perhaps orange if the Sun is low on the horizon (when sunlight travels through more of the Earth’s atmosphere, the blue wavelengths are “filtered away” while other wavelengths make it through before getting to the camera’s lens). The Sun emits light in all colors; when all these visible colors are summed together, we call this “white light.” Since yellow is the brightest wavelength from the Sun, that is the color we see in our camera’s viewer. The images included often with this column, however, come in various colors like green, blue, gold, red, orange, or even in gray-scale. Are these colored images simply a filtered look at the Sun?
Ever since Galileo Galilei’s successful improvements on the crude, vague telescope design which Hans Lippershey tried to patent in the Netherlands in 1608, scientists have developed specialized instruments with which the Sun may be observed. Observations of the Sun by way of these telescopes were made by projecting the Sun’s white light onto a “screen” so that drawings could be made of the sunspots that were now readily seen. Over the course of time, scientists discovered that they could interfere with the Sun’s light with prisms and other methods, to split apart the white light into specific visible colors. As optical technology improved, it was discovered
