1. Good Day Sunshine
Ground-based radio observations of the Sun
Because of its proximity, the Sun is the brightest radio object in the sky, and radio telescopes have therefore been used to study our home star for decades. Moreover, the Earth’s atmosphere does not distort radio waves that are less than about one meter in wavelength, so we can observe the radio Sun on a cloudy day, just as radio signals are used to communicate with satellites even when it rains or snows outside.
If you want to examine the radio Sun in fine detail, a bigger telescope is needed. Since radio waves are millions of times longer than those of light, a radio telescope needs to be at least a million times bigger than an optical telescope to obtain the same resolving power. For this reason, the first radio telescopes provided a very myopic, out-of-focus view. These views have been compared to looking at the Sun through the bottom of a glass bottle.
Nowadays, relatively small radio telescopes, separated by large distances, are combined and coordinated electronically, achieving radio images of the Sun that are as sharp as optical ones. Because it is spread out, an array of small telescopes has the property that is crucial for high resolving power, namely great size relative to wavelength.
The technique is known as interferometry because it analyzes how the waves detected at the telescopes interfere once they are added together. The simplest example is a pair of telescopes with a computer to reconstruct the waves from the combined data (Fig. 1.20).
The Very Large Array, abbreviated VLA, is an example of a modern interferometric radio array that is used to observe the Sun (Fig. 1.21). It consists of 27 radio telescopes placed along the arms of an Y-shaped array at separations of up to 34 thousand meters (Fig. 1.22). The telescopes are connected electronically and linked to a central computer, providing a total of 351 pairs of telescopes. When the telescopes are all pointed at the Sun, the received signals are combined to create images of temperature and magnetic structures on the Sun with an angular resolution of up to one tenth of a second of arc, equal to or better than any ground-based optical telescope.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University