Our Sun is one of the earliest celestial objects studied by radio astronomers. The radio emissions from the Sun provides a new window to explore and improve our understanding of the solar atmosphere and its magnetic activities. Solar radio astronomy has become an emerging field that combines solar physics and radio astronomy, where large amount of observations data of the radio Sun coupled with plasma physics has been used to understand the complex and dynamics processes of the origin of solar radio emissions.
Short History
The detection of radio waves from extraterrestrial sources such as our Sun were unsuccessful in the earlier years. This was due to the primitive technology of the radio telescope during its early stage of development and also of the absence of knowledge of the existence of a screening ionosphere. Another hold back that the Sun’s radio emission was not detected almost a decade after celestial radio waves were detected was due to the missing knowledge that solar activity affects the radio emission of the Sun, and some experiments unfortunately were carried out during the periods of low solar activity.
So it was not until 1942 that the first detection of the solar radio burst was made, by accident. During World War II, on February 26 to 28, 1942, the British army received strong interference that initially was suspected to be jamming signals transmitted by the Germans. But soon it turned out that it was caused by the Sun. James Stanley Hey noted that the radiations were between dawn and sunset, and were not observed at night. He concluded that the disturbance was of solar origin by going through the bearings and elevations of the receiving sets and found that the bearings moved throughout the day and was always within a few degrees of that of the Sun. It is also noted by Hey that the radiation appears to have been associated with the occurrence of a big sunspot group on the Sun at that time.
Earlier in 1940, Grote Reber observing the Sun at a wavelength of 187 cm reported negative results. Again, in a paper published in 1942, he characterized his results as inconclusive. However, in 1944, he finally reported the detection of solar radiation with his new and improved apparatus. Reber suggested that this long-wave radiation could be set up in the corona of the Sun. In 1945, George Clark Southworth published his results on the observation of microwave radiation coming from the Sun during the summer months of 1942 and 1943. The observation was carried out at three widely spaced wavelength in the region between 1 and 10 cm.
In the years following World War II, the development of radar antenna and receiver technology due to the war has helped to flourish the field of solar radio astronomy. The need for continuous and systematic observations of the Sun was recognized due to the great variability of the Sun’s radio emission. The two leading countries that have made significant contributions to the field then were England and Australia, where the basic techniques of observations, such as spectrographic measurement, flux measurement and polarization measurement were developed.