As radio astronomy evolved, scientists turned their attention to several objects in our own Solar System. One of these is the Sun. The radio Sun is somewhat larger than the visible solar disk, and it appears oblate or flattened along the plane of the equator. This is to say, the apparent diameter of the radio Sun is smallest through the poles and largest through the equator.
Visible solar flares are also observed with radio telescopes. Such flares have long been associated with disruption of the ionosphere of our planet, a phenomenon that wreaks havoc with radio broadcasting and communications at some frequencies. There are several different kinds of solar flares at radio wavelengths. Radio outbursts from the Sun usually portend a disturbance in the Earth's magnetic field a few hours afterward as the high-energy particles arrive and are focused toward the Earth's north and south magnetic poles. Then, at night, we see the aurora (northern lights and southern lights). We also observe an abrupt change in radio wave propagation at some frequencies.
Radio observations of the Moon and the planets have enabled astronomers to more accurately ascertain the surface temperatures, especially of planets with thick atmospheres such as the "gas giants" Jupiter, Saturn, Uranus, and Neptune.
Jupiter produces exceptionally strong radio emissions and has a fairly high temperature deep within its shroud of gas. At a wavelength of about 15 m, the EM radiation from Jupiter is almost as strong as that from the Sun. Jupiter is also a strong radio source at shorter wavelengths. Some of this radiation can be attributed to the fact that Jupiter generates considerable heat of its own, in addition to reflecting energy from the Sun. However, the internal heat of Jupiter cannot account for all the radio emissions coming from the giant planet. Several theories have been formulated in an attempt to explain the unusual levels of EM radiation coming from Jupiter. According to one idea, numerous heavy thunderstorms rage through the thick atmosphere, and the radio noise is caused by lightning. However, the noise is too intense for this idea to fully explain it. A more plausible theory is that electrons, trapped by the intense magnetic field of Jupiter and accelerated by the high rotational speed of the planet, cause a form of EM emission called synchrotron radiation.
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