Natural Sources of Electromagnetic Radiation

Cosmic Sources

An extremely important concept to understand is regarding the Earth's radiative heat balance. Although the Sun illuminates the Earth with an enormous amount of electromagnetic radiation, the Earth radiates exactly the same amount of electromagnetic energy into space!

The Earth and everything on it is a source of electromagnetic radiation. The major fraction of this radiation is in the infrared part of the spectrum, whereas the incident solar radiation is primarily in the visible part of the electromagnetic spectrum.


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Optical Radiation from the Sun

The Sun is the Earth's primary source of electromagnetic radiation. It is radiation from the Sun which sustains all life on the planet. Without radiation from the Sun, the Earth (if it existed), would have an equilibrium temperature equal to the equilibrium temperature of interstellar space (about -270o Celsius).

It takes about 8 minutes for electromagnetic radiation emitted from the Sun to reach the Earth. The major fraction of the Sun's radiation is in the "visible" part of the electromagnetic spectrum, however the Sun radiates electromagnetic energy over a very wide range of wavelengths. Some solar images are shown below, sampled at various parts of the electromagnetic spectrum.

Visible light

This the "white" light that illuminates the Earth's surface. In fact it is actually a continuous blend of colours whose wavelengths are characterized by a Planck "black body" at 6000K. When this light is passed through a prism it forms a continuous spectrum with narrow absorption lines.
Image courtesy of NASA

Ultra-violet radiation

Ultra-violet light forms the high energy component of the solar spectrum. Ultra-violet light has a wavelength shorter than those in the visible part of the solar spectrum and is not detected by human vision. Most of the Sun's very short wavelength ultraviolet radiation is filtered out by the Earth's ozone layer.
Image courtesy of NASA

Infrared radiation

Also referred to as thermal radiation. Infrared radiation is invisible to the human eye and has a wavelength longer than those in the visible part of the solar spectrum. Water vapour in the Earth's atmosphere effectively blocks most of the Sun's incoming infrared radiation. At the longest infrared wavelengths the Earth's atmosphere is somewhat more transparent, nevertheless, it is the atmosphere's opacity to infrared radiation that is largely responsible for keeping the Earth warm.
Image courtesy of NASA

X-Rays and Radio Waves from the Sun

Radio emission from the Sun

The Sun at 330MHz (wavelength about 1m). All hot objects emit radiation over a wide range of wavelengths. In this image radiowave intensity has been recovered over the surface of the Sun and converted into a pseudocolour image.

The radio image resolution is quite low because of the long wavelength of the radio waves.
Image courtesy of VLA

X-ray radiation from the Sun

The bright spots on this image are regions of intense X-ray emission. This image was taken using X-ray sensitive imaging devices and then converted into this image so that it has the appearance of an optical image.

The bright regions are related to solar flare activity which is also responsible for intense particle emissions.
Image courtesy of NASA

Terrestrial Sources: Waves (and Particles)

The gamma ray component of radioactive decay

All naturally occurring terrestrial sources arise from the radioactive decay of naturally occurring radioactive elements which are either chemically combined in the minerals that make up the soil and rock, or as gases which are radioactive daughter products of other elements, of which radon is a good example.

All radioactive materials are emitters of particle radiation and electromagnetic radiation. The particles emitted are either alpha particles or electrons. Every particle emission is accompanied by a tiny pulse of electrogmagnetic radiation which has a gamma ray signature.

For example 92U238 ( a radioactive isotope of Uranium) which has a half life of 4.51 x109a, is an alpha emitter. Each nucleus will eject an alpha particle whose energy in the 4.1-4.2MeV range accompanied by a 48keV gamma ray. Uranium however has many common isotopes which occur naturally. Their decay modes produce gamma rays with literally hundreds of different gamma ray energy signatures, ranging from less than 1 keV to slightly more than 1.14MeV. The alpha emission from the Uranium nucleus decreases its atomic number by two and its atomic weight by four, converting it into radioactive 90Th234 Thorium.

On the other hand, 88Ra226(radium) whose half-life is about 1600a, is an alpha emitter. When a radium nucleus undergoes a radioactive decay, it emits an alpha particle whose energy is in the 4.5MeV range, accompanied by a 200keV gamma ray.

Areas in which the mineral content of the soil and surrounding rocks contain radioactive elements are bathed in an extremely weak, but measurable gamma ray background. The gamma ray spectrum, that is the wavelengths (energies) of the gamma rays can be used to identify the radioisotopes responsible for the emissions and their relative concentrations.

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Prepared by the YES I Can! Science Team,