1. Good Day Sunshine
Radiation from the Sun
To understand how the Sun operates we must examine its radiation. It spreads out and carries energy in all directions. Some of the sunlight is intercepted by astronomers at Earth, who use it to deduce physical properties of the Sun. When radiation moves in space from one place to another, it behaves like trains of waves. It is called “electromagnetic” radiation because it propagates by the interplay of oscillating electrical and magnetic fields. Electromagnetic waves all travel through empty space at the same constant speed - the velocity of light. This velocity is usually denoted by the lower case letter c, and it has a value of roughly 300 000 000 meters per second. A more exact value is 299 792 458 meters per second. No energy can be transported more swiftly than the speed of light.
Different types of electromagnetic radiation differ in their wavelength, although they propagate at the same speed. Like waves on water, the electromagnetic waves have crests and troughs. The wavelength is the distance between successive crests or successive troughs (Fig. 1.2). It is usually denoted by the lower case Greek letter lambda, ?, and measured in meters.
Sometimes radiation is described by its frequency, denoted by the lower case Greek letter nu or ?. Radio stations are, for example, denoted by their call letters and the frequency of their broadcasts. The frequency of a wave is the number of wave crests passing a stationary observer each second, measure in Hertz, abbreviated Hz. One Hertz is equivalent to one cycle per second. The frequency tells us how fast the radiation oscillates, or moves up and down. The product of wavelength and frequency equals the velocity of light, so ? x ? = c. When the wavelength increases, the frequency decreases, and vice versa.
Although sunlight appears yellowish, it is actually a combination of colors that we designate white light. In the mid-17th century the English scientist Isaac Newton discovered that sunlight could be broken into its spectral colors, using a prism - a specially cut chunk of glass - to display the range of colors. White sunlight is similarly bent into its separate wavelengths when raindrops act like tiny prisms to give us a rainbow or a crystal or a single drop of dew resonate with color (Fig. 1.3). Our eyes and brain translate these wavelengths into colors.
When light is absorbed or emitted by atoms, it behaves not as a wave but as a package of energy, or a particle. These packages are given the name photons. They are created whenever a material object emits electromagnetic radiation, and they are consumed when radiation is absorbed by matter. Moreover, each elemental atom can only absorb and radiate a very specific set of photon energies.
The ability of radiation to interact with matter is determined by the energy of its photons. This photon energy depends on the wavelength or frequency of the radiation. Light waves with shorter wavelengths, or higher frequencies, correspond to photons with higher energy.
Like many hot, gaseous cosmic objects, the Sun behaves like an ideal thermal radiator called a black body. Thermal radiation arises by virtue of an objects heat, and it is characterized by a single temperature. A black body absorbs all the radiation incident on it and reflects none - hence the term black.
When the emission from a thermal radiator is arranged in order of wavelength, radiation is found at all wavelengths but with a varying intensity (Fig. 1.4). Astronomers call this a continuum spectrum since the different colors or wavelengths run together with no breaks or gaps. The wavelength, ?max, at which the thermal radiation is at a
maximum is given by: ?max = 0.003/T meters, where T is the temperature in degrees Kelvin, abbreviated °K. This expression is called the Wien displacement law. It indicates that colder objects radiate most of their energy at longer wavelengths, and hotter objects are most luminous at shorter wavelengths. In other words, as the temperature of a gas increases, most of its thermal radiation is emitted at shorter and shorter wavelengths.
The total power received at any square meter of the Earth’s surface, known as the solar insolation, is much less than the solar constant (Fig. 1.5). This is due to the absorption of sunlight in the terrestrial atmosphere, as well as the time of day. The insolation varies according to the Sun’s altitude both because of the varying angle between the normal to the Earth’s surface and the Sun’s direction and because of the varying amount of the Earth’s atmosphere that the sunlight has to shine through. When the Sun is overhead at a location, the atmosphere it shines through is least and there is therefore the lowest amount of attenuation by the atmosphere. That is essentially why the days are hottest near noontime. The total energy received at ground level is then reduced from the solar irradiance value of 1366 watts per square meter to about 1,000 watts per square meter. Of course, the Sun is not out at night, and the insolation is zero.
The atmospheric attenuation is much greater at certain wavelengths than at others due to absorption by atmospheric molecules of ozone, oxygen, water, and carbon dioxide (Fig. 1.5). The ozone molecules in our atmosphere absorb the short-wavelength ultraviolet rays from the Sun, thereby filtering out energetic radiation that can be harmful to life on Earth.
Invisible Radiation From the Sun
There is much more to the Sun than meets the eye! In addition to visible light, there is invisible radiation as well. The solar spectrum extends over an enormous range in wavelength, and different wavelength regions carry different names (Fig. 1.6). The visual or optical region contains the observed colors. The invisible domains include the infrared and radio waves, with wavelengths longer than that of red light, and the ultraviolet (UV), X-rays and gamma (?) rays that are shorter than violet light. They are all electromagnetic waves and part of the same family, and they all move in empty space at the velocity of light, but we can’t see them.
The infrared part of the electromagnetic spectrum is located between the radio wave region and visible region (Fig. 1.6). Infrared radiation was discovered in 1840 by the astronomer William Herschel, who was already world-famous for his discover of the planet Uranus. He put a beam of sunlight through a prism to spread it into tits spectral components and he noticed that a thermometer placed beyond the red edge of the visible spectrum of the Sun became warmed by an invisible portion of sunlight. A thermometer placed in this infrared sunlight showed higher temperatures than one placed in normal visible sunlight. The warmth of ha heat lamp is similarly provided by its infrared radiation.
Just beyond violet light is the short-wavelength ultraviolet part of the electromagnetic spectrum (Fig. 1.6), where photons are sufficiently energetic to tear electrons or atoms off many of the molecular constituents of the Earth’s atmosphere, particularly in the ozone layer. These energetic photons cannot reach the ground, and if they did they would cause considerable damage to out skin and eyes.
The X-ray region of the electromagnetic spectrum extend from a wavelength of one hundred billionth (10-11) of a meter, which is about the size of a atom, to the short-wavelength side of the ultraviolet (Fig. 1.6). X-ray radiation is so energetic that it is usually described in terms of the energy it carries. . The X- ray region lies between 1 and 100 kilo-electron volts, abbreviate keV, of energy. There are soft X-rays with relatively low energy and modest penetrating power, with energies of 1 to 10 keV. The hard X-rays have higher energy and greater penetrating power, at 10 to 100 keV. As a metaphor, one thinks of the large, pliant softballs and the compact, firm hardballs, used in the two kinds of American baseball games.
The atmosphere effectively absorbs most of the Sun’s ultraviolet radiation and all of its X-rays and gamma rays. To look at the Sun at these invisible wavelengths, we must loft telescopes above the atmosphere. This was done first by using balloons and sounding rockets, followed by satellites that orbit the Earth above the atmosphere. Satellite-borne telescopes now view the Sun at invisible ultraviolet and X-ray wavelengths, above the Earth’s absorbing atmosphere in a world where night is often brief or non-existent. One of these solar satellites is the Solar and Heliospheric Observatory, or SOHO for short, launched on 2 December 1995, and another is the Yohkoh, or “sunbeam”, satellite launched on 21 February 1981. They provide entirely new perspectives of the Sun, detecting the hotter parts of the solar atmosphere (Fig. 1.7) and revealing a Sun that is unlike anything we have ever seen before (Fig. 1.8).
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Copyright 2010, Professor Kenneth R. Lang, Tufts University