EM Spectrum

Tutorial 7:
Solar Radiation

Solar Radiation and the Electromagnetic Spectrum

Any object that has energy will exhibit that energy as temperature (the average kinetic energy of an object). If an object does not have energy at all, it "will not have" a temperature; actually, it will, Absolute Zero! Absolute zero is achieved when there is no molecular energy in an object. This means that even electrons stop orbiting their nuclei...at least in theory.

All objects above absolute zero radiate energy that has electrical and magnetic properties and is therefore called Electromagnetic Radiation or EMR. EMR may be understood in part as behaving as a wave. We need to know some properties of these waves. An important property shown below is Wavelength. Wavelength is measured as the distance between successive wave crests or, equivalently, between successive wave troughs.

EMR exists in a continuous spectrum of wavelengths. Hotter objects emit short wavelengths while cooler objects radiate longer wavelengths. All EMR travels at the same speed regardless of wavelength. That speed is the Speed of Light. Since the speed of light is a constant, it is symbolized as "c." C is equal to 186,000 miles per second or about 300,000 kilometers per second. EMR may travel in a vacuum (unlike sound waves) which is why solar energy can travel the 94 million miles of space to reach the earth.

Another property of EMR waves is Frequency. Frequency is simply the number of waves to pass a point per time unit (typically 1 second). Numbers of waves per second are measured in Hertz (Hz). Since all EMR travels at the same speed, the only variable to influence frequency is wavelength. The shorter the wavelength, the higher the frequency. Frequency, in turn, relates to energy level and danger level. Short wave energy is of high frequency and of high energy level which make it dangerous. You will need to know the relationships summarized below.

There is a negative relationship between wavelength and frequency.
There is a positive relationship between frequency and energy level.
There is a positive relationship between energy level and danger level.

We will use these relationships to study the overall EM Spectrum (See below).

EMR may be arranged by wavelength with the shortest wavelengths to the left and the longest at the far right. Given the relationships summarized above, we may also make the following observations about the arrangement of the EM spectrum.

Wavelength increases to the right.
Frequency decreases to the right.
Energy level decreases to the right.
Danger level decreases to the right.

The shortest wavelength, highest frequency, highest energy, and most dangerous form of EMR is Gamma radiation. Next come X-rays and then Ultraviolet (we will revisit UV below). Then comes the human Visible Spectrum. These are the wavelengths human eyes perceive as colors.

Hint: Make sure you know the sequence of colors backwards and forwards and that you associate the end colors with wavelength (short versus long), frequency (high versus low), and energy level (high versus low).

Where the visible spectrum ends, Infrared radiation begins. The longer forms of infrared are sensible, that is we can feel them as warmth or "heat." The EM spectrum ends with Radio waves.

The diagram below shows the forms of EMR radiated by the sun and the earth.

There are some important observation to make.

The sun emits radiation from the UV, visible, and infrared portions of the overall EM spectrum.
The earth emits strictly infrared radiation.

Look again at the spectrum diagram. Note that two major divisions are shown at the top of the diagram. We may use these divisions when discussing energy, the sun, and the earth. We will simply say that the sun emits shortwave energy and the earth longwave energy.

A view of ultraviolet radiation emissions from the sun: June 9, 2000.

You probably know already that ultraviolet (UV) radiation is harmful. You have most likely have had a sunburn once or twice in your life (so far!). Sunburn is caused by UV from the sun. Skin cancer is also related to excess UV exposure. One of the great environmental problems facing us is increased UV exposure due to depletion of the Ozone Layer.

Image from the Stanford Solar Center:
http://www.solar-center.stanford.edu

Click here to go to Environment Canada to learn more about ozone depletion and increased UV exposure. Try to answer the following questions.

What is Ozone?
How does ozone form naturally?
Where is ozone found naturally?
What form(s) of EMR does ozone absorb?
When did ozone depletion begin?
What is destroying earth's protective ozone layer?
What parts of the earth are not protected by ozone?
What are the effects of increased UV exposure?
What can you do to protect yourself?

Solar Wind and the Auroras

The sun emits more than just EMR. The sun is also the source of high energy particles. These high energy particles are protons and electrons and they generally flow from the sun as a constant stream that is called Solar Wind. Follow this link to a press release from the European Space Agency and answer the following questions concerning solar wind.

Where does solar wind originate?
How fast does solar wind travel?
Has solar wind always affected human activity?
In what ways does solar wind affect modern technologies?

Solar wind interacts with the earth's magnetic field in an intimate fashion (see the diagram below).

The Magnetosphere is generated by the earth's metallic cores and it serves to protect life from being destroyed by the solar wind. Note how the flow of the solar wind is deflected around the planet. Not all of the particles in the solar wind are diverted around the earth, some are pulled toward the earth by the earth's lines of magnetic force. These high energy particles interact with the earth's upper atmosphere and create the Auroras.

The auroras appear as moving patches or curtains of color in the night sky (above left). The auroras are generally restricted to high latitude regions. Regions of auroral activity form halos around the poles (above right). Follow this link and answer the following questions.

Are the aurora ovals even in width?

Where are the aurora ovals widest?

Why are the aurora ovals thickest on the night side of the earth?

Hint: Look at the solar wind and magnetosphere diagram shown earlier!!

Where do the different colors of the auroras come from?

Auroral activity will commonly increase when the sun emits extraordinarily intense bursts of Ionizing Radiation (high energy electrons and protons...the same materials that comprise the solar wind). Solar Flares or prominences (like the one shown below) are solar events that may increase auroral activity.