Environmental Engineering Reference
In-Depth Information
Figure 2.1
A black box model of Earth's energy system.
THE SUN, OUR NEAREST STAR
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The sun is a nuclear furnace located about 150 million km away from our planet.
Virtually all the energy reaching Earth comes from the sun. Although the amount Earth
receives is only some two billionth of the sun's total energy, it provides sufficient heat to
keep the planet within an equitable temperature range that allows life to thrive.
Solar energy is generated in the core by nuclear fusion where the temperatures are
estimated to be near 15 million ° C. Hydrogen nuclei collide at very high speeds, fusing
into helium nuclei and releasing vast amounts of energy. This slowly makes its way
towards the solar surface, from where it is released into space at temperatures of about
6000° C.
The rotation of the sun under these conditions generates an intense magnetic field
which in turn has an effect on solar activity. Magnetic activity has a cycle lasting about
eleven years. We can see evidence of this in the sunspots, which are huge, cooler regions
dotted about the solar surface like black spots. Although the spots are cooler, they are
most abundant during the more active phases of the solar cycle, the surrounding areas
radiating more energy during these times, more than compensating for the darker sunspot
areas. Satellite measurements of solar output confirm that this is the case. At times of
high solar activity, flares and prominences, consisting of vast jets of gas arching into the
solar atmosphere, can be ejected. They emit large quantities of energy and charged
particles that can reach Earth. As they disturb our magnetic field, there can be a great
effect on radio and satellite communications.
The level of solar activity as measured by sunspots has been recorded for a
considerable time (Figure 9 9)
During the seventeenth century
few sunspots were
