A Conducting Blanket

The Sun's visible light colors range from long-wavelength deep-dark red through the rainbow spectrum to short-wavelength violet. The shorter the wavelength of light, the higher the radiation energy. Past the violet colors of the spectrum is ultraviolet (UV) light, against which we all try to protect our skins with sunscreen and our eyes with sunglasses. This solar ultraviolet radiation is strong enough to break apart the nitrogen and oxygen molecules of our atmosphere into ions and electrons. Air becomes thinner at higher altitudes. Far above the Earth, there are too few air molecules to stop much of the UV radiation. Very close to the Earth a major portion of the UV energy is used to split oxygen molecules (O2) of the air into atoms of oxygen, which recombine to produce ozone (O3) molecules and form a layer about the Earth that is concentrated near the 25 to 30 km (16 to 19 miles) level. The presence of man-made chemicals has initiated a depletion of this fragile layer that protects us from much of the UV radiation.

At higher altitudes, to about 90 km (56 miles), the molecules that have been broken into charged ions and electrons can recombine quickly because there are so many nearby particles of opposite charge. From about 90 to 1000 km (56 to 625 miles), there are still enough molecules of nitrogen and

Electron Density (number per cubic centimeter)

FIGURE 3.16 ► A high concentration of electrons in the ionized air forms the ionosphere. Radiowaves bounce between the ionosphere and the Earth—higher frequency signals are returned from the higher electron density regions. The electron density values shown here are for midday, summertime, at mid-latitudes.

Electron Density (number per cubic centimeter)

FIGURE 3.16 ► A high concentration of electrons in the ionized air forms the ionosphere. Radiowaves bounce between the ionosphere and the Earth—higher frequency signals are returned from the higher electron density regions. The electron density values shown here are for midday, summertime, at mid-latitudes.

oxygen to absorb some UV and be broken into ions and electrons, but too few molecules to provide a substantial recombination rate. In the lower half of that region an ionized layer of electron and ions forms (various combinations of nitrogen and/or oxygen) as a conducting blanket (the ionosphere), which is profiled by the electron density (Figure 3.16). Gravity holds our atmosphere near the Earth. Therefore, above 900 km (560 miles), the lighter atoms of hydrogen and helium dominate over the heavier atoms of nitrogen and oxygen in this higher, upper-atmosphere composition.

The maximum electron density is usually found to be near 300 km (188 miles); although, the region that can carry strong currents is near 100 km (63 miles). The reason for this difference is that the electrical conductivity of the ionosphere depends on some special features, such as:

1. the suitability of the ions and electrons to recombine (recombination coefficient),

2. how often the ions and electrons collide (the collision frequency), and

3. the Earth's magnetic field strength and direction in the region.

Of course, the rising and setting of the Sun each day (our source of UV radiation) provides a daily variation in the ionization. However, not all the ionosphere goes away at night. Although the 100-km night-time ionization almost disappears, the collisions of the ions and electrons above 200 km (125 miles) in altitude are rare enough that some of the ionization slowly decreases until the start of the next day. As we might expect from the changes in Sun exposure around the Earth, there are latitude and seasonal constraints on the ionosphere's appearance. For example, summer days at polar locations can be in full daylight and winter days in full darkness.

One unique feature occurs at the magnetic dip equator ionosphere—where the Earth's main field near 100 km in altitude is directed horizontally to the Earth surface. That field direction causes the ionospheric gas conductivity to become extremely large. Any electric currents arriving in this region are channeled into a narrow ionospheric current band (called the equatorial elec-trojet) causing an enhanced field effect at the ground. Another unique feature happens in the high-latitude ionosphere where auroras occur. Bombarding particles that produce the auroras (a subject we will explore in Chapter 4) produce extra local ionization and conductivity so that strong auroral electro-jet currents flow.

Radiowave signals that are transmitted through the atmosphere can be reflected at the ionospheric conducting surfaces (Figure 3.16). The reflection depends on the radiowave frequency at which the transmitting station sends the signal and on the special nature of the conductor that is encountered. Our distant radiowave communications to locations that are not as close as our local radio stations depend on bouncing the radiowave signals between the conducting Earth and the conducting ionosphere. In this way, information can be transmitted to the opposite side of the Earth (see Figure 2.22).

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