Locating the Fields

Geomagnetism is a global phenomenon that shows differing behavior at various latitude regions (zones) of the Earth (Figure 1.17). These zones are specified by their relationship to a pair of principal regions: the auroral zone, where the Northern Lights (or their Aurora Australis companion in the Southern Hemisphere) are most commonly observed; and to the equatorial zone, where the Earth's main magnetic field is directed horizontally. Instead of delineating exactly where in the 0° to 90° north or south latitude something happens, geomagneticians use names of six latitude zones:

1. the polar caps, where the Earth's main field is nearly vertically aligned,

2. the auroral zones, regions of the most frequent northern and southern auroras,

3. the high latitudes, near enough to the auroral zones to be greatly affected during solar-terrestrial disturbances,

4. the equatorial region, within a few degrees around the magnetic dip equator, where the Earth's horizontal field creates special upper-atmosphere effects,

5. the low latitudes, just outside the equatorial region, and

6. the mid (middle) latitudes locations, between regions 3 and 5, where many of the populated countries of the world are found.

Of course, although we have marked rather precise zones on the global map in Figure 1.17, the boundary markers of these regions are, in fact, rather inexact. For example, often the auroral zone activity expands into the polar cap, and on rare occasions the auroras appear at the middle latitudes.

FIGURE 1.17 ► Generalized locations of geomagnetic study regions.

POLAR

AURORAL

HIGH

MIDDLE LOW

EQUATORIAL LOW

MIDDLE

HIGH AURORAL

POLAR

FIGURE 1.17 ► Generalized locations of geomagnetic study regions.

1.3.5 Nature's Basic Particles ► Atoms

To understand the magnetic fields on our Earth we also need a quick review of the meaning of the words atoms, elements, molecules, ions, electric current, and conductivity. Let us start with an early model for the building blocks of all matter, often called the elements. This word clearly implies an inability for something to be further subdivided into other substances. Because this term appears in many nonscientific contexts, scientists use an equivalent but more specific term, atom. All atoms have a heavy central, positively charged part called the nucleus that carries almost all the weight and characteristic properties of the atom. In the simplest model design, electrons orbit around the nucleus. These electrons are small spinning particles, each carrying one negative electric charge. The electron paths are somewhat like planets that surround the Sun of our solar system; however, electron locations are not restricted to a unique plane but, rather, occur at prescribed orbital shell distances. Depending on the number of electrons, up to seven specific shells can be formed about the atom's nucleus.

The nucleus of all atoms is composed of neutrons that have no charge and protons that are positively charged (except hydrogen, which has only one proton and no neutrons). The number of protons is equal to the number of electrons, so that the whole atom is electrically neutral (uncharged). Protons and neutrons each have about the same weight, almost 2000 times the weight of an electron. Scientists call the total number of protons and neutrons the atomic weight of the specific atom.

The number of protons (or electrons) in each atom is called the atomic number and fixes the sequential order of all the atoms: hydrogen = 1, helium = 2, lithium = 3,..., nitrogen = 7, oxygen = 8,..., sodium = 11,..., chlorine = 17,..., silver = 47,..., gold = 79,..., lead = 82,..., uranium = 92,..., and so on (Figure 1.18). At present more than 100 distinct atoms are known.

The term atom was taken from the Greek word for indivisible because early studies implied that the atoms were the limiting segment of natural materials. We now know that there are radioactive atomic particles that are not indivisible but, rather, spontaneously decay into other atoms. Also, nuclear physicists have further subdivided the atomic structure of the nucleus using extremely complex techniques and a special mathematics of quantum-mechanical modeling. Nevertheless, the three principal parts of the atoms described here essentially define the chemical properties that we need as a starting point in our tour.

The atom's electron shells are filled in order, starting with the innermost. The first shell of an atom can hold only two electrons. The second shell

OXYGEN (6 electrons, 8 protons, 8 neutrons)

FIGURE 1.18 ► The hydrogen atom has just one proton and one electron. The oxygen atom has eight protons, eight neutrons, and eight electrons. The water molecule has one oxygen and two hydrogen atoms.

OXYGEN (6 electrons, 8 protons, 8 neutrons)

FIGURE 1.18 ► The hydrogen atom has just one proton and one electron. The oxygen atom has eight protons, eight neutrons, and eight electrons. The water molecule has one oxygen and two hydrogen atoms.

becomes full with eight electrons. The third shell is also full with eight electrons, by may carry up to eighteen when another outer shell exists. The atomic outer shells, after the first, are all considered full with eight electrons (but can contain more on rare occasions). Atoms with full outer shells are quite stable (they don't combine with other atoms). For example, helium has only two electrons in a single orbit, and neon has two electrons in its inner shell and eight in its second (outer) shell.

Molecules are special combinations of atoms that have become bonded together. One important way that this bonding occurs is by the sharing of electrons so that a full outer shell of electrons is formed while the molecule itself remains electrically neutral. Hydrogen (atomic number 1) with just one electron in its single shell is ready to react with other atoms. A hydrogen atom can share its electron with another hydrogen, forming a hydrogen gas molecule and completing the two-electron first shell for each. The oxygen atom (atomic number 8), with two electrons in its first shell and only six in its outer shell, is ready to take up two electrons. A water molecule has two

Two oxygen atoms form oxygen gas

Two nitrogen atoms form nitrogen gas

Molecular oxygen and nitrogen gases make up atmospheric air

Two hydrogen atoms and one oxygen atom form water

One sodium atom and one chlorine atom form table salt

FIGURE 1.19 ► Composition of air and some examples of atoms that combine to form molecules.

hydrogen atoms and one oxygen atom. That combination allows the completion of both two-electron and eight-electron shared orbits to be formed (Figure 1.18).

Two oxygen atoms can join to share two electrons of their outer shells and form an oxygen gas molecule. Nitrogen (atomic number 7) can share an outer electron with another nitrogen to form a nitrogen gas molecule. A sodium atom (atomic number 11) with just one electron in its third shell readily shares this with a chlorine atom (atomic number 17), which has just seven electrons in its third shell, to form sodium chloride, the molecule of table salt. Our atmosphere near the Earth's surface (excluding considerable water vapor) is mainly a mixture of almost 78% molecular nitrogen and 21% molecular oxygen (Figure 1.19). There is only a relatively small amount of other molecules (such as argon, carbon dioxide, neon, and helium) in the smog-free air we breathe.

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