Ions and Current

Atoms and molecules sometimes are broken into parts that are no longer electrically neutral. These parts are called ions. The negatively charged electrons can be stripped away, leaving a positive ion. Molecules can be split into groups of positively and negatively charged ions or into electrons and positive ions. For example, the Northern Lights (auroral displays) occur when the air molecules of nitrogen and oxygen have been ionized after being bombarded with incoming particles (Figure 1.20).

FIGURE 1.20 ► A bombarding electron (e~) from a solar disturbance hits a nitrogen molecule (N2) of the high atmosphere. An electron is stripped from the outer shell of the N2 making it an excited ion and doubling the number of electrons in the region.

The releases its excited energy as auroral light (hv) in colors characteristic of N^. Similar ionizations occur from bombardment of the atmospheric oxygen molecules, producing other characteristic auroral colors.

When a stream of either all-negative or all-positive charged particles move together in a specific direction, the flow is called an electric current. By convention, the current direction is taken to be the direction that the positive ions would flow. This convention means that negatively charged electrons flowing to the right would be called a current flowing to the left. How easily the current flows in a medium naturally depends on some special characteristics of that flow region, called its conductivity. For example, the conductivity of air is a lot less than that of ocean water, the conductivity of rain water is less than that of the wet Earth, and the conductivity of copper wire is greater than all of these.

The current in a metal wire consists of electrons that are pushed along by a battery or other power source, and the specific type of metal determines the conductivity for that wire current. We are interested in the conductivity of the ionized high atmosphere. In that gaseous region, the conductivity also depends on the direction of the current with respect to the Earth's local magnetic field. This is because the moving charges, ions or electrons that compose the current, can have their direction diverted by a magnetic field.

All electric currents of moving charged particles produce their own magnetic fields. These fields flow around the axis of the current direction in a fashion that is called the right-hand rule. If you think of the fingers of your right hand as surrounding the current flow, with the thumb pointing in the direction of the electric current, then the fingers of your hand point in the direction of the magnetic field that circles the current. That is why the helical winding of the electromagnet, described earlier, produces a strong one-directional field through the central region of its wire windings (Figure 1.21).

FIELD

CURRENT

FIELD

FIELD

ELECTROMAGNET

FIELD

ELECTROMAGNET

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FIGURE 1.21 ► Electric currents flowing in a wire cause a magnetic field that circles the wire. The magnetic field can be concentrated in a single direction by a toroidal winding of the wire.

current out

FIGURE 1.21 ► Electric currents flowing in a wire cause a magnetic field that circles the wire. The magnetic field can be concentrated in a single direction by a toroidal winding of the wire.

In a gas of energetic charged particles that are moving together in a strong, generally linearly directed magnetic field, the charges will form tight spirals about the field lines and be guided along in an overall forward direction as a field-aligned, current. Such behavior is often visible in auroras as field-aligned luminosity excited by the bombarding electrons that hit the air molecules, causing them to glow (e.g., Plate 5) and marking the Earth's main field extension into space.

1.41 Our Tour of the Fields

In our guided tour I will not trouble you with the special studies of physics laboratories, nor with the magnetic fields that concern engineers working in electronic information storage and transfer. Rather, I will describe the natural magnetic fields found in our everyday environment. The sources of such fields are strong currents deep within the Earth, magnetized materials, and natural current systems above the Earth. We want to discover how these magnetic fields can affect our lives.

In subsequent chapters, I will show that our measurements of the principal field that moves our compass needles, the main field at the Earth's surface, is actually a summation of a field from sources inside the solid Earth and another field that is caused by field sources away from the Earth's surface. We will learn that the inside (internal) part mainly comes from currents flowing in the deep, liquid outer core of the Earth; from currents induced to flow in the conducting Earth because of sources above the Earth's surface; and from natural magnetized materials in the Earth's crust. The part of the magnetic field from sources away from the surface is, in large measure, due to currents flowing in our space environment.

Naturally magnetized rocks are found broadly distributed about the Earth's surface. Careful mapping of these field contributions show they fall far short of providing the major contribution to the observed Earth's main field. A magnetic mountain assumed to attract the compass needle of Columbus's time just doesn't exist. However, in a subsequent chapter we will see how measurements of the Earth's crustal fields are important for understanding the natural history of our Earth's magnetic field evolution.

Natural currents, flowing in the Earth's surrounding space, are a major source of variations observed in the surface measurements of magnetic field. These external fields induce currents to flow in both the conducting Earth and in man-made conductors such as storage tanks, pipelines, and electricity transmission lines. Some magnetic field fluctuations are also naturally generated by the motion of conductors in the Earth's large main field (in a manner similar to the hydroelectric generation of electricity by turbines that move wire through the field of a large magnet). Two examples of these natural sources are the motion of conducting atmospheric ions in the Earth's main field and the motion of conducting ocean waves in the Earth's main field.

In our next chapter we will visit some of the many consequences that all the natural fields bestow on our modern lives. We will wait until the later chapters to describe how these geomagnetic field sources are generated.

E Chapter 2

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