Chapman

Finally, the present age of geomagnetism arrived with the long lifetime of extensive publications by Sydney Chapman (Figure 1.10), who originally worked in England, but spent much of his productive life in the United States.

FIGURE 1.6 ► This 1701 chart of magnetic declination contours for the Atlantic Ocean was produced by Edmund Halley, who is more famous for his prediction of the orbit for a comet that bears his name. The plotted declinations are inaccurate because of the difficulty in obtaining longitude at sea during the years of Halley's voyage.

FIGURE 1.6 ► This 1701 chart of magnetic declination contours for the Atlantic Ocean was produced by Edmund Halley, who is more famous for his prediction of the orbit for a comet that bears his name. The plotted declinations are inaccurate because of the difficulty in obtaining longitude at sea during the years of Halley's voyage.

Chapman became the father of space magnetism, applying Maxwell's mathematics to the natural processes of the upper atmosphere and magnetic storms originating from solar mechanisms. With help from the distinguished field observer Julius Bartels of Germany, Chapman produced the first completely modern two-volume textbook, Geomagnetism, in 1940.

FIGURE 1.7 ► Michael Faraday (1791-1867), who experimented with the relationship of electric and magnetic fields, devised the first electric motor and electric current generator.
FIGURE 1.8 ► Carl Friedreich Gauss (1777-1855) used mathematical techniques to distinguish between contributions to the surface magnetic fields from sources out in space (external) and sources below (internal to) the Earth.
FIGURE 1.9 ► James Clerk Maxwell (1831-1879) devised the mathematical formulation for the physics of electricity and magnetism that is still in use today.
FIGURE 1.10 ► Sydney Chapman (1888-1970) was an early space-science pioneer and father of modern geomagnetic studies.
FIGURE 1.11 ► This is an early instrument for measurement of the Earth's main magnetic field dip angle.

1.31 Local Language Dictionary 1.3.1 Earth Fields, Steady and Changing

Now, to become fully attracted to our magnetic subject, let us explore some word meanings and mildly technical terms that we use in this guided tour. Let us start with the meaning of geomagnetism. The prefix geo- is used to identify our Earth combined with its following root word, as in geographic (related to Earth mapping) and geophysical (related to the physical properties of the Earth). Our tour has its focus on geomagnetism, the natural fields within and around the Earth. However, when the context of a sentence is clearly understood to relate to our planet, popular use has favored the shortened term magnetism, as has been done for the magnetic pole location found on global charts.

Because the Earth behaves as a great dipole magnet, the dipping angle of a special compass needle that is freely suspended at its horizontal balance location (Figure 1.11), points at different angles, out of or into the Earth, to identify global north and south magnetic dip latitudes (see also Figure 1.5). This feature is still used in paleomagnetic studies to interpret the apparent

Time

Period = 24 hours

FIGURE 1.12 ► When the amplitude of a measurement oscillates in a regular fashion, the time for one oscillation to occur is called its period. Examples of 12- and 24-hour periods are shown. The position of the first maximum (measured in degrees with respect to a 360° full oscillation) is called the phase of the oscillation.

Time

Period = 24 hours

FIGURE 1.12 ► When the amplitude of a measurement oscillates in a regular fashion, the time for one oscillation to occur is called its period. Examples of 12- and 24-hour periods are shown. The position of the first maximum (measured in degrees with respect to a 360° full oscillation) is called the phase of the oscillation.

paleolatitude locations of ancient rocks that became magnetized at their time of formation millions of years ago. The prefix paleo, meaning ancient, is used in geophysics to indicate those distant prehistoric times.

The strongest part of geomagnetic fields varies so slowly over the years that we call it the main or steady field. In contrast, a superposed natural field of much smaller amplitude, but more dramatic appearance, is rapidly changing. This variation field has some irregular amplitude-changing parts of limited duration and some parts with prescribed periods of oscillation. But what do the scientists mean by periods of oscillation? This is the time for something that changes to complete one cycle—for example, the 24-hour oscillation period of daily temperature goes from a pre-dawn minimum to the following post-noon maximum and back to the next day's minimum (Figure 1.12). The inverse of the period is the frequency, which is the number of oscillations (cycles) in one unit of time. For example, we say that the daily temperature frequency is 1 cycle per day or that some magnetic fields have a frequency of 3 cycles per second.

Geomagnetic variation fields have durations or oscillation periods from fractions of a second to many months. Researchers have found that the geomagnetic variation field amplitudes generally decrease in size as the characteristic duration or oscillation period becomes shorter. Stronger fields usually seem to be generated by sources that are spread over greater distances. In addition, the laws of physics require that the further we go away from a magnetic field source, the weaker the effect of that field becomes.

Why does the field have a rapid variation part when we thought that there was just a big dipole-like magnet that caused the Earth's field? Well, what we measure with our field instruments is a summation of all the natural fields that are at the place of measurement (Figure 1.13). As we shall see in our tour,

150 degrees West Meridian Time

FIGURE 1.13 ► A disturbance field variation at the Fairbanks, Alaska, magnetic observatory. The horizontal axis indicates the local time from midnight to 5 am. The irregular trace shows a magnetic field disturbance that varies over 1000 gammas (magnetic field units) in size.

150 degrees West Meridian Time

FIGURE 1.13 ► A disturbance field variation at the Fairbanks, Alaska, magnetic observatory. The horizontal axis indicates the local time from midnight to 5 am. The irregular trace shows a magnetic field disturbance that varies over 1000 gammas (magnetic field units) in size.

there are many different sources of magnetic fields. Scientists try to separate these sources by special analysis techniques to determine where on our Earth the fields are similar, to find out how special fields are tied to processes in space or below the Earth's surface, and to discover what physical mechanisms can cause the various periods of field oscillation to occur.

1.3.2 Forces at Work

A magnetic field can be defined by the control that is exerted on certain substances that invade a region near a magnet or a steady electric current. Of course, the word "near" is relative to the strength of the magnet or current. At the place where a field exists, this control is measured by the force, having both a pushing (pulling) strength and a direction of the action, that can move another magnet, iron, or iron-rich rocks. The strength of this magnetic field decreases with the cube of the distance from the magnetic source (Figure 1.14). For example, at 2 inches from a magnet, the magnetic field is one-eighth of the field at 1 inch. The effectiveness of this field, of course, also depends on the special magnetic characteristics of the region in which the field exists. The field effect of a magnet held in the air has a different attraction on an intruding material if the magnet and material are submersed in oil. Scientists call this regional environment characteristic the magnetic permeability.

DIPOLE FILED DECREASE WITH DISTANCE

DIPOLE FILED DECREASE WITH DISTANCE

Distance to Dipole Center

FIGURE 1.14 ► The strength of a field originating at a dipole magnet is shown to decrease rapidly with distance away from the magnet.

Distance to Dipole Center

FIGURE 1.14 ► The strength of a field originating at a dipole magnet is shown to decrease rapidly with distance away from the magnet.

Not long ago I had an interesting personal magnetic field experience. I had just purchased a new 6-foot grandfather clock for our home. To my dismay, the clock regularly stopped before the weight-winding system ran down. When the manufacturer's representative came to investigate, he discovered that magnetism was the problem. What I thought were large brass winding weights to drive the clock mechanism were really just decorative brass cans containing less expensive, heavy iron bars. Somehow, probably in shipping when the delivery box was sharply jarred, the iron bars had been accidentally magnetized. When the metal pendulum of the clock traveled near one of the weights, the magnetic field of the iron bars exerted an attracting force on the metal clock pendulum, slowing it to a stop. The clock problem was solved by replacing the magnetized iron weights with unmagnetized ones. The stopping of a clock pendulum had provided the indirect evidence of a nearby, strong attracting magnetic field.

Science teachers illustrate these invisible magnetic fields using a simple bar magnet placed just below a sheet of paper. A file is used to scrape an iron nail so that some filings fall on the paper just over the spot where the magnet is hiding. By gently flicking the paper, the newly magnetized filings align with the magnetic field to display the dipole field pattern of the bar magnet (Figure 1.15).

Direct (not oscillating) current through a wire that is wound into a helix (as thread is wound on a spool) creates what is called an electromagnet. The field

FIGURE 1.15 ► A magnetic field pattern is formed by iron filings on a sheet of paper that covers a dipole magnet.

pattern from this winding has a dipolar form, similar to that of the bar magnet. Giant, current-driven electromagnets have been manufactured with a reputed capacity for lifting 75 tons—the weight of an entire train engine. The strong fields that we examine in this book come from natural magnetized material, from electric current sources deep within the Earth, and from currents in the region of space above our planet.

1.3.3 Measuring Scales

Although the dipole moment lets us compare the field sources, we need some units for measuring how strong a magnetic field can be at any place away from the source. Space scientists confuse the public by using two equivalent units for the field strength, gamma (y) and nanotesla (nT). In this book we use the simpler Greek letter gamma (y) because it is older, it provides a convenient size for the natural fields that we examine, the Greek letter honors a famous geomagnetician (Gauss), and y is more in use by the Earth magnetism and space science communities. Nevertheless, the equivalent name, nanotesla (one-millionth of a Tesla equals 1 gamma), is the proper unit officially

0.01

0.001

0.0001

0.00001

EARTH MAIN FIELD

• AURORAL ZONE MAGNETIC STORMS

■ MID-LATITUDE MAGNETIC STORMS

■ MID-LATITUDE QUIET-DAY CHANGES

■ GEOMAGNETIC PULSATIONS

■ TYPICAL MAGNETIC SENSOR

HUMAN BRAIN

SQUID MAGNETIC SENSOR

FIGURE 1.16 ► The size of magnetic fields originating from various sources. Note that the gamma (nanotesla) scale is logarithmic (meaning that each step upward is 10 times larger than the step below).

assigned by an International System committee for standardizing the world's scientific naming system, called SI Units. The Tesla units are preferred by physicists and engineers. Another unit, the Gauss (1 Gauss =100,000 gammas), is a convenient size for paleomagnetic studies.

The Earth's main field varies from about 60,000 gammas in polar regions to about half this size near the equator. Quiet-time daily variations of the field at mid-latitudes can be tens of gammas in amplitude. The Earth's natural pulsation fields have been measured from about ten gammas to the tiny one-thousandth of a gamma, with oscillation periods ranging from several minutes to fractions of a second. Field variations from hundreds to several thousand gammas in size, and lasting from hours to a full day or more, occur during a geomagnetic storm. We will be visiting all of these phenomena in our tour. The field sizes of these and other sources are compared in Figure 1.16.

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