Locally Measured Dip Poles

So far we have discovered four possible poles from our modeling of the full global field measurements. Expeditions to the north and south magnetic poles supposedly are searching for the locations where the main field points directly into or out of the surface. There is no concern with measurements elsewhere about the Earth. Three principal local problems affect the explorer's attempt to define this "place toward which world compasses point." The first is that the explorers have local measurements only, which are typically dependent on local geological characteristics. Positions on or near the islands of northern Canada (Figure 3.4) or off the coast of Antarctica (Figure 3.12) are known to have crustal geological conductivity features that modify the locally measured fields. For example, aeromagnetic measurements of field anomalies led to the discovery of oil-bearing regions in northern Alaska. Also, at locations near a lateral change in conductivity (such as at the ocean boundaries of continents or islands) induction causes the observed field fluctuations to follow a sloping surface (called the Parkinson's vectors phenomenon).

The second problem is that the magnetic pole expedition's vertical field measurement adds together all the local fields from both above and below

Antarctica Magnetic Pole
FIGURE 3.12 ► This map of the region between Antarctica, New Zealand, and Tasmania shows the strange position for a South Magnetic Pole. Figure adapted from Atlas of Continents, Rand McNally & Company.

the Earth's surface; whereas, what is expected is a unique pole of the Earth's main (internal only) field. As we shall see shortly, there are seasonal, diurnal (24-hr cycle), and sector-effect (Section 3.5.4, p. 94) distortions of the Earth's high-latitude external (magnetospheric) quiet main field in space that are influencing the surface vertical field measurements. In addition, the arrival of energetic particles from the Sun introduces strong currents that flow in the upper atmosphere at the polar regions on nearly every day of the year. (I will explain this further in Chapter 4.) Also, the ionospheric currents (see Section 3.4, p. 88) are quite different in the long sunlit days of summertime polar expeditions than they are in the long nights of winter. With the great cost constraints and time limitations imposed on high-latitude research operations, it is highly unlikely that the summertime exploration team remains long enough at the selected site to obtain a record of the rare, fully quiet, solar-terrestrial conditions.

The third problem is that the desired internal main field can only be separated from the external field by a complete global analysis for which the single polar expedition has no data. It takes a full global internal field pattern obtained from a global network of observatories to establish the best dipole location; the "place toward which all world compasses point" cannot be obtained from just one polar location measurement. Nevertheless, for some strange reason, cartographers for major map publishing companies still indicate this spot where some high-latitude expeditions have found a Locally-Measured Dip Pole (Figure 3.13), which is our fifth and poorest candidate for that important designation as the "Magnetic Pole."

Another difficulty is the dates that the poles were measures are rarely printed on world charts, although we know that the magnetic field patterns are drifting steadily westward. The map publishers can obtain their information on the best pole positions from the International Association of Geomagnetism and Aeronomy scientists, not from overenthusiastic polar explorers.

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