Two Eccentric Axis Poles

It became evident to early geomagneticians that not only was the Earth's dipole field tilted, it was offset from the Earth's geographic center. Two procedures have been used to find the best eccentric axis dipole location. In one, the geographic axis field IGRF coefficients were given a second special mathematical analysis that let the dipole tilt and eccentricity (the distance from the Earth geographic center) to be varied until the new dipole terms of the IGRF table were maximized at the expense of all the nondipole terms. Because the quadrupole terms are the second largest multipole field-strength group, a slightly different, simpler analysis method is now typically used. That method determines what dipole tilt and eccentricity can best maximize the dipole terms while minimizing just the quadrupole terms. The Eccentric Axis Dipole field of the Earth represents the bulk of the main field energy. This model provides the simplest representation of the directing field that, on average, attracts world compasses and is essentially the guiding field affecting a charged particle as it nears the Earth.

Because the eccentric axis dipole location is away from the Earth's geographic center (Figure 3.10), the eccentric dipole field line in the polar region that is perpendicular to the Earth's surface is not the field line that traces the dipole axis. The locations where the eccentric axis itself breaks the Earth surface are called the Eccentric Axis Dipole Poles—a third candidate for our

FIGURE 3.10 ► The eccentric axis dipole center is offset from the Earth's center. That position is exaggerated here to show how the eccentric dipole axis exits the Earth surface at an angle and how two off-axis field lines (north and south) are perpendicular to the surface when they exit. Bombarding auroral electrons follow field lines that are organized with respect to the dipole axis, not the Earth's surface.

j&S

FIGURE 3.10 ► The eccentric axis dipole center is offset from the Earth's center. That position is exaggerated here to show how the eccentric dipole axis exits the Earth surface at an angle and how two off-axis field lines (north and south) are perpendicular to the surface when they exit. Bombarding auroral electrons follow field lines that are organized with respect to the dipole axis, not the Earth's surface.

"Magnetic Poles" definition. The completely different locations where the eccentric dipole fields are vertical are called the Eccentric Axis Dip Poles—a forth candidate for "Magnetic Poles". The eccentric axis pole positions have a pronounced hemisphere asymmetry; Southern Hemispheric eccentric axis poles are noticeably further from the Earth's spin axis than their Northern Hemisphere counterparts. Were it not for some other difficulties, the Eccentric Axis Dip Pole locations would be the place where the special magnetic pole search expeditions find their goal.

The main geomagnetic field changes with time. Similarly, the tilt of the eccentric dipole, its distance from the geographic Earth center, and its westward rotation about the Earth's center have been changing (Figure 3.11). Since the 1800s, the dipole center position has moved away from the Earth's geographic center, from about 250 km (156 miles) to about 530 km (331 miles) toward the northwest Pacific.

EAST LONGITUDE

FIGURE 3.11 ► The drifting position of the Earth's eccentric axis dipole pole from 1650 to 1985 is shown for the northern Arctic region. Figure adapted from A. C. Fraser-Smith.

EAST LONGITUDE

FIGURE 3.11 ► The drifting position of the Earth's eccentric axis dipole pole from 1650 to 1985 is shown for the northern Arctic region. Figure adapted from A. C. Fraser-Smith.