Global Positioning System Reference
In-Depth Information
FIGURE 1.13. Idealized geomagnetic field lines. Note the variation of magnetic north
from true north, and the inclination angle of the field lines with the earth's surface.
Another source of geomagnetic field variation is the sun. There are from
time to time huge eruptions from the solar surface that are known as coro-
nal mass ejections ; these disturb our planet's own field and occasionally
even link directly to it, resulting in large magnetic storms, during which
all bets are o√ regarding field variation predictability. The heating of our
ionosphere during these magnetic storms adversely a√ects navigation in
a couple of di√erent ways: long-range radio communications are inter-
rupted, and GPS service is degraded. (Also the aurorae—borealis and aus-
tralis—become more prominent, and electrical blackouts happen as a re-
sult of power surges along national power grids.)
The overall shape of the field is not as simple as figure 1.13 suggests:
there are many local anomalies due, for example, to iron ore deposits in the
earth's crust, but on a larger scale due to irregularities in the outer core and
to non-dipole features of the field. Thus, in the year 2000 the geomagnetic
field looked like figure 1.14. From this chart it is clear that real magnetic
field lines wander quite far from the idealized north-south direction of
figure 1.13. 25 Following a compass from the magnetic South Pole will
eventually lead you to the magnetic North Pole, but your path will not be
straight. Because every feature of the geomagnetic field changes in time,
magnetic maps soon go out of date and need to be replaced every five years.
The geophysicists' understanding of the phenomenon is now su≈ciently
25. An animation of the change in magnetic variation over the past few centuries is
provided online at Wikipedia's entry for magnetic declination.
 
 
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