Earth's magnetic field extends from the Earth's inner core to where it meets the solar wind emanating from the Sun. Earth's field orientation changes over time because it is generated by the motion of molten iron alloys in the Earth's outer core. The Magnetic North Pole wanders, fortunately slowly enough that the compass is useful for navigation. At random intervals ("averaging" several hundred thousand years) the Earth's field completely reverses (the north and south geomagnetic poles change places with each other). These reversals leave a record in rocks that allow 'paleomagnetists' to calculate past motions of continents and ocean floors as a result of plate tectonics.
Note: The region above the ionosphere, and extending several tens of thousands of kilometers into space, is called the magnetosphere. This region protects the Earth from harmful ultraviolet radiation and cosmic rays.
http://en.wikipedia.org/wiki/Earth%27s_magnetic_field
The rate of reversals in the Earth's magnetic field has varied widely over time. 72 million years ago (Ma), the field reversed 5 times in a million years. In a 4-million-year period centered on 54 Ma, there were 10 reversals; at around 42 Ma, 17 reversals took place in the span of 3 million years. In a period of 3 million years centering on 24 Ma, 13 reversals occurred. No fewer than 51 reversals occurred in a 12-million-year period, centering on 15 million years ago. Two reversals occurred during a span of 50,000 years. These eras of frequent reversals have been counterbalanced by a few "superchrons" – long periods when no reversals took place
Magnetic North Change:
http://modernsurvivalblog.com/pole-shift-2/alarming-noaa-data-rapid-pole-shift/
Magnetic South Change:
http://modernsurvivalblog.com/pole-shift-2/pole-shift-north-races-south-crawls/
Magnetic Declination:
http://www.compassdude.com/compass-declination.shtml
Magnetic Pole Record:
http://image.gsfc.nasa.gov/poetry/tour/AAmag.html
Magnetic Pole Reversals:
http://en.wikipedia.org/wiki/Geomagnetic_reversal
http://en.wikipedia.org/wiki/File:Geomagnetic_polarity_0-169_Ma.svg
http://upload.wikimedia.org/wikipedia/commons/1/13/Geomagnetic_polarity_late_Cenozoic.svg
