1. Introduction to Geomagnetic Fields - Cambridge University Press
Introduction to Geomagnetic Fields is a textbook for advanced undergraduate and graduate students of geophysics. Introduction to Geomagnetic Fields is a textbook for advanced undergraduate and graduate students of geophysics. It explains the natural magnetic fields in and surrounding the Earth that arise from a variety of electric currents. The author clearly presents these different components of the Earth¿s magnetic field with a minimum of mathematical complexity. Readers are also introduced to the techniques and instrumentation for measuring geomagnetic fields, and to the range of applications for which these measurements are used. This second edition has been fully revised to include the most recent advances in this subject area. It has been designed as a textbook for use with semester courses in geomagnetism and includes student exercises at the end of each chapter. Special appendices review relevant mathematical techniques and direct the reader to various journals, books, organizations, and websites where the latest computer programs for geomagnetism may be downloaded.

2. More Information About Geomagnetic Fields
Further Understanding of Geomagnetism The Earth's magnetic field is described by seven parameters. These are declination (D), inclination (I), horizontal intensity (H), vertical intensity (Z), total intensity (F) and the north (X) and east (Y) components of the horizontal intensity. By convention, declination is considered positive when measured east of north, inclination and vertical intensity positive down, X positive north, and Y positive east. The magnetic field observed on Earth is constantly changing. The geomagnetic field measured at any point on the Earth's surface is a combination of several magnetic fields generated by various sources. These fields are superimposed on and interact with each other. More than 90% of the field measured is generated INTERNAL to the planet in the Earth's outer core. This portion of the geomagnetic field is often refered to as the Main Field. The Main Field varies slowly in time and can be described by Mathematical Models such as the International Geomagnetic Reference Field (IGRF) and World Magnetic Model (WMM). The Main Field creates a cavity in interplanetary space called the magnetosphere, where the Earth's magnetic field dominates in the magnetic field of the solar wind. The magnetosphere is shaped somewhat like a comet in response to the dynamic pressure of the solar wind. It is compressed on the side toward the sun to about 10 Earth radii and is extended tail-like on the side away from the sun to more than 100 Earth radii. The magnetosphere deflects the flow of most solar wind particles around the Earth, while the geomagnetic field lines guide charged particle motion within the magnetosphere.

3. The Geomagnetic Field : Seconds to millions of years.
Earth's magnetic field varies on timescales from milliseconds to millions of years. Below are a couple of images of various timescales indicating how Earth's magnetic field has varied in the past

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