21st Century Essential Guide to Geomagnetism and the World Magnetic Model: U.S. Geological Survey (USGS) and National Oceanic and Atmospheric ... National Geomagnetism Program (CD-ROM)
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The Earth's magnetic field is both expansive and complicated. It is generated by electric currents that are deep within the Earth and high above the surface. All of these currents contribute to the total geomagnetic field. In some ways, one can consider the Earth's magnetic field, measured at a particular instance and at a particular location, to be the superposition of symptoms of a myriad of physical processes occurring everywhere else in the world. The challenge is to untangle the rich information content of the magnetic field so that we can better understand our planet and the surrounding space environment in which it resides. The geomagnetic field is generated by electric currents located in many different parts of the Earth. In the outer core the main part of the geomagnetic field is sustained by a naturally occurring dynamo. In the mantle currents can be induced by time-dependent variations in the ambient magnetic field. In the crust the field has both induced and permanent components. And, in the ionosphere and magnetosphere electric currents are sustained through a complicated interaction with the Sun, the heliomagnetic field, and the solar wind of charged particles. The many different, and sometimes remote, sources of the Earth's magnetic field each contribute to the total field at any one particular location, with the very different physical processes in each domain giving rise to a wide variety of time-dependent geomagnetic variations. Therefore, through the analysis of the time series of the magnetic field from different geographic locations, the different source regions, be they below or above the Earth's surface, can be studied for the purposes of scientific knowledge and understanding. The monitoring and analysis of the geomagnetic field is also important for practical applications, some of which have been made for centuries. The magnetic field can be used for orientation, navigation, and mineral and oil exploration. Of more recent interest, the infrastructure and the activities of our modern, technologically-based society can be adversely affected by rapid magnetic-field variations driven by the dynamic processes in the near- Earth space environment. This is particularly true during so-called ‘magnetic storms’, when radio communication can be difficult or impossible, global-positioning systems (GPS) can be degraded, satellite electronics can be damaged, satellite drag can be enhanced, astronaut and high-altitude pilots can be subjected to increased levels of radiation, pipe-line corrosion can be enhanced, and electric-power grids can experience voltage surges which cause blackouts. The most beautiful manifestations of geomagnetic activity are aurorae, seen prominently at high latitudes near the geomagnetic poles. Given the plurality of geomagnetic phenomena, it is not surprising that the communities concerned with magnetic data are numerous and diverse.
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