geomagnetism
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| geomagnetism | |
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| Name | Geomagnetism |
| Caption | A schematic of Earth's magnetic field structure, showing field lines and the magnetosphere. |
geomagnetism. The scientific study of the Earth's magnetic field, a complex and dynamic force that originates within the planet's interior and extends far into space. This field, which resembles that of a giant bar magnet, is generated by the motion of molten iron and nickel in the Earth's outer core, a process known as the geodynamo. It shields the planet from harmful solar wind and cosmic rays, influences animal navigation, and is fundamental to technologies like compass navigation and geophysical surveying.
Earth's magnetic field
The Earth's magnetic field is primarily dipolar, with magnetic north and south poles that are offset from the geographic poles. The field's intensity varies across the globe, being strongest near the poles and weakest near the magnetic equator. Key structures include the magnetosphere, a region dominated by the planet's magnetic field that is compressed on the sunward side by the solar wind and elongated into a magnetotail on the night side. The Van Allen radiation belts, zones of trapped charged particles, are a critical component of this magnetospheric system. The interaction between the solar wind and the magnetosphere creates phenomena like the aurora borealis and aurora australis.
Geomagnetic phenomena
Numerous transient and periodic phenomena are driven by interactions between the Earth's magnetic field and external forces. Magnetic storms are major disturbances caused by coronal mass ejections from the Sun, which can induce powerful geomagnetically induced currents in power grids and pipelines. Daily variations, such as the solar quiet variation, are driven by ionospheric currents. Auroras occur when charged particles from the solar wind are guided by field lines into the upper atmosphere, exciting gases like oxygen and nitrogen. Other phenomena include magnetic pulsations and magnetic substorms.
Measurement and observation
The systematic measurement of geomagnetic elements began with the use of the magnetic compass, with early records from China and later work by William Gilbert. Modern observation relies on a global network of observatories, such as those operated by the British Geological Survey and the United States Geological Survey. Key instruments include the magnetometer, with types like the fluxgate magnetometer and proton precession magnetometer deployed on the ground, aboard satellites like the Swarm constellation, and on ocean-going vessels. Historic voyages, like those of HMS *Challenger*, contributed to early global field mapping.
Geomagnetic models
Mathematical models are essential for describing and predicting the magnetic field. The International Geomagnetic Reference Field is a standard spherical harmonic model of the Earth's main field, updated every five years by the International Association of Geomagnetism and Aeronomy. For navigation and orientation, the World Magnetic Model is widely used. More complex models, like the CHAOS model, incorporate data from satellite missions to study core dynamics and secular variation. These models are critical for applications in aviation, submarine navigation, and spacecraft attitude control.
Causes and generation
The primary source of the field is the geodynamo in the Earth's outer core, where the convective motion of electrically conductive molten iron and nickel, combined with the planet's rotation, generates self-sustaining magnetic fields through magnetohydrodynamics. This process is influenced by heat flow from the inner core and mantle convection. The Coriolis force organizes these fluid motions into complex rolls and columns. Paleomagnetic studies of igneous rock and sedimentary rock provide evidence for the dynamo's operation over billions of years. Theoretical foundations were advanced by scientists like Walter M. Elsasser and Edward Bullard.
Effects and applications
Geomagnetism has profound practical and biological effects. It enables magnetic navigation using the compass, a technology pivotal during the Age of Discovery for explorers like Christopher Columbus. In geophysics, magnetic surveying is used for mineral exploration, locating deposits of iron ore, and archaeological prospecting. The field protects the atmosphere from erosion by the solar wind. Many organisms, including homing pigeons, monarch butterflies, and certain bacteria, use the field for magnetoreception and navigation. It is also crucial for calibrating systems in Global Positioning System satellites.
Geomagnetic variations and reversals
The magnetic field undergoes constant change. Secular variation refers to slow changes in the field's strength and direction over decades to centuries, causing the gradual drift of the north magnetic pole across the Canadian Arctic. More dramatically, the field has reversed its polarity numerous times throughout Earth's history, as recorded in the magnetic signatures of oceanic crust at mid-ocean ridges and in volcanic rock sequences like those of the Columbia River Basalt Group. These geomagnetic reversals are irregular, with the last major one being the Brunhes–Matuyama reversal approximately 780,000 years ago. Studies of these events rely on the work of institutions like the Lamont–Doherty Earth Observatory.