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Published online by Cambridge University Press: 22 October 2009
The terrestrial magnetosphere comprises the region of space where the properties of naturally occurring ionized gases are controlled by the presence of Earth's magnetic field. This very broad definition means that the terrestrial magnetosphere extends from the bottom of the ionosphere to more than ten Earth radii (Re) in the sunward direction and to several hundred Re in the antisunward direction.
The magnetosphere is formed as a result of the interaction of the supersonic, superalfvénic, magnetized solar wind with the intrinsic magnetic field of the Earth. To understand this interaction, we first briefly discuss the main characteristics of the intrinsic terrestrial magnetic field and then turn our attention to the interaction between this intrinsic field and the solar wind.
The Intrinsic Magnetic Field
A couple of hundred years ago Gauss showed that the magnetic field at the surface of the Earth can be described as the gradient of a scalar potential. In general the near Earth magnetic field can be expressed as
where Φint and Φext represent scalar potentials due to intrinsic and external sources, and Φtot is the sum of these two potentials (describing the total geomagnetic field). In general, planetary magnetic potentials are expressed as an infinite series using associated Legendre polynomials:
where θ and ϕ are geographic colatitude and east longitude, respectively.
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