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beyond 4 Earth radii and especially beyond synchronous altitude, 6.6 Earth radii, the perturbations are sufficiently large to render the harmonic expansion model invalid.

Perturbations to the Main Field. The primary source of geomagnetic field perturbations is the Sun. The Sun constantly emits a neutral plasma called the solar wind, described in Section 5.3. The action of the solar wind is to distort the Earth's field at high altitudes (8 to 10 Earth radii) so that the multipole description of Eq. (5-5) is no longer valid at those altitudes. Because the plasma is highly conductive, it will not allow the Earth's field to enter it. Thus, the plasma compresses the field ahead of it until the plasma energy density equals the magnetic field energy density at a distance of about 10 Earth radii [Haymes, 1971], At that point, the plasma breaks up so that some of the charged particles are trapped in the magnetic field. Other particles slip around the field and drag the field lines along as they pass the Earth, as shown schematically in Fig. 5-6.

A shock front, similar to a sonic boom, occurs where the solar wind first strikes the geomagnetic field because the solar wind is moving faster than the field can respond [Haymes, 1971], Just beyond the shock front is a region of magnetic turbulence called the magnetosheath. It is characterized by rapidly fluctuating field strengths and directions [Harris and Lyle, 1969]. Within the magnetosphere, the field is primarily due to geologic causes; outside, the field is due largely to the solar wind and its interaction with the geomagnetic field. The boundary between the magnetosheath and the magnetosphere is called the magnetopause. The region behind the Earth relative to the Sun, where the geomagnetic field lines first fail to close because they are being dragged along by the plasma, is called the cusp region. It occurs at a distance of 8 to 16 Earth radii and ± 25 deg geomagnetic latitude [Harris and Lyle, 1969]. That part of the geomagnetic field which is carried by the plasma is called the magnetotail. Its extent is not known, but it has been observed by Pioneer VII at 1000 Earth radii [Harris and Lyle, 1969], The plane which

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