A magnetosphere is a cavity in the solar wind flow formed by the interaction of the solar wind and interplanetary magnetic field with the intrinsic magnetic field or ionized upper atmosphere of a planetary body. In the terrestrial case, the interaction is dominated by the strong intrinsic quasi-dipolar magnetic field; this is also the case of the outer giant planets Jupiter, Saturn, Uranus and Neptune (Southwood and Kivelson, 2001). On the other hand, the inner solar system, Earth-like planets Venus and Mars possess no intrinsic magnetic field, and the magnetosphere is then mostly formed by interaction of the atmosphere and ionosphere with the solar wind flow (Lundin et al., 2001). Mercury has a small intrinsic field but no atmosphere, there the solar wind interacts directly with the surface and exosphere (Killen et al., 2001). The types of space weather phenomena at the different planets depend critically on the magnitude of the intrinsic magnetic field, on the existence and characteristics of the planetary atmosphere and ionosphere, and on the distance from the Sun determining the properties of the driving solar wind and IMF. In the following, we only treat the terrestrial magnetosphere, which is characterized by a dense atmosphere and ionosphere and by a strong intrinsic geomagnetic field, but no internal plasma sources other than the ionosphere.
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