As first proposed by Axford (1962) and Kellogg (1962), all planetary bodies having either a magnetosphere or a highly conducting ionosphere have also a bow shock associated with the deflection of the solar wind around them. This shock wave develops because the information needed to deflect the solar wind plasma around the obstacle travels at a velocity that is less than that of the solar wind flow. In general three waves are needed:

It is the last, fast magnetosonic wave that creates the bow shock also in front of the Earth 's magnetosphere. The other two waves are present in the magnetosheath.

In the upstream region, i.e., in front of the bow shock, is the so-called foreshock region. This region is important for the magnetospheric physics because of a wealth of wave activity that can be found inside of it, and because it creates, together with the most typical IMF direction close to the inner planets, a strong dusk-dawn asymmetry around the magnetopause (upstream foreshock in the dawn side is much larger than the downstream foreshock in the dusk side).

The waves in the foreshock region are coming from several sources:

For example, the compressional ULF waves observed in the foreshocks of several planets are due to backstreaming ions. The frequency of these waves is controlled by the strength of the IMF, and in Earth 's case they are labelled as Pc 3 pulsations with periods about 30-40 s.

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