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:
- slow magnetosonic wave
- intermediate wave
- fast magnetosonic wave
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:
- Some of the waves are generated in the bow shock and propagate upstream.
- Other waves are generated by electrons and ions accelerated at the bow shock and reflected back into the solar wind or leaked from the magnetosheath back upstream. These backstreaming particles generate waves through various instabilities and these waves are then convected with the solar wind flow toward the shock.
- Still other waves originate as newly created ions scatter and thermalize both in the extended coronas surrounding comets and in the exospheres of unmagnetized planets.
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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