Space between the Sun and its planets is not empty as had been generally thought until the 1950s. It is filled by a tenuous magnetized plasma, which is a mixture of ions and electrons flowing away from the Sun: the solar wind. In fact, the Sun’s outer atmosphere is so hot that not even the Sun’s enormous gravity can prevent it from continually evaporating. The escaping plasma carries the solar magnetic field along, out to the border of the heliosphere where its dominance finally ends.
The solar wind (and the IMF carried with it) proves to be one key link between the solar atmosphere and the Earth system. Although the energy transferred by the solar wind is minuscule compared to both sunlight and those energies involved in Earth’s atmosphere, the solar wind is capable of pin-pricking the Earth system which eventually may react in a highly nonlinear way. There are indications of effects reaching down as far as the troposphere, and our increasingly sophisticated high-tech civilization can indeed notice them and does, at times, even suffer from them. That is why the role of the Sun and the solar wind as the drivers of space weather have gained particular attention in the recent past.
Generally, the solar wind flow is diverted around Earth by its magnetosphere that is maintained by the Earth’s intrinsic magnetic field. Solar wind particles cannot enter, unless there occurs a process called magnetic reconnection of interplanetary and planetary magnetic field lines. That may happen if the northward pointing Earth field on the front of the magnetosphere is hit by solar wind carrying a southward pointing interplanetary field. In such case, significant geomagnetic disturbances of various kinds are initiated (see, e.g., Tsurutani et al., 1988). Note that usually the IMF near the Earth does not have northward or southward pointing components. It is the intention of this section to describe the various effects by which the IMF can be tilted such that major Bz south excursions actually do occur.
The status of knowledge on the solar wind before 1972 had been very well summarized in the textbook by Hundhausen (1972). Then, from the mid 1970s on, a new class of space missions (Skylab, Helios, Voyager, and Ulysses) equipped with a new generation of instruments had initiated a new epoch in solar and heliospheric research. Numerous important discoveries were made and are documented in the literature. Major reviews can be found, e.g., in Zirker (1977); Schwenn and Marsch (1990, 1991); Kohl and Cranmer (1999); Srivastava and Schwenn (2000); Balogh et al. (2001). A comparable step forward occurred in the mid 1990s when the Yohkoh, SOHO, WIND, ACE, and TRACE spacecraft went into operation. Reviews can readily be found, e.g., in the series of SOHO Workshop Proceedings published in the ESA SP series. I further recommend the reviews in Living Reviews in Solar Physics “Kinetic Physics of the Solar Corona and Solar Wind” by Marsch (2006) and “The Solar Wind as a Turbulence Laboratory” by Bruno and Carbone (2005) and the article by Marsch et al. (2003).
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