Solar Wind: Global Properties

The most fundamental problem in solar system research is still unsolved: how can the Sun with a surface temperature of only 5800 K heat up its atmosphere to more than a million K? In fact, the solar atmosphere is so hot that not even the Sun’s enormous gravity can contain it. Part of it is continuously evaporating into interplanetary space: the SOLAR WIND. As a highly ionized magnetized plasma it dominates a huge volume around the Sun which is called the heliosphere. Only far beyond the outermost planets is a transition into the interstellar gas flowing through our Galaxy expected to occur. On the rare occasions of solar eclipses, every observer is stunned by the high degree of structure in the then uncovered CORONA. No wonder that the solar wind emerging from there is similarly inhomogeneous and creates a complicated three-dimensional shape of the plasma heliosphere. Interactions between outflowing streams of different speeds and solar transient phenomena cause further complications. Thus, the solar wind as we see it from the Earth’s orbit is characterized by an enormous variability in all its basic properties. It is this very variability that allows the solar wind to have a surprisingly large impact on the Earth. Some effects even make it down to the ground! The solar wind proves to be one key link between the solar atmosphere and the Earth. Although the energy transferred by the solar wind is miniscule compared to both sunlight and those energies involved in the Earth’s atmosphere, the solar wind is capable of pin-pricking the Earth system until it eventually reacts in a highly nonlinear way. In this article the chief characteristics of the solar wind in context with its sources in the corona will be presented. The next section first describes the basic properties of the solar wind as observed in the comparatively simple situation around solar activity minimum. Next the corona and inner heliosphere are discussed in their full 3D extent. The fate of the stream structure as it evolves with increasing distance from the Sun is then dealt with. Finally we summarize the current knowledge of the different types of solar wind, ending with the admission that none of them is really explained yet.