Matter, in our daily experience, can be divided into solids, liquids, and gases. Manipulation of matter has shaped our scientific world view as well as our intuitive understanding of its different states and their behavior. But moving upwards from the surface of the Earth, our environment changes and no longer fits into this picture: starting at a height of about 80 km, the atmosphere contains an ionized particle component, the ionosphere. With increasing height, the relative importance of the neutral component decreases and ionized matter becomes dominant. Farther out in the magnetosphere and in interplanetary space almost all gas is ionized: the hard electromagnetic radiation from the Sun immediately ionizes almost all matter. Space therefore is dominated by a plasma, the “fourth state of matter”.
KeywordsSolar Wind Coronal Mass Ejection Energetic Particle Interstellar Medium Stellar Wind
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