The first evidence of the solar wind surfaced after comet tail deflections were observed by L. Biermann in 1951. A cometary ion tail is oriented along the difference between the cometary and solar wind velocities, whereas the dust tail is in the antisunward direction. The ion tail directions indicated the existence of an outflow of ionized gas from the Sun (the solar wind) and allowed estimates of solar wind speed. Spacecraft observations have established that at 1 AU, the solar wind has a typical ion number density of about 7 cm−3 and is composed of about 95% protons and 5% helium and other minor ions. A typical speed is 450 km/s. At this speed, ions travel from the Sun to 1 AU in about 4 days. The impact of the solar wind on planets with magnetic fields (Earth, Jupiter, Saturn, Uranus, Neptune) causes phenomena such as magnetospheres, aurorae, and geomagnetic storms, whereas at objects lacking magnetospheres (Mars, Venus, comets), atmospheric neutrals undergo charge exchange and are picked up by the solar wind flow.
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