The solar wind, consisting of charged particles, extends about 100 AU from the Sun and is the source of auroras and geomagnetic storms. Its high speed and temperature are mysteries, and it wanes at the termination shock. The Sun emits 6.7 billion tons of solar wind per hour, but loses only 0.01% of its mass over time.
The solar wind is a stream of charged particles that shoots out of the Sun, or any star, in all directions. It consists mainly of protons and free electrons (plasma) with energies of about 1 keV (kilo-electron-volt). This fairly energetic but solar wind is usually harmless due to its low density. It extends outward about 100 AU (astronomical units, Earth-Sun distances), about three times as far from the Sun as Neptune’s orbit, at which point it collides with the interstellar medium. The region where this wind is dominant is known as the heliosphere.
It is not fully understood how the solar wind escapes the Sun and travels outward. It is partly due to the extremely high temperature of the corona, the uppermost layer of the solar atmosphere, which varies between 1 and 3 million Kelvin (1 and 3 million degrees Celsius, 1.8 and 5.4 million Fahrenheit), reaching occasional peaks of 10 million Kelvins. The high temperature of the corona is an unsolved question in physics itself, but the speed of the wind as it is ejected from the Sun – between 400 and 700 km/s – is another mystery. Even allowing for the high temperature of the corona, these particles must be given additional kinetic energy somewhere to escape the Sun as fast as they do. The magnetic fields generated by free electrons can help accelerate protons away from the Sun.
The solar wind is the source of various phenomena visible from Earth, including auroras (northern lights and southern lights), geomagnetic storms, the most severe of which can damage power grids and endanger astronauts, and tails at comet plasma. The Sun emits about 6.7 billion tons of solar wind per hour, which sounds like a lot, but becomes practically nothing as it spreads across the vast expanse of space. A landmass of wind is only ejected every 150 million years, and the Sun has lost only 0.01% of its mass during its 4.57 billion years. Other stars, notably Wolf-Rayet stars, lose much more mass to the solar wind over time. While the Sun would take 50 trillion years to expel all of its mass through the wind, a Wolf-Rayet star only takes about 100,000.
The solar wind is the main phenomenon in space for a great distance, but not forever. The influence of this wind begins to waver at the termination shock, about 75 AU from the Sun, where its speed decreases from supersonic to subsonic. The Voyager 1 space probe reached its termination shock on May 23-24, 2005. Data sent back from its sensors gave scientists a better idea of how dynamics change when solar wind is not the primary influence on the environment local space.
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