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# 11.19 The Solar Wind

The Sun also affects the Earth through the solar wind. The solar wind occurs as particles that were blasted out of flares and spots rush outward through interplanetary space. The solar coronal plasma, having been heated to nearly 2 million Kelvin by the violence of photospheric convection, also expands rapidly into space (limited only by magnetic forces acting on charged particles). Together these effects cause the solar wind — an outrush of gas past the Earth and beyond the outer planets. Near the Earth, the solar wind travels at velocities near 400 kilometers per second and sometimes reaches 1000 kilometers per second. The gas has cooled only to 200,000 Kelvin, but it is so thin that it transmits no appreciable heat to the Earth. According to spacecraft data, the solar wind extends significantly farther than Neptune's orbit.

Composite image of the Sun, the corona, and the solar wind, as seen by the SOHO satellite. Click here for original source URL.

The Sun is made of mostly hydrogen and helium and that the atoms are hot enough to be highly ionized (they have their electrons stripped off). It follows that the solar wind is composed of protons, electrons, and helium nuclei. In addition to the solar wind, solar radiation itself exerts an outward force on small dust particles. This effect, which is greater on small particles, is called radiation pressure. Together these cause the forces that blow comet ion tails away from the Sun.
The solar wind "blows a bubble" of rarified high temperature gas beyond the planets and into the interstellar medium. Because the solar wind is not uniform and the interstellar medium is not uniform, this is not a spherical bubble. The full extent of the Sun's influence is called the heliosphere. The point at which the solar wind runs out of strength in pushing against the interstellar medium is called the heliopause. It was an exciting moment when Voyager 1 measured a sudden increase in plasma density after traveling for 35 years and 11 billion miles from Earth. This most distant human artifact has officially left the Solar System and is heading to the stars